Fumaric hydrogenase

Prosthetic groups Flavine Adenine Dinucleotide (FAD) Glycine oxidase, Fumaric hydrogenase, Xanthine oxidase... 

The following enzymes have been shown to be flavoproteins D-amino acid oxidases, Z-amino acid oxidases, xanthine oxidases, aldehyde oxidases, the glucose dehydrogenase, notatin, the diamine oxidase, and fumaric hydrogenase. Details concerning these enzymes may be found in the review articles of TheorelP and Krebs. 

Midpoint potential values are useful quantitites for defining the role of the various centers in the system. In some instances, these values have even been used to predict the location of the centers in the electron transfer chain, assuming that the potential increases along the chain from the electron donor to the electron acceptor. In several oxidoreductases, however, the measured potential of some centers was found to be clearly outside the range defined by the donor and the acceptor, which raised an intriguing question as to their function. This was observed, for instance, in the case of the [4Fe-4S] (Eni = -320 mV) center in E. coli fumarate reductase (249), the [3Fe-4S] + (Era = -30 mV) center in D. gigas hydrogenase (207), and the low-potential [4Fe-4S] + + (E, = 200 and -400 mV) centers in E. 

Fig. 3 Speculative metabolic schemes of the main pathways in carbohydrate metabolism in N. ovalis Abbreviations AcCoA, acetyl-CoA, Cl, complex I, Citr, citrate, FRD, fumarate reductase, FUM, fumarate, Hyd, hydrogenase, a-KG, a-ketoglutarate, MAL, malate, OXAC, oxaloacetate, PDH, pyruvate dehydrogenase, PEP, phosphoenolpyruvate carboxyk-inase, PYR, pyruvate, RQ, rhodoquinone, SUCC, succinate, SUCC-CoA, succinyl-CoA...

The adaptation to an anaerobic lifestyle with the aid of hydrogenosomes required the acquisition of an (oxygen-sensitive) hydrogenase. The evolution of fumarate respiration in N. ovalis shows that an adaptation to life in anaerobic environments can occur in steps - by evolutionary tinkering. 

Such a metabolism ( fumarate respiration ) is well known from anaerobic mitochondria (Tielens et al. 2002 Tielens and van Hellemond, Chap. 6 in this volume), but is unique in combination with a hydrogenase that might compete with the fumarate reductase for the same substrates. This hydrogenase of N. ovalis represents a novel type of [Fe]-only or [FeFe]-hydrogenase that allows H2 formation to be coupled directly to the reoxidation of NADH. The [Fe]-hydrogenase is linked covalently with a protein, which possesses NAD and FMN binding sites, and a ferredoxin-like FeS module that allows transfer... 

Several terminal electron acceptors are coupled to H2 oxidation. These include O2, nitrate, sulfate, carbon dioxide, fumarate, and halogenated organics. Electron donors that couple to proton reduction include pyruvate and carbon monoxide. The electron donors or acceptors for the D. desulfu-ricans Fe-only hydrogenase is cytochrome c3 or c6 (Guerlesquin et al., 1994 Verhagen et al., 1994). 

X-ray crystal structures are available for cubane-type [3Fe-4S] centers in 3Fe and 7Fe Fds, aconitase, NiFe-hydrogenases, succinate dehydrogenase, fumarate... 

The [2Fe 2S], [3Fe S], and [4Fe S] clusters that are found in simple Fe S proteins are also constituents of respiratory and photosynthetic electron transport chains. Multicluster Fe S enzymes such as hydrogenase, formate dehydrogenase, NADH dehydrogenase, and succinate dehydrogenase feed electrons into respiratory chains, while others such as nitrate reductase, fhmarate reductase, DMSO reductase, and HDR catalyze the terminal step in anaerobic electron transport chains that utihze nitrate, fumarate, DMSO, and the CoB S S CoM heterodisulfide as the respiratory oxidant. All comprise membrane anchor polypeptide(s) and soluble subunits on the membrane surface that mediate electron transfer to or from Mo cofactor (Moco), NiFe, Fe-S cluster or flavin active sites. Multiple Fe-S clusters define electron transport pathways between the active site and the electron donor or... 

The electrons then are used by a hydrogenase enzyme to produce Ha gas. Desuljovibrio can also convert succinate, fumarate, malate, oxaloacetate, or lactate into pyruvate by reactions which resemble fragments of the Krebs cycle (360). 

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