Question

In step 4 of the citric acid cycle, the reduction of NAD+ to NADH is coupled to the generation of CO2 and the formation of a high-energy thioester bond. The energy of the thioester bond is harnessed in step 5. What is the energy used for?

a) ATP synthesis
b) Oxidative phosphorylation
c) Substrate-level phosphorylation
d) Carbon fixation

Answer 1

The energy from the thioester bond hydrolysis in step 5 of the citric acid cycle is used for substrate-level phosphorylation to produce GTP, which can then be converted to ATP.

Step-by-step explanation:

In step 5 of the citric acid cycle, the energy released by the hydrolysis of the high-energy thioester bond in succinyl-CoA is used for the process known as substrate-level phosphorylation. This specific reaction involves the formation of guanosine triphosphate (GTP) from guanosine diphosphate (GDP) and inorganic phosphate, which is catalyzed by the enzyme succinyl-CoA synthetase. GTP can then transfer its terminal phosphate group to adenosine diphosphate (ADP) to generate ATP, highlighting the direct generation of high-energy phosphate compounds in the cycle. The correct answer to the energy usage in step 5 is c) Substrate-level phosphorylation.