Fermentation Pathway—Pyruvate to Lactate Conversion Which specific enzyme catalyzes the reduction of pyruvate to lactate during anaerobic glycolysis (lactic acid fermentation)?

Difficulty: Easy

Correct Answer: Lactate dehydrogenase

Explanation:


Introduction / Context:
When oxygen is limited, many tissues (e.g., exercising muscle, red blood cells) regenerate NAD+ by reducing pyruvate to lactate. Identifying the correct enzyme ensures conceptual clarity between fermentative pathways and mitochondrial oxidative metabolism.


Given Data / Assumptions:

  • Substrate: pyruvate.
  • Cofactor cycling: NADH → NAD+.
  • Outcome: lactate formation in cytosol.


Concept / Approach:
Lactate dehydrogenase (LDH) catalyzes pyruvate + NADH + H+ → lactate + NAD+. This reaction is reversible and central to the Cori cycle (lactate back to glucose in liver). Other listed enzymes act in unrelated steps or organisms (e.g., pyruvate decarboxylase to acetaldehyde in yeast alcoholic fermentation).


Step-by-Step Solution:

Identify fermentative need: regenerate NAD+ to sustain glycolysis.Recall the enzyme: LDH reduces pyruvate to lactate using NADH.Conclude that lactate dehydrogenase is the correct catalyst.


Verification / Alternative check:
Clinical marker: serum LDH rises with tissue damage; lactate accumulation during hypoxia confirms pathway activity.


Why Other Options Are Wrong:

  • Pyruvate carboxylase: pyruvate → oxaloacetate in gluconeogenesis.
  • Pyruvate dismutase: not a standard human enzyme in this context.
  • Pyruvate decarboxylase: pyruvate → acetaldehyde (yeast), not lactate.
  • Malate dehydrogenase: interconverts malate and oxaloacetate.


Common Pitfalls:
Mixing up human lactic fermentation with yeast alcoholic fermentation; confusing LDH with pyruvate dehydrogenase.


Final Answer:
Lactate dehydrogenase

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