Tài liệu Báo cáo khoa học: Reaction mechanisms of thiamin diphosphate enzymes: redox reactions pdf

MINIREVIEW

Reaction mechanisms of thiamin diphosphate enzymes:

redox reactions

Kai Tittmann

Albrecht-von-Haller-Institut fu¨r Pflanzenwissenschaften und Go¨ttinger Zentrum fu¨r Molekulare Biowissenschaften, Georg-August-Universita¨t

Go¨ttingen, Germany

Introduction

The oxidative decarboxylation of 2-keto acids, such

as pyruvate, branched-chain keto acids and ketoglu￾tarate, is a key reaction of intermediary metabolism

in virtually all organisms and is catalyzed by thiamin

diphosphate (ThDP)-dependent enzymes [1]. In view

of the central metabolic role of pyruvate, the various

biochemical reactions involving pyruvate are the

most intensely studied and are well understood.

Thus, they serve as prototypical reactions for the

enzymic oxidative conversion of 2-keto acids. Hence,

the present review mainly focuses on the reaction

mechanisms of ThDP enzymes that directly oxidize

pyruvate. Special emphasis is devoted to the nature

and reactivity of transient intermediates, the coupling

of oxidation–reduction and acyl group transfer and

electron transfer between cofactors. The review

includes a discussion of general aspects of enzyme

catalyzed pyruvate oxidation, in addition to individ￾ual sections on the different ThDP enzymes that act

on pyruvate.

Pathways of pyruvate oxidation by

ThDP enzymes

Generally, there are at least four major different path￾ways of ThDP enzyme catalyzed pyruvate oxidation.

Keywords

electron transfer; flavin; intermediate;

iron-sulfur cluster; lipoamide; oxidative

decarboxylation; phosphorolysis; pyruvate;

radical; X-ray crystallography

Correspondence

K. Tittmann, Albrecht-von-Haller-Institut fu¨r

Pflanzenwissenschaften und Go¨ttinger

Zentrum fu¨r Molekulare Biowissenschaften,

Georg-August-Universita¨t Go¨ttingen, Ernst￾Caspari-Haus, Justus-von-Liebig-Weg 11,

D-37077 Go¨ttingen, Germany

Fax: +49 551 39 5749

Tel: +49 551 39 14430

E-mail: [email protected]

(Received 7 November 2008, revised 3

February 2009, accepted 13 February 2009)

doi:10.1111/j.1742-4658.2009.06966.x

Amongst a wide variety of different biochemical reactions in cellular car￾bon metabolism, thiamin diphosphate-dependent enzymes catalyze the oxi￾dative decarboxylation of 2-keto acids. This type of reaction typically

involves redox coupled acyl transfer to CoA or phosphate and is mediated

by additional cofactors, such as flavins, iron-sulfur clusters or lipoamide

swinging arms, which transmit the reducing equivalents that arise during

keto acid oxidation to a final electron acceptor. EPR spectroscopic and

kinetic studies have implicated the intermediacy of radical cofactor

intermediates in pyruvate:ferredoxin oxidoreductase and an acetyl phos￾phate-producing pyruvate oxidase, whereas the occurrence of transient

on-pathway radicals in other enzymes is less clear. The structures of pyru￾vate:ferredoxin oxidoreductase and pyruvate oxidase with different enzymic

reaction intermediates along the pathway including a radical intermediate

were determined by cryo-crystallography and used to infer electron tunnel￾ing pathways and the potential roles of CoA and phosphate for an intimate

coupling of electron and acyl group transfer. Viable mechanisms of reduc￾tive acetylation in pyruvate dehydrogenase multi-enzyme complex, and of

electron transfer in the peripheral membrane enzyme pyruvate oxidase

from Escherichia coli, are also discussed.

Abbreviations

AcThDP, 2-acetyl-ThDP; HEThDP, 2-(1-hydroxyethyl)-ThDP; PDHc, pyruvate dehydrogenase multi-enzyme complex; PFOR,

pyruvate:ferredoxin oxidoreductase; POX, pyruvate oxidase; Q8, ubiquinone 8; ThDP, thiamin diphosphate.

2454 FEBS Journal 276 (2009) 2454–2468 ª 2009 The Author Journal compilation ª 2009 FEBS