When enzymes are optimized for biotechnological purposes, the goal often is to increase stability or catalytic efficiency. However, many enzymes reversibly convert their substrate and product, and if one is interested in catalysis in only one direction, it may be necessary to prevent the reverse reaction. In other cases, reversibility may be advantageous because only an enzyme that can operate in both directions can turnover at a high rate even under conditions of low thermodynamic driving force. Therefore, understanding the basic mechanisms of reversibility in complex enzymes should help the rational engineering of these proteins. Here, we focus on NiFe hydrogenase, an enzyme that catalyzes H(2) oxidation and production, and we elucidate the mechanism that governs the catalytic bias (the ratio of maximal rates in the two directions). Unexpectedly, we found that this bias is not mainly determined by redox properties of the active site, but rather by steps which occur on sites of the proteins that are remote from the active site. We evidence a novel strategy for tuning the catalytic bias of an oxidoreductase, which consists in modulating the rate of a step that is limiting only in one direction of the reaction, without modifying the properties of the active site.

Hydrogenases are highly active enzymes for hydrogen production and oxidation. [NiFeSe] hydrogenases, in which selenocysteine is a ligand to the active site Ni, have high catalytic activity and a bias for H production. In contrast to [NiFe] hydrogenases, they display reduced H inhibition and are rapidly reactivated after contact with oxygen. Here we report an expression system for production of recombinant [NiFeSe] hydrogenase from Desulfovibrio vulgaris Hildenborough and study of a selenocysteine-to-cysteine variant (Sec489Cys) in which, for the first time, a [NiFeSe] hydrogenase was converted to a [NiFe] type. This modification led to severely reduced Ni incorporation, revealing the direct involvement of this residue in the maturation process. The Ni-depleted protein could be partly reconstituted to generate an enzyme showing much lower activity and inactive states characteristic of [NiFe] hydrogenases. The Ni-Sec489Cys variant shows that selenium has a crucial role in protection against oxidative damage and the high catalytic activities of the [NiFeSe] hydrogenases.

Nickel-containing hydrogenases, the biological catalysts of H 2 oxidation and production, reversibly inactivate under anaerobic, oxidizing conditions. We aim at understanding the mechanism of (in)activation and what determines its kinetics, because there is a correlation between fast reductive reactivation and oxygen tolerance, a property of some hydrogenases that is very desirable from the point of view of biotechnology. Direct electrochemistry is potentially very useful for learning about the redox-dependent conversions between active and inactive forms of hydrogenase, but the voltammetric signals are complex and often misread. Here we describe simple analytical models that we used to characterize and compare 16 mutants, obtained by substituting the position-74 valine of the O 2 -sensitive NiFe hydrogenase from Desulfovibrio fructosovorans. We observed that this substitution can accelerate reactivation up to 1,000-fold, depending on the polarity of the position 74 amino acid side chain. In terms of kinetics of anaerobic (in)activation and oxygen tolerance, the valine-tohistidine mutation has the most spectacular effect: The V74H mutant compares favorably with the O 2 -tolerant hydrogenase from Aquifex aeolicus, which we use here as a benchmark.electrocatalysis | direct electron transfer | protein film voltammetry | hydrogen T he nickel-iron hydrogenases that have been crystallized and/ or thoroughly studied so far are very similar from a structural point of view: They all are either soluble heterodimers or heterodimers isolated from a membrane-associated complex. The amino acids that surround the NiFe active site are conserved (1) and yet the kinetic properties of these enzymes are diverse. For example, some NiFe hydrogenases can oxidize and produce H 2 , whereas others preferentially catalyze one direction of the reaction (2-4). Another property of some hydrogenases that has attracted considerable interest is their sensitivity (and sometimes their resistance) to O 2 . This interest stems from the fact that hydrogenases could be used for H 2 oxidation in fuel cells or H 2 production in photo-electrochemical cells if they were functional under aerobic conditions (5).The NiFe hydrogenases that have been studied first, referred to as "standard," were purified from Allochromatium vinosum or Desulfovibrio species. Upon exposure to O 2 , they convert into two inactive forms called NiA and NiB, where an oxygenated ligand bridges the Ni and the Fe. The NiB and NiA states can be reactivated by reduction, the former more quickly than the latter (6). The membrane-bound NiFe hydrogenases from, e.g., Ralstonia eutropha or Aquifex aeolicus are reversibly inhibited by O 2 and termed "O 2 resistant." Apparently, this resistance results from (i) the enzyme reacting with O 2 to form only the NiB state and (ii) this NiB state reactivating much more quickly than in standard hydrogenases (4, 7). The most patent differences between O 2 -resistant and O 2 -sensitive hydrogenases are the structures and redox properties of the FeS clu...

We studied the mechanism of aerobic inactivation of Desulfovibrio fructosovorans nickel-iron (NiFe) hydrogenase by quantitatively examining the results of electrochemistry, EPR and FTIR experiments. They suggest that, contrary to the commonly accepted mechanism, the attacking O(2) is not incorporated as an active site ligand but, rather, acts as an electron acceptor. Our findings offer new ways toward the understanding of O(2) inactivation and O(2) tolerance in NiFe hydrogenases.