Tungsten-containing aldehyde oxidoreductases
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The main objective of this research was the molecular, biochemical and kinetic characterization of the members of the AOR family of enzymes and attempt to elucidate the role these enzymes play in the physiology of Pyrococcus furiosus. Three different types of tungsten-containing aldehyde oxidizing enzymes aldehyde ferredoxin oxidoreductase (AOR), formaldehyde ferredoxin oxidoreductase (FOR) and glyceraldehyde-3-phosphate ferredoxin oxidoreductase (GAPOR) have previously been isolated from this microorganism. Although P. furiosus AOR had been extensively characterized, relatively little was known about the related enzyme FOR. The molecular catalytic and structural properties of this enzyme are presented. Detailed kinetic analyses of FOR indicate that C4 – C6 dialdehyde or acid-substituted aldehyde may serve as the physiological substrate for this enzyme. Since such semialdehydes are involved in the metabolic pathways of certain amino acids, FOR is proposed to have a role in amino acid metabolism of P. furiosus. The oxygen-inactivated forms of FOR, AOR and GAPOR from P. furiosus could be reactivated to 100% of original specific activity by incubation with sodium sulfide under reducing conditions. Loss of sulfide from the W coordination sphere presumably accounts for the loss in specific activity of these enzymes on exposure to oxygen. However, the exact nature of this ‘restored’ sulfide is not clear at this point. Although, P. furiosus cells grown with S° have 3-5 fold higher intracellular concentrations of acid- labile sulfide and approximately 2-fold higher polysulfide than those grown without S°, these are associated with the high molecular weight fraction of the cell-free extract and not available in the free form. Based on these results, sulfide-activation does not appear to be a physiological reaction. A survey of the complete genome sequences of numerous microorganisms reveals that tungstoenzymes of the AOR family appear to be widespread among the hyperthermophilic archaea and some bacteria. Two hitherto unknown members have also been identified in the P. furiosus genome. Termed wor4 and wor5, these genes are proposed to encode putative tungstoenzymes. One of these tungstoproteins WOR 4 has been isolated from P. furiosus cell-free extract and characterized.