Question

How many ATP will be produced during the production of 1 molecule of acetyl CoA from one molecule of pyruvic acid?
A. 3 ATP
B. 5 ATP
C. 8 ATP
D. 38 ATP

Answer 1

Hint: Let’s revise, pyruvate is the product obtained from the breakdown of glucose in the cytoplasm. On further oxidation, pyruvate is oxidized to Acetyl CoA and this reaction is catalysed by a highly organized multienzyme called Pyruvate dehydrogenase complex. Acetyl Co-A then enters into Krebs cycle in the mitochondrial matrix in presence of oxygen.

Complete answer:
(a) Two molecules of pyruvate are produced by the breakdown of glucose (glycolysis) in the cytoplasm. These pyruvates still contain a large amount of energy that is left to be utilized still. 4 ATP molecules are produced in the process and two of them are consumed while converting glucose to glucose-6-phosphate and fructose-6-phosphate to fructose-1,6-phosphate. Pyruvate oxidation is the next step in the series of obtaining energy from these molecules in the form of ATPs.
(b) Compared to preceding glycolysis and following Kreb’s cycle, pyruvate oxidation is relatively shorter acting as a key connector between glycolysis and cellular respiration.
(c) Pyruvate oxidation converts the 3 Carbon molecule (Pyruvate) into a 2 carbon molecule (Acetyl CoA) with the addition of Coenzyme A, producing an NADH (Nicotinamide Adenine Dinucleotide) and releasing a carbon atom in form of carbon dioxide.
(d) So technically no ATPs are generated directly as a result of pyruvate oxidation.
(e) However, NADH produced in the reaction is equivalent to 3 ATP molecules upon complete oxidation and it is an extremely good electron donor in biological redox reactions. Similarly, FADH2 is equivalent to 2 ATP molecules.
(f) The NADH generated as the result of pyruvate oxidation later undergoes oxidative phosphorylation where this NADH acts as an electron carrier that generates 3 ATP for each pair of electrons that are transported to the electron transport chain.

Hence, the correct answer is (A) 3 ATP.

Note: The Acetyl CoA is the fuel of cellular respiration which is utilized in the citric acid cycle (Kreb’s cycle). Pyruvic acid is a keto acid i.e., it contains both carboxyl and keto- functional groups. when 1 molecule pyruvic acid enters into mitochondria, it undergoes 3 decarboxylation and 5 oxidations and produces 15 ATP molecules (3 in Link reaction and 12 in Krebs cycle). Therefore, for 1 glucose molecule, Krebs cycle produces 24 ATP in two turns. A total of 38 ATP molecules are generated 4 from the substrate level phosphorylation and 34 from oxidative phosphorylation.