30. Enzymes with a kca/ Kn ratio of about 10* M\'s1 are considered to show optim

Question

30. Enzymes with a kca/ Kn ratio of about 10* M's1 are considered to show optimal catalytic efficiency. Fumarase, which catalyzes the reversible-dehydration reaction fumarate + H20 malate has a ratio of turnover number to the Michaelis-Menten constant, (kcat /Km) of 1.6 X 108 for the substrate fumarate and 3.6 X 107 for the substrate malate. Because the turnover number for both substrates is nearly identical, what factors might be involved that explain the different ratio for the two substrates? 31. Methanol (wood alcohol) is highly toxic because it is converted to formaldehyde in a reaction catalyzed by the enzyme alcohol dehydrogenase: NAD+methanol->NADH+H +formaldehyde Part of the medical treatment for methanol poisoning is to administer ethanol (ethyl alcohol) in amounts large enough to cause intoxication under normal circumstances. Explain this in terms of what you know about examples of enzymatic reactions.

Explanation / Answer

30. If the turnover number is nearly identical for both substrates, then the Km for malate must bemuch larger than for fumarate. Similar turnover numbers suggest no significant differences in rate ofconversion of substrate to product, but the different Km values could possibly be explained by astronger binding affinity of the enzyme for fumarate than for malate or some other aspect of thereaction mechanism that affects Km.

31. Ethanol competes with methanol for the substrate binding site of alcohol dehydrogenase,slowing the conversion of methanol to formaldehyde by acting as a competitive inhibitor (this allowsmore time for methanol to be removed by the kidneys rather than being converted to formaldehyde).

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