Metabolism
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Breathless after sprinting
Key points from this exercise:
Under conditions of maximum exertion the rate of uptake of oxygen into muscle in not adequate to meet the requirement for ATP synthesis, so in addition to as much aerobic metabolism as the available oxygen will permit, muscle metabolises glucose anaerobically. This has a yield of 2 x ATP per glucose metabolised.
The anaerobic metabolism of glucose requires reduction of pyruvate to lactate, in order to re-oxidise the NADH formed in glycolysis.
Lactate is released from muscle and taken up by the liver, where it is used for gluconeogenesis.
Three steps in glycolysis are irreversible under physiological conditions:
the reaction of hexokinase is reversed by glucose 6-phosphatase in gluconeogenesis
the reaction of phosphofructokinase is reversed by fructose 1,6-bisphosphatase in gluconeogenesis
the reaction of pyruvate kinases is reversed by a two-step pathway, requiring 1 x ATP and 1 x GTP, in gluconeogenesis
The synthesis of 1 x glucose from 2 x lactate requires 6 x ATP equivalents.
Oxygen debt is the additional consumption of oxygen required to permit the metabolism of sufficient lactate (or other metabolic fuel) to yield the ATP that is needed to convert the lactate formed during anaerobic glycolysis back to glucose.
There would be futile cycling between fructose 6-phosphate and fructose 1,6-bisphosphate, with hydrolysis of ATP and generation of heat, if both phosphofructokinase and fructose disphosphate were active to the same extent at the same time. These two enzymes are regulated reciprocally, but both have some activity at all times. This permits rapid and precise control over the rate of glycolysis or gluconeogenesis.