Hydrogenase activation states

The catalytic site of [NiFe] and [NiFeSe] hydrogenases in oxidised inactive and reduced active states... 

Figure 7.5 Overview of the several states of the active site in the [NiFe] hydrogenase from A. v/nosum.Transitions can be invoked by redox titrations in the presence of redox mediators and involve both electrons (e ) and protons (H ) as indicated on the left side of the blocks. H2, in the absence of mediators, can rapidly react only with enzyme in the active states (lower block), as indicated on the right side. Dotted arrows indicate a very slow reaction.Very similar states are found in the D. gigas enzyme.

For A. vinosum hydrogenase, activation requires not only reduction, but an elevated temperature is essential as well. When enzyme in the NiJ state is reduced at 30°C or... 

We should also remember that not all of the states that we see when freezing the enzyme (Section 7.4) are necessarily part of the mechanism. The most stable enzyme molecule is a dead one, so we must be aware that some of the spectroscopic signals represent damaged molecules. In the [NiFe] hydrogenases, the NiA and NiB states probably are not involved in the catalytic cycle, because they react slowly, if at all, with H2. In the mechanism shown in Fig. 8.3, it is assumed that the relevant active states are NiSR, NiA and NiR. 

Davidson, G., Choudhury, S. B., Gu, Z., Bose, K., Roseboom, W., Albracht, S. P. and Maroney, M. J. (2000) Structural examination of the nickel site in chromatium vinosum hydrogenase Redox state oscillations and structural changes accompanying reductive activation and CO binding. Biochemistry, 39, 7468-79. 

Hydrogen tunnelling, 44 276 Hydrogen vapor formation from, 14 133 Hydrogenarsenito complexes, osmium, 37 288 Hydrogenase 3, function, 38 416 Hydrogenases, 32 298, 299, 38 397-419 catalytic properties activity states, 32 317-320 mechanism of catalysis, 32 320-321 coordination state of nickel... 

EPR spectra of CO oxidoreductase under non reducing conditions showed a spectrum at g = 2.21, 2.11, and 2.02, which, by analogy with spectra observed in nickel-containing hydrogenases, was attributed to Ni(III) (96). The spectrum was of low intensity and it was not established whether it represents an active state of the enzyme. 

Bruschi M, De Gioia L, Zampella G, Reiher M, Fantucci P, Stein M,(2004). A theoretical study of spin states in Ni-S4 complexes and models of the [NiFe] hydrogenase active site. J. Biol. Inorg. Chem. 9 873-884... 

Foerster S, van Gastel M, Brecht M, Lubitz W. An orientation-selected ENDOR and HYSCORE study of the Ni-C active state of Desulfovibrio vulgaris Miyazaki F hydrogenase. J Biol Inorg Chem. 2005 10(1) 51 62. 

Liu, T. and M. Darensbourg. 2007. A Mixed-Valent, Fe(II)Fe(I), Diiron Complex Reproduces the Unique Rotated State of the [FeFe] Hydrogenase Active Site. Journal of the American Chemical Society 129(22) 7008-7009. 

This structure mimics the resting state of the [FeFe]hydrogenase active site. The enzyme holds this conformation in position throughout proton/electron coupling/ decoupling reactions. 

Mixed Valent, Fe(II)Fe(I), Diiron Complexes Reproduce the Unique Rotated State of the [FeFe]Hydrogenase Active Site... 

The active sites of [FeNi]-hydrogenases give rise to a number of interesting EPR spectra. There are three EPR active states labeled A, B and C. Each of these states produces a rhombic EPR spectrum. Data for a variety of different [FeNi]-hydrogenases are collected in Table 1. These signals have been assigned to the Ni ion on the basis of Ni-labeling studies [30]. 

In the case of the [FeNij-hydrogenase of Desulfovibrio gigas IR absorption in the 1900-2100 cm region has been assigned for each of the redox-active states [44], These data are collected in Table 2. This study revealed the presence of an EPR silent state labeled SU that exists as a mixture of two protonation states. The changes in IR properties have been used to determine the electrochemical potentials required for the changes in redox state. These data are presented in the following section. 

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