O2 Reactions at the Six-iron Active Site (H-cluster) in [FeFe]-Hydrogenase

Abstract

[FeFe]-hydrogenase (H 2 ase) 4 proteins are the most active biological catalysts for the production of molecular hydrogen (H 2 ) from proton reduction, with reported turnover rates of up to 10 4 s Ϫ1 (1, 2). Therefore, these enzymes are of high interest for biotechnology, aiming at the generation of H 2 as a renewable fuel (1, 3-5). However, a severe limitation for such applications is the rapid inactivation of [FeFe]-H 2 ases by dioxygen (O 2 ) (6, 7). Understanding the mechanism of O 2 -induced inactivation may allow the improvement of enzyme features to yield increased O 2 tolerance (e.g. by genetic engineering, which has already been demonstrated for NiFe hydrogenases) (8 -10).[FeFe]-H 2 ases are found in certain bacteria and green algae (11, 12). All of these enzymes contain an active site that consists of an inorganic iron complex, which is denoted as the H-cluster (13-15). The [FeFe]-H 2 ase enzymes from anaerobic bacteria in addition bind several iron-sulfur (FeS) clusters, serving as a relay for electron transfer to and from the active site (13-15).[FeFe]-H 2 ases from green algae represent the minimal unit for biological H 2 production because they contain only the H-cluster, whereas accessory FeS clusters are absent (16). This feature renders these enzymes most suitable for spectroscopic investigations on O 2 -induced inactivation (17), focusing on the active site reactions. The general structure of the H-cluster has been unraveled by crystallography, x-ray absorption spectroscopy (XAS), EPR, and FTIR spectroscopy on H 2 ase protein from various organisms (18). The H-cluster structure in both bacteria and green algae appears to be similar overall (16,19,20). It features a [4Fe4S] cubane cluster, which is bound by four cysteine residues to the protein and is linked by one of them to a binuclear iron unit (2Fe H ) (Fig. 1). The latter carries two cyanide (CN Ϫ ) ligands and three carbon monoxide (CO) ligands (21, 22) and presumably an azadithiolate group (adt; (SCH 2 ) 2 NH) in the metal-bridging position (21,23,24). Both types of diatomic ligands are probably derived from a biosynthetic pathway starting with tyrosine (25-27). The nitrogen atom of the adt has been proposed to be involved in proton transfer at the active site (21, 28). The H-cluster structure is assembled in a complex reaction involving three maturation proteins (29 -34). H 2 formation has been proposed to involve the binding and reduction of a single proton, which transiently creates a hydride ligand, either located in a bridging position between the two iron ions or terminally bound at the distal iron ion of the 2Fe H moiety (Fig. 1). After a second protonation step, H 2 is released from the H-cluster (35)(36)(37) 4 The abbreviations used are: H 2 ase, hydrogenase; adt, azadithiolate; EXAFS, extended x-ray absorption fine structure; NaDT, sodium dithionite; ROS, reactive oxygen species; XANES, x-ray absorption near edge structure; XAS, x-ray absorption spectroscopy; FT, Fourier transform.

Stepwise isotope editing of [FeFe]-hydrogenases exposes cofactor dynamics

“…However, configurations with (semi) bridging or equatorial H-species were considered as well (12,14,36) and may result from structural flexibility of the H-cluster (36). Such structural dynamics may facilitate apical or equatorial ligand binding at the distal iron ion and may also be relevant for O 2 inactivation of the enzymes via reactive oxygen species formation (24,29,30,60,61). Our protocol for selective preparation of H ox with eight distinct isotopic labeling patterns introduces spectroscopic probes at individual positions at the cofactor.…”

Mechanism of O2 diffusion and reduction in FeFe hydrogenases

“…According to X-ray absorption measurements, exposing the enzyme from Cr to O 2 damages the 4Fe4S subcluster; this suggested that O 2 binding to Fe d results in the formation of a reactive oxygen species (ROS) that diffuses towards the 4Fe4S subcluster and destroys it. 16,18 According to the DFT studies of Reiher 19,20 and Pachter, 21 this ROS could be the OOH radical or H 2 O 2 . However, this mechanism conflicts with a recent report according to which the O 2 -damaged enzyme from Cr harbors an intact 4Fe4S subsite and no 2Fe subcluster, and the observation that the O 2 -damaged enzyme is repaired upon insertion of a synthetic analogue of the 2Fe subcluster.…”

Differential Expression of the Chlamydomonas [FeFe]-Hydrogenase-Encoding HYDA1 Gene Is Regulated by the COPPER RESPONSE REGULATOR1

“…As a response to anaerobiosis the unicellular green alga Chlamydomonas reinhardtii develops a complex fermentative metabolism (Gfeller and Gibbs, 1984;Hemschemeier and Happe, 2005;Mus et al, 2007;Hemschemeier et al, 2008b;Philipps et al, 2011) with molecular hydrogen (H 2 ) as one key metabolite. Hydrogen is produced by an oxygen-sensitive [FeFe]-hydrogenase (Stripp and Happe, 2009;Lambertz et al, 2011) that is linked to the photosynthetic electron transport chain via ferredoxin PETF (Winkler et al, 2009). Chlamydomonas expresses two hydrogenaseencoding genes, [FeFe]-hydrogenase isoform1 (HYDA1) and HYDA2 (Forestier et al, 2003), but the HYDA1 protein seems to be the primarily active hydrogenase isoform in the green alga (Godman et al, 2010;Meuser et al, 2012).…”