9. Which one of the following enzymes present in erythrocytes helps bypass the first step of ATP formation in glycolysis?
(1) Bisphosphoglycerate mutase
(2) Phosphoglycerate kinase
(3) Glyceraldehyde 3-phosphate dehydrogenase
(4) Phosphofructose mutase

Erythrocytes (red blood cells) rely exclusively on glycolysis for their energy needs because they lack mitochondria. Their metabolic pathways have unique adaptations to balance energy production with oxygen transport efficiency. One such adaptation is a glycolytic bypass involving the enzyme bisphosphoglycerate mutase (BPGM), which helps erythrocytes bypass the first ATP-producing step in glycolysis.

This article explains the role of bisphosphoglycerate mutase in erythrocyte metabolism, how it bypasses the ATP formation step, and why this is crucial for red blood cell function.

Glycolysis and ATP Formation in Erythrocytes

Glycolysis is the metabolic pathway that converts glucose into pyruvate, generating ATP in the process. In most cells, two key steps produce ATP:

• The conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate, catalyzed by phosphoglycerate kinase (PGK).

• The conversion of phosphoenolpyruvate to pyruvate, catalyzed by pyruvate kinase.

Erythrocytes depend solely on glycolysis for ATP because they lack mitochondria and cannot perform oxidative phosphorylation.

The Rapoport-Luebering Shunt: A Unique Glycolytic Bypass

Erythrocytes possess a specialized pathway called the Rapoport-Luebering shunt, which involves bisphosphoglycerate mutase (BPGM). This shunt diverts 1,3-bisphosphoglycerate to form 2,3-bisphosphoglycerate (2,3-BPG), a molecule critical for regulating hemoglobin’s oxygen affinity.

How BPGM Bypasses the First ATP-Forming Step

• Normally, 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by phosphoglycerate kinase, generating ATP.

• In erythrocytes, BPGM converts 1,3-bisphosphoglycerate into 2,3-bisphosphoglycerate.

• The 2,3-BPG is then converted back to 3-phosphoglycerate by a phosphatase activity of BPGM, bypassing the ATP-generating step catalyzed by PGK.

• This bypass means the first ATP formation step is effectively skipped in this shunt.

This unique pathway allows erythrocytes to regulate oxygen delivery via 2,3-BPG levels while modulating ATP production.

Why Is This Bypass Important?

• Oxygen Delivery: 2,3-BPG binds to hemoglobin and decreases its affinity for oxygen, facilitating oxygen release to tissues.

• Energy Balance: By bypassing the ATP-generating step, erythrocytes can fine-tune ATP production without compromising oxygen delivery.

• Metabolic Flexibility: This shunt provides a mechanism to regulate glycolytic flux and energy production in response to physiological needs.

Evaluating the Enzyme Options

Given the question about which enzyme in erythrocytes helps bypass the first ATP formation step in glycolysis, here is the analysis:

1. Bisphosphoglycerate mutase (BPGM)

   • Correct. BPGM catalyzes the formation of 2,3-BPG and bypasses the phosphoglycerate kinase step, which is the first ATP-generating step.

2. Phosphoglycerate kinase (PGK)

   • Incorrect. PGK catalyzes the ATP-producing step that is bypassed.

3. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH)

   • Incorrect. GAPDH catalyzes an earlier step in glycolysis involving NADH production, not ATP formation bypass.

4. Phosphofructose mutase

   • Incorrect. This enzyme is not involved in glycolysis or erythrocyte metabolism.

Conclusion

The enzyme bisphosphoglycerate mutase uniquely enables erythrocytes to bypass the first ATP-forming step in glycolysis through the Rapoport-Luebering shunt. This adaptation balances energy production with oxygen delivery, critical for red blood cell function.

Correct answer: (1) Bisphosphoglycerate mutase