Well into exercise, the ratio of ATP/AMP in muscle cells decreases. What effect does this ratio reduction have on the activity of phosphofructokinase and pyruvate kinase?

Question

BlackTom AI · Accepted Answer

To understand the question, we need to connect the ATP/AMP ratio with the regulation of two glycolytic enzymes, phosphofructokinase-1 (PFK-1) and pyruvate kinase (PK).

Option 1: 'Both enzymes are increased in activity due to lack of allosteric binding of ATP.' This is correct mechanistically. When ATP levels fall relative to AMP, AMP accumulates and ATP-dependent allosteric inhibition of PFK-1 is relieved while activation by AMP occurs, driving PFK-1 activity up to accelerate glycolysis. Similarly, pyruvate kinase is inhibited by ATP via an allosteric site; a drop in ATP reduces this inhibitory binding, lifting the brake on PK and increasing its activity. Additionally, PK is stimulated downstream by fructose-1,6-bisphosphate provided by the upregulated PFK-1 step, further boosting PK activity. In summary, a lower ATP/AMP ratio promotes higher activities of both enzymes.

Option 2: 'Phosphokinase activity is increased and pyruvate kinase is decreased.' While PFK-1 would be activated, PK would not be decreased; it is relieved from ATP inhibition, not further suppressed. This option misstates the PK response under energy demand.

Option 3: 'Both enzymes decrease in activity due to lack of allosteric binding of ATP.' This contradicts the physiology. Lack of ATP inhibition (i.e., less ATP binding) actually increases enzyme activity, not decreases it. Decreased ATP binding relieves inhibition rather than suppressing activity.

Option 4: 'Both enzymes will remain at the same level of activity to maintain normal glucose metabolism.' During intense exercise, the cell shifts to higher glycolytic flux to meet energy demand. Therefore, activities of PFK-1 and PK do not stay the same; they adapt upward to accelerate glycolysis. This option ignores the energetic signal from the changing ATP/AMP ratio.

In short, the energy-depleted state (low ATP, higher AMP) modulates allosteric sites to enhance glycolytic throughput, consistent with increased activities of both PFK-1 and PK as described in Option 1.

Well into exercise, the ratio of ATP/AMP in muscle cells decreases. What effect does this ratio reduction have on the activity of phosphofructokinase and pyruvate kinase? 单项选择题

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