Question

In the conversion of pyruvic acid to acetyl coenzyme A, pyruvic acid is:
A. Oxidized
B. Reduced
C. Isomerised
D. Inactivated

Answer 1

Hint: Pyruvic acid has a carboxylic acid and a ketone functional group, making it the most basic of the alpha-keto acids. Pyruvic acid can be produced from glucose, converted back to carbohydrates through gluconeogenesis, or converted to fatty acids via an acetyl-CoA reaction. It can also be used to make the amino acid alanine, and it can be fermented to produce ethanol or lactic acid. While oxygen is present (aerobic respiration), pyruvic acid provides energy to cells through the citric acid cycle (also known as the Krebs cycle), and when oxygen is not present (anaerobic respiration), it ferments to produce lactate (lactic acid).

Complete explanation:
Option A: Pyruvate reaches mitochondria through a transport protein, where it undergoes oxidative decarboxylation (carbon dioxide oxidation as well as removal) to produce $C{O_2}$, which then combines with sulphur, which has coenzyme A, to form acetyl CoA.
Option A is correct.
Option B: The coenzymes $NA{D^ + }$, \[FAD\], and coenzyme Q are reduced as the enzymes of the Krebs cycle oxidise fuel molecules to carbon dioxide.
So, option B is incorrect.
Option C: In the tricarboxylic acid cycle, aconitase catalyses the stereo-specific isomerization of citrate to isocitrate through cis-aconitate, a non-redox-active mechanism.
So, option C is incorrect.
Option D: With the exception of succinate dehydrogenase, all dehydrogenases in the citric acid cycle produce \[NADH\], which inhibits pyruvate dehydrogenase, isocitrate dehydrogenase, -ketoglutarate dehydrogenase, and citrate synthase. Pyruvate dehydrogenase is inhibited by acetyl-CoA, while alpha-ketoglutarate dehydrogenase and citrate synthase are inhibited by succinyl-CoA.
So, option D is incorrect.

Hence, Option A is the correct answer.

Note:
In order for pyruvate, a glycolysis product, to join the next pathway, it must go through a series of transformations to become acetyl Coenzyme A. The molecule acetyl CoA is converted to oxaloacetate, which then joins the citric acid cycle (Krebs cycle). Pyruvate dehydrogenase complex catalyses pyruvate's irreversible oxidative decarboxylation into acetyl CoA and \[C{O_2}\]. NAD is a pyruvate oxidizer that is also reduced to \[NADH\]. It's called oxidative decarboxylation since one $C{O_2}$ molecule is removed from the pyruvate during oxidation.