Crystallization and preliminary crystallographic analysis of the pyruvate-ferredoxin oxidoreductase from Desulfovibrio africanus

Decarboxylation of pyruvate to acetaldehyde for ethanol production by hyperthermophiles

Pyruvate decarboxylase (PDC encoded by pdc) is a thiamine pyrophosphate (TPP)-containing enzyme responsible for the conversion of pyruvate to acetaldehyde in many mesophilic organisms. However, no pdc/PDC homolog has yet been found in fully sequenced genomes and proteomes of hyper/thermophiles. The only PDC activity reported in hyperthermophiles was a bifunctional, TPP- and CoA-dependent pyruvate ferredoxin oxidoreductase (POR)/PDC enzyme from the hyperthermophilic archaeon Pyrococcus furiosus. Another enzyme known to be involved in catalysis of acetaldehyde production from pyruvate is CoA-acetylating acetaldehyde dehydrogenase (AcDH encoded by mhpF and adhE). Pyruvate is oxidized into acetyl-CoA by either POR or pyruvate formate lyase (PFL), and AcDH catalyzes the reduction of acetyl-CoA to acetaldehyde in mesophilic organisms. AcDH is present in some mesophilic (such as clostridia) and thermophilic bacteria (e.g., Geobacillus and Thermoanaerobacter). However, no AcDH gene or protein homologs could be found in the released genomes and proteomes of hyperthermophiles. Moreover, no such activity was detectable from the cell-free extracts of different hyperthermophiles under different assay conditions. In conclusion, no commonly-known PDCs was found in hyperthermophiles. Instead of the commonly-known PDC, it appears that at least one multifunctional enzyme is responsible for catalyzing the non-oxidative decarboxylation of pyruvate to acetaldehyde in hyperthermophiles.

Metal centers in the anaerobic microbial metabolism of CO and CO2

Carbon dioxide and carbon monoxide are important components of the carbon cycle. Major research efforts are underway to develop better technologies to utilize the abundant greenhouse gas, CO(2), for harnessing 'green' energy and producing biofuels. One strategy is to convert CO(2) into CO, which has been valued for many years as a synthetic feedstock for major industrial processes. Living organisms are masters of CO(2) and CO chemistry and, here, we review the elegant ways that metalloenzymes catalyze reactions involving these simple compounds. After describing the chemical and physical properties of CO and CO(2), we shift focus to the enzymes and the metal clusters in their active sites that catalyze transformations of these two molecules. We cover how the metal centers on CO dehydrogenase catalyze the interconversion of CO and CO(2) and how pyruvate oxidoreductase, which contains thiamin pyrophosphate and multiple Fe(4)S(4) clusters, catalyzes the addition and elimination of CO(2) during intermediary metabolism. We also describe how the nickel center at the active site of acetyl-CoA synthase utilizes CO to generate the central metabolite, acetyl-CoA, as part of the Wood-Ljungdahl pathway, and how CO is channelled from the CO dehydrogenase to the acetyl-CoA synthase active site. We cover how the corrinoid iron-sulfur protein interacts with acetyl-CoA synthase. This protein uses vitamin B(12) and a Fe(4)S(4) cluster to catalyze a key methyltransferase reaction involving an organometallic methyl-Co(3+) intermediate. Studies of CO and CO(2) enzymology are of practical significance, and offer fundamental insights into important biochemical reactions involving metallocenters that act as nucleophiles to form organometallic intermediates and catalyze C-C and C-S bond formations.

Structural and kinetic studies of the pyruvate-ferredoxin oxidoreductase/ferredoxin complex from Desulfovibrio africanus

The pyruvate-ferredoxin oxidoreductase (PFOR)/ferredoxin (Fd) system of Desulfovibrio africanus has been investigated with the aim of understanding more fully protein-protein interaction and the kinetic characteristics of electron transfer between the two redox partners. D. africanus contains three Fds (Fd I, Fd II and Fd III) able to function as electron acceptors for PFOR. The complete amino acid sequence of Fd II was determined by automatic Edman degradation. It revealed a striking similarity to that of Fd I. The protein consists of 64 residues and its amino acid sequence is in agreement with a molecular mass of 6822.5 Da as measured by electrospray MS. Fd II contains five cysteine residues of which the first four (Cys11, Cys14, Cys17 and Cys54) are likely ligands for the single [4Fe-4S] cluster. A covalently cross-linked complex between PFOR and Fd I or Fd II was obtained by using a water soluble carbodiimide. This complex exhibited a stoichiometry of one ferredoxin for one PFOR subunit and is dependent on the ionic strength. The second-order rate constants for electron transfer between PFOR and Fds determined electrochemically using cyclic voltammetry are 7 x 107 M-1.s-1 for Fd I and 2 x 107 M-1.s-1 for Fd II and Fd III. The Km values of PFOR for Fd I and Fd II measured both by the electrochemical and the spectrophotometric method have been found to be 3 microM and 5 microM, respectively. The three-dimensional modelling of Fd II and surface analysis of Fd I, Fd II and PFOR suggest that a protein-protein complex is likely to be formed between aspartic acid/glutamic acid invariant residues of Fds and lysine residues surrounding the distal [4Fe-4S] cluster of PFOR. All of these studies are indicative of the involvement of electrostatic interactions between the two redox partners.