Isotope effects in 3-dehydroquinate synthase and dehydratase. Mechanistic implications.
作者： S.L. Rotenberg , D.B. Sprinson
DOI: 10.1016/S0021-9258(17)38060-2
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摘要: The mechanism of 3-dehydroquinate synthase was explored by incubating partially purified enzyme with mixtures [1-14C]3-deoxy-D-arabino-heptulosonic acid 7-phosphate (DAHP) and one the specifically tritiated substrates [4-3H]DAHP, [5-3H]DAHP, [6-3H]DAHP, (7RS)-[7-3H]DAHP, (7R)-[7-3H]DAHP, or (7S)-[7-3H]DAHP. Kinetic secondary 3H isotope effects were calculated from 3H:14C ratios obtained in unreacted DAHP, 3-dehydroquinate, 3-dehydroshikimate. not incorporated medium into indicating that a carbanion (or methyl group) at C-7 is formed. A kinetic effect kH/k3H 1.7 observed C-5, afforded support for involving oxidation C-5 NAD. similar found C-6 owing to removal proton elimination phosphate, which reasonably assumed be next step synthase. Hydrogen DAHP lost cyclization reaction, enol formed phosphate participated directly an aldolase-type reaction carbonyl C-2. In dehydration 3-dehydroshikimate (7R) (7RS)- (7R)-[7-3H]DAHP lost, 7R occupies 2R position dehydroquinate. Hence occurs inversion configuration C-7. = 2.3 conversion (2R)-[2-3H]dehydroquinate dehydroshikimate. loss enzyme-dehydroquinate imine contributed rate limitation reaction.
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