Q40E - Organic Chemistry (Mcmurry)

Question

The hydrolysis of a biological thioester to the corresponding carboxylate is often more complex than the overall result might suggest. The conversion of succinyl CoA to succinate in the citric acid cycle, for instance, occurs by initial formation of an acyl phosphate, followed by reaction with guanosine diphosphate (GDP, a relative of adenosine diphosphate [ADP]) to give succinate and guanosine triphosphate (GTP, a relative of ATP). Suggest mechanisms for both steps.
 

Step-by-Step Solution

Answer

The mechanism for the above conversion can be explained.

Step 1: Mechanism for conversion of succinyl CoA to acyl phosphate

The negatively charged oxygen’s nucleophilic attack of phosphate ion on the carbonyl carbon of succinyl CoA will give an intermediate. Then the next step is an elimination of intermediate to give acyl phosphate. The proton can be taken to become HSCoA.

Step 2: Mechanism for conversion of acyl phosphate to guanosine diphosphate

The negatively charged oxygen’s nucleophilic attack of GDP on P acyl phosphate which ion gives an intermediate. The elimination of intermediate GTP for the production of succinate anion in second step.