Thursday, April 13, 2006

HOWTO:Low Mem requirment of Ubuntu


How to install an
Ubuntu-Desktop on low memory systems

(Pentium II and III Processor, 32-256 MB RAM)


Thomas HINTERBERGER contributed a chapter,
which details how to install iceWM on Debian-Sarge; scroll to the end
of this document, if you want to see it.

BnB Logo

Last updated: 26 Feb

This document has been written for Ubuntu Warty - so it is somehow
outdated. Well, we have done some additions for Hoary but probably
you'll find better guides in these days. Here are some (we have not
tested them!):

CongoriverWant to do a journey
Congo with Bino na Biso?

Check out
Kotambola! (only German yet - sorry!)

But you can write us, we
will answer in English :-))

More images from Congo: ../work/Fotos/index.html

What this Howto is about

Ubuntu installs by default
with the Gnome Desktop and other memory-intensive applications. So if
you do not have a fast and powerful machine (Pentium 4 and 512 MB of
RAM) your system will be quite slow. The following document explains
the steps, how to install Ubuntu and a GUI on a low memory system, so
that you can use it as a Workstation for your daily work. It is aimed
to the average SOHO-user (Small Office/Home Office).


Screenshot (Click to


# command

Type command as root (you can do
a sudo su- before, in order to get the root-prompt. Or you put sudo
before the command.

$ command

Type command as user.

Red text

Parts which should be improved.
Please let me know, if you have usefull suggestions.

1 The Basesystem

Get Ubuntu CD-ROM see

After inserting the CD-ROM into the
drive, boot and when asked to press Enter, type custom before. (Not linux custom!)

Hint for Hoary; In the Hoary installer there is no
'custom' install option. It appears they've replaced it with 'server'
which is a minimum install.

The result will be a minimal-system
with less than 300 MB on the HD and only a textprompt (no GUI).

2 Postinstall the GUI

I did it in the following order, but
probably the order is not important:

$ sudo su -

This puts you into a root-shell (#), so no more sudo
is necesary.

# vi

you are not familiar with
vi you can use nano or
any other texteditor instead.)

Enable the universe-repository by removing the Hashmarks (=# (2 times))

apt-get update

apt-get install icewm

apt-get install xserver-xfree86

apt-get install x-window-system-core

apt-get install xdm

apt-get install numlockx

apt-get install xterm

The result is a system with X and iceWM
as windowmanager. You log in as user and on the prompt:

$ startx

starts the GUI. (After the first
reboot, xdm autostarts and puts you directly into the GUI-mode)

So far this system needs 468 MB on your

From Frank Martelli [frank at
foodsavvy dot com]: Just a note when
dealing with the ppc version
of Ubuntu -- for some

reason, icewm (silently) requires /usr/lib/ A very
frustrating bug, as X starts lets you login and then (without any

returns to the login screen.


ln -s /usr/lib/ /usr/lib/

3 Basic Applications

We consider
a Mailreader, Webbrowser, PDF-Reader and Officesuite as basic:


apt-get install acroread

This adds 26 MB to the harddrive
(compared to 3,2 MB of gPDF, but gPDF laks crucial features like search
and copy of text.)


We include Flashplayer and
Acrobat-Plugin for optimal compatibility with todays WWW. (The plugins
only need 2,5 MB)

apt-get install mozilla flashplayer-mozilla acroread-plugin

Fills up the harddrive to 555MB


At the date of writing this Howto you
have two options. Choose one of them (either-or, not
both!). This will probably change constantly, as OpenOffice envolves -
but the principle will stay the same (just the versions will change).

Either: Install OpenOffice 1.1.2 from apt-repository

apt-get install

This installs Openoffice 1.1.2

Or: Install OpenOffice 1.1.3 from

I installed OpenOffice 1.1.3 from the
tar, provided by
After untaring the tree I cd into the tree and type:


3.3 Final thoughts

Finally we have now 770 MB for the
bare system on the harddrive, so if we consider to have 128 MB minimum
for the swap, we need harddrives between 1 and 1,5 GB for such

4 More Applications



Size on HD


install openssh-server



install gthumb


Acrobat Reader

install acroread

26 MB


install gpdf

gpdf has no search-feature and you can not copy&paste parts of text
out of PDFs)


OOo, German
Language Pack



OOo, German Help



CUPS Daemon

install cupsys



install gnome-cups-manager



install cupsomatic-ppd

22 MB

(This is too much - any ideas how to reduce this?!)

The Gimp

install gimp



install cdrecord

1,2 MB


install mkisofs

860 kB

nedit (Editor)

install nedit


alicq (ICQ-Client)

install alicq


gaim (ICQ-Client)

install gaim



install guitar

3,3 MB

und unzip

apt-get install zip

install unzip

550 kB


install rdesktop


(Graphisches Frontend für rdesktop und VNC)

install tsclient



not work yet!

install cdrtoaster

106 kB


not work yet !

install simplecdrx

5,1 MB


not work yet!

install gtoaster

6 MB

xine-ui (Multi Media Player)

apt-get install xine-ui

12 MB


apt-get install w32codecs

28,4 MB


Does not work

install mozilla-mplayer


apt-get install mplayer-fonts

7,3 MB

apt-get install

11 MB

mc (Midnightcommander,

apt-get install mc

5 MB

emelfm (Filemanager)

apt-get install emelfm

582 kB



Find a working frontend for cdrecord.

Find a working Mplayer-package.

4 Future plans

If we go down to 64 or 32 MB of RAM we
won't install OpenOffice. I recommend Abiword and Gnumeric instead.
Also the Mozialla web broser could be a problem on such systems. Better
to go with Firefox and Thunderbird/Pine.

5 Comparison

If we compare Ubuntu's ability to adapt
to low-memory-systems with Fedora we will be surprised:


Fedora Core 3 [MB]

Ubuntu 4.10 [MB]

Minimal standard



Minimal trimmed



w/ X, iceWM



w/ Mozilla


add. 58

w/ OpenOffice 1.1.3


add. 184

6 More help

Install and configure Ubuntu:

Installing Low-Memory-Systems:

7 Feedback

Please send comments and Feedback to

8 Alternative Installation with Debian Sarge


After installing, I had the feeling,
that the Debian Icewm has some problems with the Ubuntu System (tar was
not working, the same with Acroread and OpenOffice - the paths looked
different). Not knowing Icewm very well, I just decided to start from a
Debian Netinstall CD-Image (even floppy-disks are possible - you
download it from ).
It is very small - including X with Icewm 298 MB. I think, that Icewm
works better with the original Debian - after installation all programs
was working immediately (but perhaps it is just the newer Version of
Icewm - 2.20 - you also get Firefox 1.0, Mozilla 1.7.5 and gimp 2.2)

you can follow the guide from Ingo - I
will note only the steps you have to add.

after installation you have no "sudo
su"- so just type su and the root-password to get root.

add the follwing lines to your

deb sarge main

deb sarge/non-US main

deb-src sarge main

deb-src sarge/non-US main

# netselect-apt finds the most quick of all

after # apt-get install
xterm you add

apt-get install sudo

apt-get install emelfm

when you are now the
first time in Icewm, verify with emelfm or with the shell, that there
is a file /etc/sudoers - if not, you have to reboot. - rootshell: #
shutdown -r now

echo "[username] ALL = NOPASSWD: /sbin/shutdown, /sbin/poweroff,

/sbin/halt, /sbin/reboot, /bin/cdrecord" >> /etc/sudoers

shutdown -r now

after rebooting you
should now be able to use all commands, when you press strg+alt+del. If
you don't do this, you have to write all the time # shutdown -r now to
a rootshell, to shut down the system proper.

instead of this, you can
edit /etc/sudoers, but only with the command


!!!! don't use another
editor !!!!
- for visudo: strg o = saving, strg x = exit

you should have the
following lines:

root ALL=(ALL) ALL

[username] ALL = NOPASSWD: /sbin/shutdown,
/sbin/poweroff, /sbin/halt, /sbin/reboot, /bin/cdrecord

The rest is the same like in Ingos description - the only thing - I
would use bluefish instead of nedit - it is 4 MB more, but you have a
wonderful HTML - Editor.

The configuration of X is not as good and comfortable, as with
Ubuntu - I did a little trick: run the Ubuntu live CD or Knoppix (if
you have only 64MB, you have to do a swap partition before - I did not
test it with Ubuntu, but Knoppix runs with 64MB) - make the screen and
fonts well looking- copy the /etc/X11/XF86Config-4 from the live CD to
a disk (or mail it), bring it to your /home directory in Icewm- save
your existing /etc/X11/XF86Config-4.

# cp XF86Config-4 /etc/X11/XF86Config-4

with my system, it was working perfect (both - Knoppix-config and

one more tip: emelfm is working as a file browser, like Nautilus or
Konqueror - it is not visible from the first moment, it is hard
to configure, but it does.

Back to


1 2/1/05

Monday, February 27, 2006

Post-Translational Modification
Secreted and Membrane-Associated Proteins
Proteolytic Cleavage of Proteins
Mechanism of Sugar Linkage
Lysosomal Targeting of Enzymes
Clinical Significances of Glycoproteins
Defects in Glycoprotein Degradation
Vitamin C-Dependent Modifications
Vitamin K-Dependent Modifications
Selenoproteins Return to Medical Biochemistry Page

Secreted and Membrane-Associated Proteins

Proteins that are membrane bound or are destined for excretion are synthesized by ribosomes associated with the membranes of the endoplasmic reticulum (ER). The ER associated with ribosomes is termed rough ER (RER). This class of proteins all contain an N-terminus termed a signal sequence or signal peptide. The signal peptide is usually 13-36 predominantly hydrophobic residues. The signal peptide is recognized by a multi-protein complex termed the signal recognition particle (SRP). This signal peptide is removed following passage through the endoplasmic reticulum membrane. The removal of the signal peptide is catalyzed by signal peptidase. Proteins that contain a signal peptide are called preproteins to distinguish them from proproteins. However, some proteins that are destined for secretion are also further proteolyzed following secretion and, therefore contain pro sequences. This class of proteins is termed preproproteins.
Mechanism of synthesis of membrane bound or secreted proteins. Ribosomes engage the ER membrane through interaction of the signal recognition particle, SRP in the ribosome with the SRP receptor in the ER membrane. As the protein is synthesized the signal sequence is passed through the ER membrane into the lumen of the ER. After sufficient synthesis the signal peptide is removed by the action of signal peptidase. Synthesis will continue and if the protein is secreted it will end up completely in the lumen of the ER. If the protein is membrane associated a stop transfer motif in the protein will stop the transfer of the protein through the ER membrane. This will become the membrane spanning domain of the protein.back to the top

Proteolytic Cleavage

Most proteins undergo proteolytic cleavage following translation. The simplest form of this is the removal of the initiation methionine. Many proteins are synthesized as inactive precursors that are activated under proper physiological conditions by limited proteolysis. Pancreatic enzymes and enzymes involved in clotting are examples of the latter. Inactive precursor proteins that are activated by removal of polypeptides are termed, proproteins.
A complex example of post-translational processing of a preproprotein is the cleavage of prepro-opiomelanocortin (POMC) synthesized in the pituitary (see the Peptide Hormones page for discussion of POMC). This preproprotein undergoes complex cleavages, the pathway of which differs depending upon the cellular location of POMC synthesis.
Another is example of a preproprotein is insulin. Since insulin is secreted from the pancreas it has a prepeptide. Following cleavage of the 24 amino acid signal peptide the protein folds into proinsulin. Proinsulin is further cleaved yielding active insulin which is composed of two peptide chains linked togehter through disulfide bonds..
Still other proteins (of the enzyme class) are synthesized as inactive precursors called zymogens. Zymogens are activated by proteolytic cleavage such as is the situation for several proteins of the blood clotting cascade. back to the top


Membrane associated carbohydrate is exclusively in the form of oliogsaccharides covalently attached to proteins forming glycoproteins, and to a lesser extent covalently attached to lipid forming the glycolipids. Glycoproteins consist of proteins covalently linked to carbohydrate. The predominant sugars found in glycoproteins are glucose, galactose, mannose, fucose, GalNAc, GlcNAc and NANA. The distinction between proteoglycans and glycoproteins resides in the level and types of carbohydrate modification. The carbohydrate modifications found in glycoproteins are rarely complex: carbohydrates are linked to the protein component through either O-glycosidic or N-glycosidic bonds. The N-glycosidic linkage is through the amide group of asparagine. The O-glycosidic linkage is to the hydroxyl of serine, threonine or hydroxylysine. The linkage of carbohydrate to hydroxylysine is generally found only in the collagens. The linkage of carbohydrate to 5-hydroxylysine is either the single sugar galactose or the disaccharide glucosylgalactose. In ser- and thr-type O-linked glycoproteins, the carbohydrate directly attached to the protein is GalNAc. In N-linked glycoproteins, it is GlcNAc.
O-linkage to GalNAc
N-linkage to GlcNAc
The predominant carbohydrate attachment in glycoproteins of mammalian cells is via N-glycosidic linkage. The site of carbohydrate attachment to N-linked glycoproteins is found within a consensus sequence of amino acids, N-X-S(T), where X is any amino acid except proline. When an analysis of proteins in the public databases is carried out, it can be shown that approximately 65% of all the proteins contain at least one occurrence of the Asn-X-Ser/Thr consensus. N-linked glycoproteins all contain a common core of carbohydrate attached to the polypeptide. This core consists of three mannose residues and two GlcNAc. A variety of other sugars are attached to this core and comprise three major N-linked families:
1. High-mannose type contains all mannose outside the core in varying amounts.
2. Hybrid type contains various sugars and amino sugars.
3. Complex type is similar to the hybrid type, but in addition, contains sialic acids to varying degrees.
Structures of oligosaccharides of the 3 major classes of glycoprotein.Open squares: GlcNAc; open circles: mannose; open diamonds: galactose; filled squares: fucose; filled triangles: sialic acid the greek symbols a and b followed by numbers refers to the type of linkage.
Most proteins that are secreted, or bound to the plasma membrane, are modified by carbohydrate attachment. The part that is modified, in plasma membrane-bound proteins, is the extracellular portion of the protein that is modified. Intracellular proteins are less frequently modified by carbohydrate attachment. However, the attachment of carbohydrate to intracellular proteins confers unique functional activities on these proteins. Linkage of carbohydrate to cytosolic and/or nuclear proteins occurs via O-linkage and involves attachment of GlcNAc to serine or threonine residues. The linkage is catalyzed by the enzyme O-GlcNAc transferase, OGT. Several transcription factors and RNA polymerase II have been shown to be modified by O-GlcNAc linkage. back to the top

Mechanism of Carbohydrate Linkage to Protein

The protein component of all glycoproteins is synthesized from polyribosomes that are bound to the endoplasmic reticulum (ER). The processing of the sugar groups occurs cotranslationally in the lumen of the ER and continues in the Golgi apparatus for N-linked glycoproteins. Attachment of sugars in O-linked glycoproteins occurs post-translationally in the Golgi apparatus. Sugars used for glycoprotein synthesis (both N-linked and O-linked) are activated by coupling to nucleotides. Glucose and GlcNAc are coupled to UDP and mannose is coupled to GDP.
O-linked sugars: The synthesis of O-linked glycoproteins occurs via the stepwise addition of nucleotide-activated sugars directly onto the polypeptide. The nucleotide-activated sugars are coupled to either UDP, GDP (as with mannose) or CMP (for instance, NANA). The attachment of sugars is catalyzed by specific glycoprotein glycosyltransferases. Evidence indicates that each specific type of carbohydrate linkage in O-linked glycoproteins is the result of a different glycosyltransferase.
N-linked sugars: As indicated earlier, the three major classes of N-linked carbohydrate modifications are high-mannose, hybrid and complex. The major distinguishing feature of the complex class is the presence of sialic acid, whereas the hybrid class contains no sialic acid.
In contrast to the step-wise addition of sugar groups to the O-linked class of glycoproteins, N-linked glycoprotein synthesis requires a lipid intermediate: dolichol phosphate. Dolichols are polyprenols (C80-C100) containing 17 to 21 isoprene units, in which the terminal unit is saturated.
DolicholThe black bracket denotes the isoprene unit.The phosphate in dolichol phosphate is attached to the hydroxyl.
As indicated, the formation of the GlcNAc-b-Asn linkage in proteins occurs in the endoplasmic reticulum (ER) through cotranslational addition of a preassembled carbohydrate core structure that is delivered via the carbohydrate-dolichol lipid intermediate. The preassembled carbohydrate core structure comprises three terminal residues of glucose attached to a branched cluster of nine mannose residues that are in turn attached to two GlcNAc residues attached to dolicholpyrophosphate. The structure is abbreviated Glc3Man9GlcNAc2-P-P-dolichol. This structure is commonly referred to as the lipid-linked oligosaccharide (LLO), whereas the oligosaccharide structure itself is termed the en bloc oligosaccharide. In mammalian cells the importance of the terminal glucose residues is evident from the fact that transfer of Man9GlcNAc2-P-P-dolichol is some 25-times less efficient than the complete structure. In addition, structures that contain three terminal glucose residues, but not the complete Man9GlcNAc2 structure, are efficiently transferred to protein by oligosaccharyltransferase. Synthesis of the en bloc dolichol-P-P-oligosaccharide unit begins on the cytoplasmic face of the ER membrane and prior to transfer to the protein, the structure 揻lips?to the luminal side.
Pathway by which the synthesis and transfer of the lipid-linked oligosaccharide unit takes place at the membrane of the ER.
Immediately following transfer of the en bloc oligosaccharide unit to the protein, processing and alteration of the composition of the oligosaccharide ensues and continues as the protein passes through the ER then into and through the Golgi apparatus. Initially, the terminal glucose is removed through the action of glucosidase I (GI), a membrane bound enzyme recognizing a1,2-linked glucose. The remaining two glucose residues are then removed by glucosidase II (GII), a soluble enzyme recognizing a1,3-inked glucose. After removal of the glucose residues, the action of a-mannosidases removes several mannose residues as the protein progresses to the Golgi. The action of the various glucosidases and mannosidases leaves N-linked glycoproteins containing a common core of carbohydrate consisting of three mannose residues and two GlcNAc. Through the action of a wide range of glycosyltransferases and glycosidases a variety of other sugars are attached to this core as the protein progresses through the Golgi. These latter reactions generate the three major families of N-linked glycoproteins described above. back to the top

Lysosomal Targeting of Enzymes

Enzymes that are destined for the lysosomes (lysosomal enzymes) are directed there by a specific carbohydrate modification. During transit through the Golgi apparatus a residue of GlcNAc-1-phosphate (GlcNAc-1-P) is added to the carbon-6 hydroxyl group of one or more specific mannose residues that have been added to these enzymes. The GlcNAc is activated by coupling to UDP and is transferred by UDP-GlcNAc:lysosomal enzyme GlcNAc-1-phosphotransferase (GlcNAc-phosphotransferase), yielding a phosphodiester intermediate: GlcNAc-1-P-6-Man-protein. A second reaction (catalyzed by GlcNAc 1-phosphodiester-N-acetylglucosaminidase) removes the GlcNAc leaving mannose residues phosphorylated in the 6 position: Man-6-P-protein. A specific Man-6-P receptor (MPR) is present in the membranes of the Golgi apparatus. Binding of Man-6-P to this receptor targets proteins to the lysosomes.
Two distinct MPRs have been identified and both are members of the P-type lectin family. Both are type I integral membrane glycoproteins that contain an N-terminal extracellular domain, a single transmembrane domain and a C-terminal cytoplasmic domain. One receptor is large with a molecular weight of approximately 300kDa, the other receptor is smaller with a molecular weight of approximately 46kDa. Structural similarities between these two receptors indicates they are derived from a single ancestral gene with the larger receptor arising through multiple gene duplications. The extracellular portion of the larger receptor contains 15 repeating elements, each of which is highly similar to the extracellular domain of the smaller receptor. Both receptors exist as dimers embedded in the membrane.
The large receptor binds two moles of Man-6-P and the smaller binds one mole of Man-6-P per subunit, thus 4 and 2 moles of Man-6-P per dimer, respectively. The bovine and murine versions of the smaller receptors require divalent cations for ligand binding and thus the receptor has been termed the cation-dependent Man-6-P receptor (CD-MPR). However, the human counterpart may not require cations for its activity. The larger receptor does not require divalent cations for ligand binding and is therefore, commonly referred to as the cation-independent Man-6-P receptor (CI-MPR). However, the CI-MPR has been shown to bind the nonglycosylated polypeptide hormone, insulin-like growth factor II (IGF-II) and as such the larger MPR is more frequently identified as IGF-II/MPR. The IGF-II/MPR is available at the cell surface and its role in binding IGF-II is to target this hormone for degradation in the lysosomes. In addition to IGF-II, the IGF-II/MPR has been shown to bind a diverse array of Man-6-P-containing proteins as well as several nonglucosylated proteins. Although IGF-II/MPR and CD-MPR exhibit distinct activities, both receptors function to target newly synthesized lysosomal enzymes to the lysosomes. back to the top

Clinical Significances of Glycoproteins

Glycoproteins on cell surfaces are important for communication between cells, for maintaining cell structure and for self-recognition by the immune system. The alteration of cell-surface glycoproteins can, therefore, produce profound physiological effects, of which several are listed below.
1. The ABO blood group antigens are the carbohydrate moieties of glycolipids on the surface of cells as well as the carbohydrate portion of serum glycoproteins. When present on the surface of cells the ABO carbohydrates are linked to sphingolipid and are therefore of the glycosphingolipid class. When the ABO carbohydrates are associated with protein in the form of glycoproteins they are found in the serum and are referred to as the secreted forms. Some individuals produce the glycoprotein forms of the ABO antigens while others do not. This property distinguishes secretors from non-secretors, a property that has forensic importance such as in cases of rape. For more information of blood group antigens, including ABO visit the blood groups page at the SCARF site.
Structure of the ABO blood group carbohydrates,R represents the linkage to protein in the secreted forms, sphingolipid in the cell-surface bound square = GlcNAc, open diamond = galactose, filled square = fucose, filled diamond = GalNAc, filled diamond = sialic acid (NANA)
2. The truncation of erythrocyte surface glycoproteins leads to cell clumping, as in congenital dyserythropoietic anemia type II. Also referred to as HEMPAS (hereditary erythroblastic multinuclearity with positive acidified-serum test).
3. Several viruses, bacteria and parasites have exploited the presence of cell-surface carbohydrates, principally associated with protein (glycoproteins), using them as portals of entry into the cell.
A. Human immunodeficiency virus (HIV), the causative agent of AIDS, gains entry into cells of the immune system by attaching to a class of cellular receptors known as the chemokine receptors, most notably CXCR4 and CCR5. For more information on chemokines and their receptors visit the C.O.P.E site.
B. Members of the poxvirus family of viruses gain entry into cells, most frequently migratory leukocytes, by attaching to chemokine receptors including CCR1, CCR5 and CXCR4 (Science [1999] vol. 286 pp. 1968-1971).
C. Dystroglycan (DG) is a component of the dystrophin-glycoprotein complex. It is a laminin receptor encoded by a single gene and cleaved by postranslational processing into two proteins, peripheral membrane a-DG and transmembrane b-DG. a-DG interacts with laminin-2 in the basal lamina and b-DG binds to dystrophin containing cytoskeletal proteins in muscle and peripheral nerves. DG is involved in agrin- and laminin-induced acetylcholine receptor clustering at neuromuscular junctions, morphogenesis, early development, and the pathogenesis of muscular dystrophies. Recent evidence (Science (1998) vol. 282 pp. 2076-2079 and 2079-2081) demonstrates that a-DG present on Schwann cell membranes is the receptor for Mycobacterium leprae and also serves as the receptor for the arenavirus class of pathogens. Arenaviruses cause hemorrhagic fever in humans. Lymphocytic choriomeningitis virus (LCMV), Lassa fever virus (LFV), Oliveros and Mobala (all members of the arenavirus family) all bind to a-DG. The specificity of this interaction was demonstrated by the resistance to LCMV infection of cells harboring a null mutation in DG.
D. Rhinoviruses utilize attachment to ICAM-1 (intercellular adhesion molecule-1) to gain entry into cells.
E. The pathogenic human parvovirus, B19, attaches to the erythrocyte-specific cell-surface globoside identified as erythrocyte P antigen to infect erythrocytes.
F. The malarial parasite Plasmodium vivax, binds to the erythrocyte chemokine receptor known as the Duffy blood group antigen (also known as the erythrocyte receptor for interleukin-8) to infect erythrocytes.
G. The MN blood group system is a well-characterized set of erythrocyte surface antigens that represent the variable carbohydrate modifications of the trans-membrane glycoprotein, glycophorin. Glycophorin is the cellular receptor for influenza virus as well as the receptor for erythrocyte invasion by the malarial parasite Plasmodium falciparum.
H. Helicobacter pylori is the bacterium responsible for chronic active gastritis and gastric and duodenal ulcers; it is also the causative agent for one of the most common forms of cancer in humans, adenocarcinoma. This bacterium attaches to the Lewis blood group antigen on the surfaces of gastric mucous cells.
I. Rabies virus binds to cells through interactions with neural cell adhesion molecule (N-CAM).
J. The receptor for fibroblast growth factor (FGF) has been reported to be the portal of entry for human herpes virus Type I. Recent new evidence indicates that the portal of entry for human herpes simplex Type I viruses is 3-O-sulfated heparan sulfate (Cell 99:13-22, 1999).
K. Human herpesvirus 6 (HHV-6) infection occurs in virtually all persons within the first 2 years of life and persists the entire lifetime. In immunocompromised patients HHV-6 causes opportunistic infections and is the causative agent of exanthema subitum. HHV-6 has been linked to multiple sclerosis and to the progression of AIDS. The cellular receptor for HHV-6 is the cell-surface type-I glycoprotein, CD46 (Cell 99:817-827, 1999).
4. Some glycoproteins are tethered to the membrane by a lipid linkage: the protein is attached to the carbohydrate through phosphatidylethanolamine (PE) linkage, and the carbohydrate is in turn attached to the membrane via linkage to phosphatidylinositol (PI), which anchors the structure within the membrane. The linkage is called a glycosylphosphotidylinositol (GPI) anchor, and proteins that are anchored in this way are termed glypiated proteins. The disease, paroxysmal nocturnal hemoglobinuria, results from the loss of the erythrocyte surface glycoprotein, decay-accelerating factor, (DAF). DAF prevents erythrocyte lysis by complement. When this factor is lost from the erythrocyte surface, abnormal hemolysis occurs, with the end result of hemoglobin accumulation in the urine.
The GPI linkage of the T-cell marker Thy-1Line represents the outer surface of the membraneSquiggles represent the lipid portion of the GPI linkage embedded in the membraneOpen circles = mannose, filled diamonds = GalNAc, filled squares = fucoseFilled pentagons = ethanolamine, solid circles with P = phosphates
Other important GPI linked proteins are the enzymes acetylcholinesterase, intestinal and placental alkaline phosphatase and 5'-nucleotidase, the cell adhesion molecule N-CAM (neural cell adhesion molecule) and the T-cell markers Thy-1 and LFA-3 (lymphocyte function associated antigen-3).
5. The proper degradation of glycoproteins has medical relevance. Degradation occurs within lysosomes and requires specific lysosomal hydrolases, termed glycosidases. Exoglycosidases remove sugars sequentially from the non-reducing end and exhibit restricted substrate specificities. In contrast, endoglycosidases cleave carbohydrate linkages from within and exhibit broader substrate specificities. Several inherited disorders involving the abnormal storage of glycoprotein degradation products have been identified in humans. These disorders result from defects in the genes encoding specific glycosidases, leading to incomplete degradation and subsequent over-accumulation of partially degraded glycoproteins. As a general class, such disorders are known as lysosomal storage diseases and include the diseases known as mucolipidoses that result from incomplete degradation of the carbohydrate portions of glycolipids.
6. Defects in the proper targeting of glycoproteins to the lysosomes can also lead to clinical complications. Deficiencies in the enzyme responsible for the transfer of GlcNAc-1-P to Man residues (GlcNAc phosphotransferase) in lysosomal enzymes leads to the formation of dense inclusion bodies formation in the fibroblasts. Two disorders related to deficiencies in the targeting of lysosomal enzymes are termed I-cell disease (mucolipidosis II) and pseudo-Hurler polydystrophy (mucolipidosis III, also called mucolipidosis-HI). I-cell disease is characterized by severe psychomotor retardation, skeletal abnormalities, coarse facial features, painful restricted joint movement, and early mortality. Pseudo-Hurler polydystrophy is less severe; it progresses more slowly, and afflicted individuals live to adulthood.
Enzyme Defects in Degradation ofAsn-GlcNAc Type Glycoproteins

Enzyme Deficiency
progressive mental retardation, delayed speech and motor development, coarse facial features
primarily neurological defects, speech impairment
mental retardation, dystosis multiplex, hepatosplenomegaly, hearing loss, delayed speech
GM1 Gangliosidosis
also identified as a glycosphingolipid storage disease
GM2 Gangliosidosis(Sandhoff-Jatzkewitz disease)
b-N-acetylhexosaminidases A and B
also identified as a glycosphingolipid storage disease
Sialidosis(also identified as Mucolipidosis I)
myoclonus, congenital ascites, hepatosplenomegaly, coarse facial features, delayed mental and motor development
progressive motor and mental deterioration, growth retardation, coarse facial features, recurrent sinus and pulmonary infectionsback to the top
OMIM links for additional Defects in Glycoprotein Degradation


Many proteins are modified at their N-termini following synthesis. In most cases the initiator methionine is hydrolyzed and an acetyl group is added to the new N-terminal amino acid. Acetyl-CoA is the acetyl donor for these reactions. Some proteins have the 14 carbon myristoyl group added to their N-termini. The donor for this modification is myristoyl-CoA. This latter modification allows association of the modified protein with membranes. The catalytic subunit of cyclicAMP-dependent protein kinase (PKA) is myristoylated. back to the top


Post-translational methylation occurs at lysine residues in some proteins such as calmodulin and cytochrome c. The activated methyl donor is S-adenosylmethionine. back to the top


Post-translational phosphorylation is one of the most common protein modifications that occurs in animal cells. The vast majority of phosphorylations occur as a mechanism to regulate the biological activity of a protein and as such are transient. In other words a phosphate (or more than one in many cases) is added and later removed.
Physiologically relevant examples are the phosphorylations that occur in glycogen synthase and glycogen phosphorylase in hepatocytes in response to glucagon release from the pancreas. Phosphorylation of synthase inhibits its activity, whereas, the activity of phosphorylase is increased. These two events lead to increased hepatic glucose delivery to the blood.
The enzymes that phosphorylate proteins are termed kinases and those that remove phosphates are termed phosphatases. Protein kinases catalyze reactions of the following type:
ATP + protein <----> phosphoprotein + ADP
In animal cells serine, threonine and tyrosine are the amino acids subject to phosphorylation. The largest group of kinases are those that phsophorylate either serines or threonines and as such are termed serine/threonine kinases. The ratio of phosphorylation of the three different amino acids is approximately 1000/100/1 for serine/threonine/tyrosine.
Although the level of tyrosine phosphorylation is minor, the importance of phosphorylation of this amino acid is profound. As an example, the activity of numerous growth factor receptors is controlled by tyrosine phosphorylation. back to the top


Sulfate modification of proteins occurs at tyrosine residues such as in fibrinogen and in some secreted proteins (eg gastrin). The universal sulfate donor is 3'-phosphoadenosyl-5'-phosphosulphate (PAPS).
Since sulfate is added permanently it is necessary for the biological activity and not used as a regulatory modification like that of tyrosine phosphorylation. back to the top


Prenylation refers to the addition of the 15 carbon farnesyl group or the 20 carbon geranylgeranyl group to acceptor proteins, both of which are isoprenoid compounds derived from the cholesterol biosynthetic pathway. The isoprenoid groups are attached to cysteine residues at the carboxy terminus of proteins in a thioether linkage (C-S-C). A common consensus sequence at the C-terminus of prenylated proteins has been identified and is composed of CAAX, where C is cysteine, A is any aliphatic amino acid (except alanine) and X is the C-terminal amino acid. In order for the prenylation reaction to occur the three C-terminal amino acids (AAX) are first removed and the cysteine activated by methylation in a reaction utilizing S-adenosylmethionine as the methyl donor.
Important examples of prenylated proteins include the oncogenic GTP-binding and hydrolyzing protein Ras and the g-subunit of the visual protein transducin, both of which are farnesylated. Numerous GTP-binding and hydrolyzing proteins (termed G-proteins) of signal transduction cascades have g-subunits modified by geranylgeranylation. back to the top

Vitamin C-Dependent Modifications

Modifications of proteins that depend upon vitamin C as a cofactor include proline and lysine hydroxylations and carboxy terminal amidation. The hydroxylating enzymes are identified as prolyl hydroxylase and lysyl hydroxylase. The donor of the amide for C-terminal amidation is glycine.
The most important hydroxylated proteins are the collagens. Several peptide hormones such as oxytocin and vasopressin have C-terminal amidation. back to the top

Vitamin K-Dependent Modifications

Vitamin K is a cofactor in the carboxylation of glutamic acid residues. The result of this type of reaction is the formation of a g-carboxyglutamate (gamma-carboxyglutamate), referred to as a gla residue.

Structure of a gla residue
The formation of gla residues within several proteins of the blood clotting cascade is critical for their normal function. The presence of gla residues allows the protein to chelate calcium ions and thereby render an altered conformation and biological activity to the protein. The coumarin-based anticoagulants, warfarin and dicumarol function by inhibiting the carboxylation reaction. back to the top


Selenium is a trace element and is found as a component of several prokaryotic and eukaryotic enzymes that are involved in redox reactions. The selenium in these selenoproteins is incorporated as a unique amino acid, selenocysteine, during translation. A particularly important eukaryotic selenoenzyme is glutathione peroxidase. This enzyme is required during the oxidation of glutathione by hydrogen peroxide (H2O2) and organic hydroperoxides.

Structure of the selenocysteine residue
Incorporation of selenocysteine by the translational machinery occurs via an interesting and unique mechanism. The tRNA for selenocysteine is charged with serine and then enzymatically selenylated to produce the selenocysteinyl-tRNA. The anticodon of selenocysteinyl-tRNA interacts with a stop codon in the mRNA (UGA) instead of a serine codon. The selenocysteinyl-tRNA has a unique structure that is not recognized by the termination machinery and is brought into the ribosome by a dedicated specific elongation factor. An element in the 3' non-translated region (UTR) of selenoprotein mRNAs determines whether UGA is read as a stop codon or as a selenocysteine codon. back to the top
Return to Medical Biochemistry Page
Michael W. King, Ph.D / IU School of Medicine / miking at
Last modified: Monday, 22-Aug-2005 08:08:08 EST

post-translational modifications

General Approach
Most protein samples exhibit some degree of modification.
There are the "natural" post translational modifications, such as phosphorylation and glycosylation. There are the accidental modifications which are artefacts of sample handling, such as oxidation. Finally, there are the modifications deliberately introduced during sample work-up, such as cysteine derivatisation. In most cases, it is only the deliberate modifications which are known about for certain at the time of doing a search.
It might be assumed that the search software could allow for those modifications which are described in sequence entry annotations. However, writing code to parse these sequence annotations would be a major task. Indeed, many post-translational modifications are not specified in a way which can be readily translated into specific mass differences. For example, noting that a residue is an actual or potential glycosylation site is not much help. Even a simple modification, such as phosphorylation, is rarely quantitative, so that it would be necessary to include mass values for all permutations of occupied and unoccupied sites.
And, of course, protein sequences derived translated from nucleotide sequences contain no information on post translational modifications.
The solution adopted here is to allow modifications to be specified in two different ways: fixed modifications and variable modifications.
Fixed modifications are applied universally, to every instance of the specified residue or terminus. There is no computational overhead associated with a fixed modification, it is simply equivalent to using a different mass for the modified residue or terminus. For example, selecting Carboxymethyl (C) means that all calculations will use 161 Da as the mass of cysteine.
Variable modifications are those which may or may not be present. Mascot tests all possible arrangements of variable modifications to find the best match. For example, if Oxidation (M) is selected, and a peptide contains 3 methionines, Mascot will test for a match with the experimental data for that peptide containing 0, 1, 2, or 3 oxidised methionine residues. This greatly increases the complexity of a search, resulting in longer search times and reduced specificity, so variable modifications should be used sparingly.
The list of modifications used by Mascot is taken directly from the Unimod database. For further details of individual modifications, please refer to Unimod. Note that Unimod is an community supported resource. If you want to add a new modification to Unimod, you can do so, and you then become the curator of the new record. The Mascot modifications list is updated from Unimod each weekend.
Only selected modifications are displayed by default in the Mascot search form. If you want to see the complete list, you must go to the search form defaults page and tick the checkbox for 'Show all mods.'.
Other lists of modifications
DeltaMass is a comprehensive list of modifications, sorted by mass.
RESID database contains detailed descriptions of many post-translational modifications.