You have two purified samples of protein Y: the wild-type (nonmutated) protein and a mutant version with a single amino acid substitution
When washed through the same gel-filtration column, mutant protein Y runs through the column more slowly than the normal protein. Which of the following changes in the mutant protein is most likely to explain this result?
(a) the loss of a binding site on the mutant-protein surface through which protein Y normally forms dimers
(b) a change that results in the mutant protein acquiring an overall positive instead of a negative charge
(c) a change that results in the mutant protein being larger than the wild-type protein
(d) a change that results in the mutant protein having a slightly different shape from the wild-type protein
(a) Dimers formed by a normal protein will run through the gel-filtration column faster than a mutant protein Y monomer. Choice (b) is unlikely, because gel-filtration columns separate proteins on the basis of size, not charge or affinity for small molecules. Choice (c) is unlikely, because if the mutant protein were larger than normal it would be less able to enter the porous beads and would run through the column faster than the normal protein. Choice (d) is unlikely, because a small change in shape without a change in size would be unlikely to have a major effect on the behavior of a protein in a gel-filtration column.
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