Answer:
Inorganic phosphate reacts at the thioester of succinyl-CoA to yield a mixed phosphoanhydride.
A histidine residue from the protein is phosphorylated by succinyl phosphate.
The phosphate group is transferred from a histidine residue of the phosphorylated enzyme intermediate to the nucleotide.
Step-by-step explanation:
The conversion of succinyl-CoA to succinate and ATP (or GTP) starts with the help of enzyme known as succinyl-CoA synthetase which has two sub units named as alpha and beta. Alpha subunit comprises phosphorylated histidine residue and binding site for CoA while beta sub unit is responsible for providing specificity for either GTP/ATP.
In the very first step, the phosphorylated histidine residue of this enzyme reacts to replace CoA of succinyl-CoA which is already bound to the enzyme to yield enzyme-bound succinyl phosphate.
In the next step, this succinyl phosphate donates its phosphate group to the histidine residue of the enzyme thereby producing high energy phosphohistidyl enzyme and succinate.
In the last step, the phosphate group from the histidine residue of the enzyme is transferred to the terminal phosphate of GDP or ADP which leads to the formation of either of the two nucleotides, GTP or ATP .
Here, it may be noted that in animals there exist two isozymes of succinyl-CoA synthetase, one of them is specific to ATP while another one is specific to GTP.