35 Anaerobic Metabolism in Animals

Key Idea: Lactic acid fermentation is an anaerobic process that passes electrons directly to pyruvate as the final electron acceptor. It occurs in the muscles of mammals.

Lactic acid fermentation

` Mammalian skeletal muscle can produce ATP anaerobically using lactic acid fermentation. In this pathway, the electron acceptor is pyruvate, the end product of glycolysis. The pyruvate is reduced to lactic acid, which dissociates to form lactate and H+ . ` The conversion of pyruvate to lactate is reversible.

The pyruvate-lactate interconversion is catalysed by an enzyme (E) called lactate dehydrogenase.

Hence it is called the lactate 'shuttle'. ` Importantly, this pathway operates alongside the aerobic system (even when oxygen is present) to enable greater intensity and duration of muscle activity. It is an important mechanism for balancing the distribution of substrates and waste products, especially when pyruvate is building up faster than it can be metabolised. ` Lactate can be metabolised in the muscle itself or it can enter the circulation and be taken up by the liver to replenish carbohydrate stores. It moves from its site of production to regions within and outside the muscle where it can be respired aerobically. ` During moderate exercise, lactate released from other tissues is the main fuel source for the heart.

Possibly up to 75% of the lactate produced in muscle during exercise is used in this way. There is also evidence to support the hypothesis that lactate is an important fuel source in the brain.

Glucose

C6H12O6 2 ADP 2 ATP yield

2 x pyruvate CH3COCOOH

2 NAD+ 2 NADH + H+

Lactic acid

CH3CHOHCOO- +H+

+

NAD+

E

Pyruvate CH3COCOOH + NADH +H+

Lactic acid fermentation (animal tissues)

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1. What is the key difference between alcoholic fermentation and lactic acid fermentation?

2. (a) Explain the importance of the lactate shuttle during moderate exercise:

(b) Study the pathway of lactic acid fermentation above and explain how the lactate produced in working muscle could provide fuel for aerobic respiration in other tissues:

3. Compare the efficiency of lactic acid fermentation to aerobic respiration in terms of ATP produced: