We mentioned lactate dehydrogenase (LDH, the enzyme that generates lactate from

Question

We mentioned lactate dehydrogenase (LDH, the enzyme that generates lactate from pyruvate in lactic acid fermentation) earlier in the class when we discussed isozymes. It is a tetrameric enzyme composed of two subunit types, M and H The enzyme form in the heart is H4 and the muscle form is M4. Heart cells are capable of very high rates of the citric acid cycle and are full of mitochondria (where the citric acid cycle occurs), much more so than muscle cells. The heart isozyme of LDH is strongly inhibited allosterically by pyruvate, while the muscle isozyme is not. Explain why this differing regulation makes sense considering the function of these two tissue types (2-4 sentences)

Explanation / Answer

The major difference in M and H forms of LDH is alanine at catalytic site in M subunit is replaced by glutamine in subunit. The two isozymes differ as high levels of pyruvate allosterically inhibit the H4 but not the M4 isozyme because of difference in sequence and structure. The M4 isozyme functions optimally in an anaerobic environment, whereas the H4 isozyme does so in an aerobic environment. Since in muscle the energy requirements is high glycolysis is the favoured pathway hence Pyruvate does not have high inhibiting effect on LDH, and Lactate is made from Glucose ( 2 Pyruvates coming from Glucose). While in Heart Cells Gluneogenesis is the favoured pathway LDH is easily inhibited by Pyruvate hence favouring the reverse reaction of making Glucose ( via Pyruvate) from Lactate.

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