Succinyl coenzyme A synthetase

Succinyl coenzyme A synthetase (SCS, also known as succinyl-CoA synthetase or succinate thiokinase or succinate-CoA ligase) is an enzyme that catalyzes the reversible reaction of succinyl-CoA to succinate. The enzyme facilitates the coupling of this reaction to the formation of a nucleoside triphosphate molecule (either GTP or ATP) from an inorganic phosphate molecule and a nucleoside diphosphate molecule (either GDP or ADP). It plays a key role as one of the catalysts involved in the citric acid cycle, a central pathway in cellular metabolism, and it is located within the mitochondrial matrix of a cell.

Chemical reaction and enzyme mechanism

Succinyl CoA synthetase catalyzes the following reversible reaction:

Succinyl CoA + Pi + NDP ↔ Succinate + CoA + NTP

where Pi denotes inorganic phosphate, NDP denotes nucleotide diphosphate (either GDP or ADP), and NTP denotes nucleotide 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.