Enzyme aldehyde ferredoxin oxidoreductase

Chan, M.K., S. Mukund, A. Kletzin, M.W. Adams, and D.C. Rees. 1995. Structure of a hyperthermophilic tungstopterin enzyme, aldehyde ferredoxin oxidoreductase. Science 267 1463-1469. 

The aldehyde ferredoxin oxidoreductase from the hyperthermophile Pyrococcus furiosus was the first molybdopterin-dependent enzyme for which a three-dimensional structure became available.683,684 The tungstoenzyme resembles that of the related molybdo-enzyme (Fig. 16-31). A similar ferredoxin-dependent enzyme reduces glyceraldehyde-3-phosphate.685 Another member of the tungstoenzyme aldehyde oxidoreductase family is carboxylic acid reductase, an enzyme found in certain acetogenic clostridia. It is able to use reduced ferredoxin to convert unactivated carboxylic acids into aldehydes, even though E° for the acetaldehyde/acetate couple is -0.58 V.686... 

The crystal structure of aldehyde ferredoxin oxidoreductase (AOR) from the hyperthermophile Pyrococcus furiosus was the first of any molybdenum or tungsten enzyme (excepting nitrogenase) (42). The AOR was adopted as the parent name for the family of tungsten enzymes. The structure of formaldehyde ferredoxin oxidoreductase (FOR) has recently been solved (44). 

Although less prominent than their molybdenum counterparts, several tungsten-containing enzymes have been isolated and characterized (2, 10, 20, 23, 24a). A notable aspect of the biochemical role of these enzymes is that many occur in hyperthermophilic archea that live at temperatures of 100 °C. The known tungsten enzymes can be classified into three functional and phylogen-etically distinct families, the representative members being aldehyde ferredoxin oxidoreductase formaldehyde ferredoxin oxidoreductase and gyceraldehyde-3-phosphate ferredoxin oxidoreductase (23). 

As stated, the RTF is the inactive form of aldehyde ferredoxin oxidoreductase (AOR). The active AOR enzyme may be isolated by rapid purification under anaerobic conditions using buffer containing dithio-threitol and glycerol (282). AOR catalyzes Eq. (28) and is postulated to contain a W—SH group not present in the RTF. 

The deprotonation and addition of a base to thiazolium salts are combined to produce an acyl carbanion equivalent (an active aldehyde) [363, 364], which is known to play an essential role in catalysis of the thiamine diphosphate (ThDP) coenzyme [365, 366]. The active aldehyde in ThDP dependent enzymes has the ability to mediate an efScient electron transfer to various physiological electron acceptors, such as lipoamide in pyruvate dehydrogenase multienzyme complex [367], flavin adenine dinucleotide (FAD) in pyruvate oxidase [368] and Fc4S4 cluster in pyruvate ferredoxin oxidoreductase [369]. 

FIGURE 9.2 Physiology of ABE fermentation metabolism of Clostridium acetobutylicum with the respective enzymes and products. CoA, coenzyme A Ldh, lactate dehydrogenase Pdc, pyruvate decarboxylase Pfor, pyruvate ferredoxin oxidoreductase Fdred, ferredoxin reduced Thl, thiolase Hbd, p-hydroxybutyryl-CoA dehydrogenase Crt, crotonase Bed, butyryl-CoA dehydrogenase Etf, electron transfer flavoprotein Pta, phosphotransacetylase Ack, acetate kinase Ptb, phosphotransbutyrylase Buk, butyrate kinase Ctf A/B, acetoacetyl-CoA acyl-CoA transferase Adc, acetoacetate decarboxylase AdhE, aldehyde/alcohol dehydrogenase Bdh, butanol dehydrogenase. 

Figure 16-31 (A) Structure of molybdopterin cytosine dinucleotide complexed with an atom of molybdenum. (B) Stereoscopic ribbon drawing of the structure of one subunit of the xanthine oxidase-related aldehyde oxidoreductase from Desulfo-vibrio gigas. Each 907-residue subunit of the homodimeric protein contains two Fe2S2 clusters visible at the top and the molybdenum-molybdopterin coenzyme buried in the center. (C) Alpha-carbon plot of portions of the protein surrounding the molybdenum-molybdopterin cytosine dinucleotide and (at the top) the two plant-ferredoxin-like Fe2S2 clusters. Each of these is held by a separate structural domain of the protein. Two additional domains bind the molybdopterin coenzyme and there is also an intermediate connecting domain. In xanthine oxidase the latter presumably has the FAD binding site which is lacking in the D. gigas enzyme. From Romao et al.633 Courtesy of R. Huber.

A few representative Mo enzymes are listed in Table 18-C-6. Note that most have very high molecular weights, and this has delayed structure determinations. In the case of DMSO-reductase, however, the lower molecular weight has made possible a structure determination. The Mo site is found to be as shown in 18-C-XXVII. In a tungsten-containing enzyme, ferredoxin aldehyde oxidoreductase, the metal site is as shown in 18-C-XXVIII. 

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