Q (B-06): This enzyme catalyzes the SLP reaction of the Krebs Cycle in which GDP is phosphorylated to GTP.
A (B-06): (h) Succinyl Co A synthase (aka Succinyl CoA synthetase, aka Succinyl thiokinase)
In the succinyl CoA synthetase reaction, the thioester bond between HS-CoA and the succinyl group is hydrolyzed.
Since it is a rich in energy bond, the energy released is enough for synthesizing GTP from GDP + (P).
This GTP is equivalent, from the energetic point of view, to ATP. In fact, GTP can transfer the (P) group to ADP to form ATP:
GTP + ADP ————–à GDP + ATP
Since ATP can be produced from this reaction, without participation of the respiratory chain, this process is called Substrate Level Phosphorylation (SLP) in contrast to the Oxidative Phosphorylation (ATP synthesis using the energy released in the Electron Transport Chain).
A few other reactions in metabolism are also coupled with ATP synthesis without participation of the respiratory chain. They are considered also SLP reactions.