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- many secretory and membrane proteins are formed from more than one polypeptide chain (ie they
have QUATERNARY STRUCTURE). The polypeptides of these will associate before the proteins are
transported to the golgi.
They can be held together by covalent forces - disulfide bridges (e.g. antibodies which have inter and
intra chain bonds) or noncovalent forces e.g. INFLUENZA HA protein which is a trimer of 3 identical
polypeptides
Two forms of covalent modification happen in the ER- SS bonds and one of the 2 types of
glycosylation
Lecture 23
November-25-13
5:06 PM
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- covalent bonds between cysteines,- do NOT affect protein folding since they must be in proximity
-remember that ER proteins do not return to cytosol - cytosolic proteins don’t have S-S bonds, only
membrane, secretory and organelle proteins
- disulfide bonds will form spontaneously, often as proteins come off the ribosome and into the ER
- when formation is sequential (eg antibodies 1 + 2, 3+ 4 etc) usually the correct bonds form the first
time
- nonsequential formation (e.g. insulin) often the first bonds formed are not the correct ones
- bonds will spontaneously break and reform, but it takes time
- enzyme called protein disulfide isomerase will break disulfide bonds and thus will allow the correct
ones to form
- can happen several times, because the incorrect bond formation may have prevented proper
folding of the protein, so the protein then needs to unfold and refold - SEEKS LOWEST ENERGY LEVEL
Disulfide bond - Hold protein in 3D conformation
ER ensure that disulfide bond is formed in correct spatial orientation
PDI break disulfide bond and rearrange the structure so it is in correct orientation
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Chain A and B - stabilized structure
Internal peptidase cuts chain C leaving only chain A and B together
- protein glycosylation is not generally found in cytosolic proteins - if it is, only a single sugar residue
- function of glycosylation depends on the protein - protect from degradation, retain in ER until
folding is proper, serve as a transport signal, form part of glycocalyx (polysaccharide coat of some
cells), serve as recognition signal
- two types of glycoslyation - N-linked and O-linked. Discuss O-linked later since it occurs in the golgi
- N linked - common pathway, a common precursor is added, and then modifications to the
precursor take place in the ER and also later on in the golgi (talk about it more when we reach the
golgi)
- as the protein is being synthesized these residues are attached at a sprecific sequence. The
glycosylation is not sequential addition of monosaccharides - a particular branched oligosaccharide
with 3 glucose, 9 mannose, 2 N-acetyl glucosamine is transferred from the lipid by eoligosaccharide
transferase enzyme
- modified as soon as added by removal of glucose and one of the mannose (specific enzymes for
EACH)
- glucose MAY be signal that the sugar is complete and ready for transfer
- further modifications occur later
X = amino acid
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Document Summary

Many secretory and membrane proteins are formed from more than one polypeptide chain (ie they have quaternary structure). The polypeptides of these will associate before the proteins are transported to the golgi. They can be held together by covalent forces - disulfide bridges (e. g. antibodies which have inter and intra chain bonds) or noncovalent forces e. g. influenza ha protein which is a trimer of 3 identical polypeptides. Two forms of covalent modification happen in the er- ss bonds and one of the 2 types of glycosylation. Covalent bonds between cysteines,- do not affect protein folding since they must be in proximity. Remember that er proteins do not return to cytosol - cytosolic proteins don"t have s-s bonds, only membrane, secretory and organelle proteins. Disulfide bonds will form spontaneously, often as proteins come off the ribosome and into the er. When formation is sequential (eg antibodies 1 + 2, 3+ 4 etc) usually the correct bonds form the first time.

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