In a study of mutations that reduce the activity of a cytoplasmic enzyme, a mutation is found that results in substitution of an alanine residue for the serine residue normally present on position 127 in the enzyme, this mutation is most likely to reduce the activity of the enzyme by preventing which of the following?
A) Attachment of famesyl groups
B) Attachment of N linked oligosaccharides
C) Formation of disulfide bonds
D) Phosphorylation of the enzyme
E) Targeting of the enzyme to the correct cellular location
Pls answer with explanation
Alanine = nonpolar
Serine = Polar (OH group)
So it's likely to cause the thing to mis-fold, as in the inside would now be outside.
A) Attachment of famesyl (farnesyl?) group does occur at an alanine residue, BUT that's for a membrane signal protein isoprenoid
--now more than likely this is not going to be an answer... this is WAY WAY beyond a medical students biochemistry knowledge...
B) N-linked oligosaccharide modification occurs on an asparagine residue usually used as a post-translational modification with the use of dolichol to add sugars onto GAGs
C) The formation of disulfide bonds could be disturbed if there is a miss folding, but more than likely not, and usually occurs at cysteine residues. Usually found in secretory proteins, rather than enzymes.
D) Phosphorylation actually occurs more frequently as Serine and Threonine sites because of the OH group, and since the stem is asking which prevents the following, I'll rule this question out.
So by process of elimination we arrive at E. (my best choice)
Also I believe in the whole polar / non-polar thing, but then again... C seems to be viable, but less so because of the whole cytosolic enzyme thing, although there are enzymes that have disulfide bonds in the cytoplasm, again somewhat of a gray area with research, so less likely to be the answer.
the question is testing u on phosphorylation de phosphorylation of serine residues in cytoplasmic enzymes (EX: Glycogen phosphorylase) so here mutation has replaced serine with alanine so no serine no phosphorylation no activation of that enzyme
edited by kishanvuddanda on 6/5/2011
Bad news, I choosed E and it is marked wrong in the feedback reply from "NB...E self assessment " so it has to be something else ....