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The pH dependence of the kinetic parameters of ketol acid reductoisomerase indicates a proton shuttle mechanism for alkyl migration.

Abstract：

:The enzyme ketol acid reductoisomerase catalyzes the second common reaction in the biosynthesis of the branched chain amino acids. The reaction is complex as an alkyl migration and a ketone reduction apparently occur as separate steps during the conversion of acetolactate to 2,3-dihydroxy-3-methylbutyrate. This paper reports on the pH dependence of the kinetic parameters of the enzyme. The pH variation of log(V/K) for acetolactate was fit to an equation describing a bell-shaped curve, indicating an acid and a base catalyst for the reaction. In the reverse direction, V/K for 2,3-dihydroxy-3-methylbutyrate is constant over the pH range 8 to 10 and decreases below pH 8 with the ionization of two catalytic groups. The pH dependence of the V/K values for reduction of the kinetically competent intermediate and analogs of this intermediate are also described by a bell-shaped curve. The pH dependence of the V/K for alkyl migration of this intermediate indicates a single base catalyst for this reaction. We observe no deuterium kinetic isotope effect on V or V/K for the reaction of acetolactate at any pH. We observe a pH-dependent kinetic isotope effect on V/K for the reduction of the intermediate, the magnitude of which is metal ion dependent. Larger KIE's are observed in the presence of Mn2+ as opposed to Mg2+. In the reverse reaction there is a pH-dependent kinetic isotope effect on V/K. Based on the pH dependence of the kinetic parameters and the kinetic isotope effects, we propose a base-catalyzed proton shuttle mechanism for the alkyl migration reaction followed by an acid-assisted ketone reduction by NADPH.

journal_name

Arch Biochem Biophys

journal_title

Archives of biochemistry and biophysics

authors

Mrachko GT,Chunduru SK,Calvo KC

doi

10.1016/0003-9861(92)90710-e

subject

Has Abstract

pub_date

1992-05-01 00:00:00

pages

446-53

issue

2

eissn

0003-9861

issn

1096-0384

pii

0003-9861(92)90710-E

journal_volume

294

pub_type

杂志文章

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