The enzyme glycogen semi aldehyde transcarboxylase (GSTC) is involved in glycoge

Question

The enzyme glycogen semi aldehyde transcarboxylase (GSTC) is involved in glycogen metabolism. GSTC can be inactivated by adding a sulfate to Senne102, which is located far away from the active site catalytic residues. The enzyme that catalyzes sulfate addition to GSTC is called a sulfotransferase. There are several mutants of GSTC that have altered activities. For each of the mutations below, explain the observed changes in enzyme activity. Your answers to all of the questions should be consistent with each other. a. GSTC enzymes that have the mutation S102D are always inactive, and can?t be regulated by sulfotransferase. Why? b. GSTC enzymes that have the mutation S102K are always active, and can?t be regulated by sulfotransferase, Why? c. GSTC enzymes that have the mutation S102T have high levels of activity. While the activity of the S102T mutant can be decreased by coincubation with sulfotransferase, the activity is never as low as wild-type GSTC treated with sulfotransferase. Why?

Explanation / Answer

The Sulfotransferase in this reaction, transfers a sulfate group to the residue Ser102; hence it transfers a sulfate group to the hydroxyl group present on the side-chain of Serine.

But Threonine has a side chain slightly greater than that of Serine (longer by a –CH2 unit), so the concerned enzyme Sulfotransferase will find it difficult to transfer sulfate group to the –OH group on the side chain of Thr as its active site is specific for the transfer of Sulfate to side-chain of Ser.

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