Question

In step 3 of the citric acid cycle, the oxidation of isocitrate and the production of

CO2 are coupled to the reduction of NAD+ generating NADH and an α-
ketoglutarate molecule. In the isocitrate molecule shown in Figure Q13-47, which
carbon is lost as CO2 and which is converted to a carbonyl carbon?______
(a) 4 and 6
(b) 6 and 5
(c) 5 and 4
(d) 6 and 4

Answer 1

In step 3 of the citric acid cycle, carbon 6 of isocitrate is lost as CO2 and carbon 5 is converted to a carbonyl carbon when forming α-ketoglutarate, making the correct answer (b) 6 and 5.

Step-by-step explanation:

In step 3 of the citric acid cycle, isocitrate undergoes oxidative decarboxylation. During this process, the carboxyl group at the carbon number 3 (often labeled as the 6th carbon when counting from the carboxyl end in citrate) of isocitrate is lost as CO2. Simultaneously, the carbon that was adjacent to this carboxyl group (carbon number 2 or the 5th carbon) is oxidized, forming the carbonyl group of the resulting α-ketoglutarate molecule. Consequently, the correct answer to the student's question is (b) 6 and 5, where carbon 6 is lost as CO2 and carbon 5 is converted to a carbonyl carbon.