BIO206 Lecture 23.pdf

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George S Espie

Lecture 23November2513506 PMmany 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 golgiThey can be held together by covalent forces disulfide bridges eg antibodies which have inter and intra chain bonds or noncovalent forces eg INFLUENZA HA protein which is a trimer of 3 identical polypeptidesTwo forms of covalent modification happen in the ERSS bonds and one of the 2 types of glycosylation Book Notes Page 1 covalent bonds between cysteinesdo NOT affect protein folding since they must be in proximity remember that ER proteins do not return to cytosol cytosolic proteins dont have SS bonds only membrane secretory and organelle proteinsdisulfide bonds will form spontaneously often as proteins come off the ribosome and into the ERwhen formation is sequential eg antibodies 12 3 4 etc usually the correct bonds form the first timenonsequential formation eg insulin often the first bonds formed are not the correct onesbonds will spontaneously break and reform but it takes timeenzyme called protein disulfide isomerase will break disulfide bonds and thus will allow the correct ones to formcan 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 LEVELDisulfide bond Hold protein in 3D conformationER ensure that disulfide bond is formed in correct spatial orientation PDI break disulfide bond and rearrange the structure so it is in correct orientation Book Notes Page 2 Chain A and B stabilized structureInternal peptidase cuts chain C leaving only chain A and B togetherprotein glycosylation is not generally found in cytosolic proteins if it is only a single sugar residuefunction 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 signaltwo types of glycoslyation Nlinked and OlinkedDiscuss Olinked later since it occurs in the golgiN 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 golgias the protein is being synthesized these residues are attached at a sprecific sequenceThe glycosylation is not sequential addition of monosaccharides a particular branched oligosaccharide with 3 glucose 9 mannose 2 Nacetyl glucosamine is transferred from the lipid byeoligosaccharide transferase enzymemodified 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 transferfurther modifications occur laterXamino acid Book Notes Page 3
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