where pi denotes inorganic phosphate, ndp denotes nucleoside diphosphate (either gdp or adp), and ntp denotes nucleoside triphosphate (either gtp or atp). as mentioned, the enzyme facilitates coupling of the conversion of succinyl coa to succinate with the formation of ntp from ndp and pi. the reaction has a biochemical standard state free energy change of -3.4 kj/mol. the reaction takes place by a three-step mechanism which is depicted in the image below. the first step involves displacement of coa from succinyl coa by a nucleophilic inorganic phosphate molecule to form succinyl phosphate. the enzyme then utilizes a histidine residue to remove the phosphate group from succinyl phosphate and generate succinate. finally, the phosphorylated histidine transfers the phosphate group to a nucleoside diphosphate, which generates the high-energy carrying nucleoside triphosphate.