Question

In the presence of oxygen, the three-carbon compound pyruvate can be catabolized in the citric acid cycle. First, however, the pyruvate 1) loses a carbon, which is given off as a molecule of CO2, 2) is oxidized to form a two-carbon compound called acetate, and 3) is bonded to coenzyme A.
These three steps result in the formation of
A) acetyl CoA, O2, and ATP.
B) acetyl CoA, FADH2, and CO2.
C) acetyl CoA, FAD, H2, and CO2.
D) acetyl CoA, NADH, H+, and CO2.
E) acetyl CoA, NAD+, ATP, and CO2.

Answer 1

Answer:

D. acetyl CoA, NADH, H+, and CO2.

Explanation:

Pyruvate formed by the glycolytic breakdown of glucose enters Kreb's cycle in the form of acetyl CoA. Conversion of pyruvate into acetyl CoA occurs in the mitochondrial matrix and the reaction catalyzed by the pyruvate dehydrogenase complex (PDH). It is an oxidative decarboxylation of pyruvate.

The PDH complex is composed of multiple copies of three enzymes. The enzyme complex catalyzes an irreversible oxidation process in which the carboxyl group of pyruvate is removed as a molecule of CO2 and the two remaining carbons become the acetyl group of acetyl-CoA. Here, NAD+ serves as the electron acceptor and is converted into NADH.