Question

Cytosolic pyruvate is the end product of glycolysis. Its fate depends on the metabolic status of the cell. If there is plenty of oxygen, the aerobic pathway dominates. If oxygen is insufficient, the anaerobic pathway dominates. Associate each of the following characteristics with either the aerobic or anaerobic fate of pyruvate.

Catalyzed by lactate dehydrogenase
Catalyzed by pyruvate dehydrogenase complex
Oxidizes NADH
Oxidizes pyruvate
Produces acetyl CoA
Produces lactate
Recycles NADH to NAD+
Redirects glucose oxidation to the mitochondrion
Reduces NAD+
Reduces pyruvate

Answer 1

The glycolysis pathway occurs in the cytoplasm of the cell. In both aerobic respiration and anaerobic respiration the glucose is degraded by the glycolysis pathway. The glucose first enters into the glycolysis degraded into the pyruvate.

Anaerobic respiration: A series of metabolic steps used to extract the energy from the organic molecules like glucose is called cellular respiration. If the cellular respiration occurs in the absence of oxygen, it is called anaerobic cellular respiration. Lactic acid and alcoholic fermentation types of anaerobic respiration which involves generation of NAD+ molecules from the NADH2.

Aerobic respiration: occurrence of cellular respiration in the presence of oxygen is called aerobic cellular respiration. The NADH2 generated in glycolysis & Kreb’s cycle enters into the electron transport chain oxidized into the NAD+.

Case 1: Oxygen is abundant and the aerobic pathway dominates.

a. Catalyzed by pyruvate dehydrogenase complex

In the presence of sufficient amount of oxygen (aerobic respiration), the pyruvate is converted to acetyl-Coenzyme A by the pyruvate dehydrogenase complex.

b. Oxidizes NADH

In the presence of plenty of oxygen, the electrons from the NADH transferred to the oxygen by the electron transport chain which is located in the inner mitochondrial membrane.

c. Oxidizes pyruvate

In the presence of plenty of oxygen, the pyruvate is oxidized to acetyl-Coenzyme A by the pyruvate dehydrogenase complex.

e. Produces acetyl CoA

In the presence of sufficient amount of oxygen, the pyruvate dehydrogenase complex converts the pyruvate to acetyl-Coenzyme A.

f. Redirects glucose oxidation to the mitochondrion         

In the presence of plenty of oxygen, the glucose degraded into two pyruvate molecules which later on converted to acetyl-Coenzyme A which then enter into the mitochondria where the acetyl-Coenzyme A is completely degraded.

g. Reduces the NAD+

In the presence of plenty of oxygen, the glucose molecules are readily oxidized and NAD+ is converted to the NADH by the various enzymes of the glycolysis and Krebs cycle enzymes.

h. Reduces pyruvate

In the presence of oxygen, more NADH is produced by the reduction of lactate to pyruvate which is easily oxidized through ETC and generate the ATP.

Case 2: If oxygen is insufficient, the anaerobic pathway dominates

a. Catalyzed by lactate dehydrogenase

In order to regenerate the NAD+ to drive the glycolysis, the pyruvate is converted to lactate by the lactate dehydrogenase enzyme.

b. produces the lactate

In order to regenerate the NAD+ to drive the glycolysis, the pyruvate is converted to lactate by the lactate dehydrogenase enzyme.

c. Recycles NADH to NAD⁺

in order to regenerate the NAD+ to drive the glycolysis in anaerobic conditions lactate dehydrogenase enzyme converts the pyruvate to lactate and the 2NADH molecules are oxidized to the 2NAD+ molecules.