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The domain structure of protein Z, which is composed of 180 amino acids, is shown in the upper part of the figure below. Protein Z is palmitoylated at a cysteine residue (the third amino acid) through the mechanism shown in the box.

enter image description here

Which of the following diagrams shows the correct topology of protein Z in the plasma membrane?

enter image description here

I think the second diagram is correct because cytoplasmic side has reducing environment and so SH will not be allowed to react this way. Am I right ?

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I think you're right - disulfide bonds don't usually occur in cytoplasmic proteins either for that reason. –  shigeta Jan 11 at 11:39
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up vote 2 down vote accepted

I don't think there's any oxidation or reduction going on in this reaction; certainly there is no net reduction or involvement of NADH cofactor or anything like that. Furthermore, I think that it isn't too hard (esp. since these are enzyme-catalyzed reactions) to imagine that the mechanism is simple: nucleophilic attack of the carbon of the thioester by the cysteine, forming a thioketal intermediate, resulting in ejecting the CoASH as a leaving group. No reduction or oxidation.

Instead of oxidation or reduction, I would think about where exactly are the components of each reaction present. In order for the palmitoylation reaction to occur, you need: the protein Z N terminus, palmitoyl-CoA, and protein acyltransferase. Where are these components more likely to be present, inside or outside of the cell? Especially given that CoASH has several negatively charged phosphate groups.

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I would say - inside the cell because of the enzymes and the substrates required. But how is negative charge connected with this ? –  biogirl Jan 11 at 13:03
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This review (bio.ijs.si/~krizaj/group/Predavanja%202011/…) indicates that thioacylation of the type depicted in the question is indeed a cytoplasmic process. –  Alan Boyd Jan 11 at 14:58
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@biogirllajja, a common strategy for keeping cofactors inside the cell is to give them net charge, particularly negative charge since phosphate is the most common charged species around that can be easily added to molecules. The net charge prevents the diffusion of the cofactor across the plasma membrane. For example, one of the main reasons glucose is rapidly converted to UDP-glucose upon entering the cell is to prevent its passive diffusion out of the cell. –  A. Kennard Jan 12 at 9:42
    
@A.Kennard Ok! Thank you so much. –  biogirl Jan 12 at 10:29
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