An altered version of glycogen synthase is produced in the laboratory, with one of its amino acid residues substituted for another. Unlike normal glycogen synthase, the activity of the mutant protein is unaffected by glycogen synthase kinase-3. Which mutation is MOST likely to produce this effect?
A) Cys $\rightarrow$ Ser
B) Ile $\rightarrow$ Thr
C) Ser $\rightarrow$ Ala
D) Cys $\rightarrow$ Met
E) Thr $\rightarrow$ Ser

Question

Quizplus · Accepted Answer

Glycogen synthase kinase-3 (GSK-3) phosphorylates specific serine residues on glycogen synthase, inhibiting its activity. A mutation from serine (Ser) to alanine (Ala) would remove the phosphorylation site, rendering the enzyme's activity unaffected by GSK-3, as alanine cannot be phosphorylated.

An Altered Version of Glycogen Synthase Is Produced in the Laboratory