Exchange reactions hydrogenase

The hydrogenase-like proteins are known to catalyze the H/D exchange reaction, typical of hydrogenase (58). It remains to be determined to what extent the hydrogen binding site of these proteins re-... 

Other transformations supplied by these enzymes are para-ortho-hydrogen conversion, and the exchange reaction between H2 and protons of water.409-412 The hydrogenase enzymes found in various microorganisms are very different in their protein structure and in the types of electron carrier they use. 

N. A. Zorin, B. Dimon, J. Gagnon, J. Gaillard, P. Carrier, P. M. Vignais (1996) Inhibition by iodoacetamide and acetylene of the H-D exchange reaction catalyzed by Thiocapsa roseopersicina hydrogenase. Eur. J. Biochem., 241 675-681... 

Isotope-exchange reactions can provide incisive information about the mechanisms by which hydrogenases catalyse their reactions. The ratios of products formed, and in the rates of reaction with different isotopes, can be measured. Different hydrogenases show significantly different proportions of and H HO produced in reactions (3) and (4), and this effect has been used to identify the different types of hydrogenases in whole cells, without the need for purification (Berber et al. 1987). 

This mechanism easily accounts for the other reactions catalysed by hydrogenase. Exchange of the hydron and/or hydride with another hydron from the water, and reversal of step 1 would explain the H - F1 exchange reactions of hydrogenase (Chapter 5). 

H/D exchange reactions and mechanistic aspects of the hydrogenases 05CCR(249)1677. 

The proportions x and y depend on the type of hydrogenase and the pH (13). Hydrogenases also catalyze the interconversion of ortho- and para-hydrogen, which differ in the relative orientations of the proton spins (13). The exchange reactions do not require electron transfer to an external electron acceptor. They show that the enzymes catalyze heterolytic cleavage of H2 into a hydron (proton), H+ and hydride, H . 

A fourth assay measures the rate of interconversion of ortho and para hydrogen. Since any type of rupture of the hydrogen-hydrogen bond that permits random recombination will be detected by this assay, it was suggested that this is the most specific measurement of hydrogenase. However, the finding that a D2O medium substitutes the exchange reaction (HD formation) for the ortho-para conversion indicates that... 

There has been considerable effort in the last few years toward achieving an understanding of hydrogenase enzyme systems which have the ability to activate H2 for exchange with water, para-ortho conversion, and reduction reactions when coupled to an electron carrier E such as NAD+, cytochrome c3, or ferredoxins (7, p. 396 470-473). Reaction (81) can be catalyzed in either direction ... 

Knowledge of the active site allows for speculation on the mechanism of H2-D20 exchange which these Fe4 systems catalyze 473,483). Ruthe-nium(III) systems catalyze such an exchange via a ruthenium(III) hydride intermediate (7, p. 73 Section II,A), as exemplified in reactions (82) and (83), and iron hydrides must be involved in the hydrogenase systems. Ruthenium(III) also catalyzes the H2 reduction of ruthenium(IV) via reaction (82), followed by reaction (84) (3), and using these ruthenium systems as models, a very tentative scheme has been proposed 473) for... 

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