Hydride Binding to the Active Site of [FeFe]-Hydrogenase

Chernev, Petko; Lambertz, Camilla; Brünje, Annika; Leidel, Nils; Sigfridsson, Kajsa G.V.; Kositzki, Ramona; Hsieh, Chu‐Chin; Yao, Shenglai; Schiwon, Rafael; Drieß, Matthias; Limberg, Christian; Happe, Thomas; Haumann, Michael

doi:10.1021/ic502047q

Cited by 57 publications

(119 citation statements)

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“…Binding of hydrogen species in apical position at Fe d of the H-cluster is believed to be essential for catalysis (1). 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).…”

Section: Discussionmentioning

confidence: 99%

“…The mechanism of catalysis at the active site cofactor (H-cluster) needs to be elucidated. Further information on functional intermediates is required (11)(12)(13)(14)(15)(16) and expected to emerge from spectroscopic studies on H-cluster constructs carrying siteselective isotopic reporter groups (17)(18)(19)(20). Protein crystallography has identified the H-cluster as a six-iron complex (21)(22)(23) (19).…”

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“…H ox is believed to be the starting state in the H 2 conversion cycle of [FeFe]-hydrogenases (1). Selective 13 CO editing of H ox thus may provide access to key catalytic H-cluster intermediates (14). Introduction of 13 CO groups also facilitates analysis of structure-function relationships using quantum chemical calculations.…”

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

Senger

Mebs

Duan

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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The six-iron cofactor of [FeFe]-hydrogenases (H-cluster) is the most efficient H 2 -forming catalyst in nature. It comprises a diiron active site with three carbon monoxide (CO) and two cyanide (CN − ) ligands in the active oxidized state (H ox ) and one additional CO ligand in the inhibited state (H ox -CO). The diatomic ligands are sensitive reporter groups for structural changes of the cofactor. Their vibrational dynamics were monitored by real-time attenuated total reflection Fouriertransform infrared spectroscopy. Combination of 13 CO gas exposure, blue or red light irradiation, and controlled hydration of three different [FeFe]-hydrogenase proteins produced 8 H ox and 16 H ox -CO species with all possible isotopic exchange patterns. Extensive density functional theory calculations revealed the vibrational mode couplings of the carbonyl ligands and uniquely assigned each infrared spectrum to a specific labeling pattern. For H ox -CO, agreement between experimental and calculated infrared frequencies improved by up to one order of magnitude for an apical CN − at the distal iron ion of the cofactor as opposed to an apical CO. For H ox , two equally probable isomers with partially rotated ligands were suggested. Interconversion between these structures implies dynamic ligand reorientation at the H-cluster. Our experimental protocol for site-selective 13 CO isotope editing combined with computational species assignment opens new perspectives for characterization of functional intermediates in the catalytic cycle.[FeFe]-hydrogenase | isotope editing | infrared spectroscopy | density functional theory | cofactor dynamics T he reduction of protons to form molecular hydrogen (H 2 ) is catalyzed by [FeFe]-hydrogenases (1, 2). With a turnover rate of up to 10,000 H 2 molecules per second in a thermodynamically reversible reaction (3-5), [FeFe]-hydrogenases inspired synthetic hydrogen catalysts (6-8) and renewable fuel technology applications (9, 10). The mechanism of catalysis at the active site cofactor (H-cluster) needs to be elucidated. Further information on functional intermediates is required (11-16) and expected to emerge from spectroscopic studies on H-cluster constructs carrying siteselective isotopic reporter groups (17)(18)(19)(20) Formation of H ox -CO does not affect the formal redox state of the H-cluster, but leads to increased spin delocalization over the diiron site (28). CO binding inhibits H 2 turnover and protects the enzyme against O 2 and light-induced degradation (24,29,30).The vibrational modes of the CO and CN − ligands at the diiron site are well accessible by infrared (IR) spectroscopy because they are separated from protein backbone and liquid water bands.Infrared spectroscopy therefore has pioneered elucidation of the molecular structure of the H-cluster and identification of several redox states (24, 31). In particular, the CO stretching frequencies are highly sensitive to structural isomerism, redox transitions, ligand binding, and isotope exchange (11,12,15,18,24,31,32). 13 CO editing ...

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Section: Discussionmentioning

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

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Duan

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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“…13 The more prevalent μ -hydrides are also competent electrocatalysts for hydrogen evolution, 14 but they probably do not utilize biomimetic mechanisms, 7,15 although the matter is debated. 16 …”

Section: Acid–base Reactions Of Fe2(sr)2(co)6–xlxmentioning

confidence: 99%

Diiron Azadithiolates as Models for the [FeFe]-Hydrogenase Active Site and Paradigm for the Role of the Second Coordination Sphere

2015

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CONSPECTUS The [FeFe] hydrogenases (H2ases) catalyze the redox reaction that interconverts protons and H2. This area of biocatalysis has attracted attention because the metal-based chemistry is unusual, and the reactions have practical implications. The active site consists of a [4Fe–4S] cluster bridged to a [Fe2(μ-dithiolate)(CN)2(CO)3]z center (z = 1− and 2−). The dithiolate cofactor is [HN(CH2S)2]2−, called the azadithiolate ([adtH]2−). Although many derivatives of Fe2(SR)2(CO)6–xLx are electrocatalysts for the hydrogen evolution reaction (HER), most operate by slow nonbiomimetic pathways. Biomimetic hydrogenogenesis is thought to involve intermediates, wherein the hydride substrate is adjacent to the amine of the adtH, being bonded to only one Fe center. Formation of terminal hydride complexes is favored when the diiron carbonyl models contain azadithiolate. Although unstable in the free state, the adt cofactor is stable once it is affixed to the Fe2 center. It can be prepared by alkylation of Fe2(SH)2(CO)6 with formaldehyde in the presence of ammonia (to give adtH derivatives) or amines (to give adtR derivatives). Weak acids protonate Fe2(adtR)(CO)2(PR3)4 to give terminal hydrido (term-H) complexes. In contrast, protonation of the related 1,3-propanedithiolate (pdt2−) complexes Fe2(pdt)(CO)2(PR3)4 requires strong acids. The amine in the azadithiolate is a kinetically fast base, relaying protons to and from the iron, which is a kinetically slow base. The crystal structure of the doubly protonated model [(term-H)Fe2(HadtH)(CO)2(dppv)2]2+ confirms the presence of both ammonium and terminal hydrido centers, which interact through a dihydrogen bond (dppv = cis-C2H2(PPh2)2). DFT calculations indicate that this H---H interaction is sensitive to the counterions and is strengthened upon reduction of the diiron center. For the monoprotonated models, the hydride [(term-H)Fe2(adtH)(CO)2(dppv)2]+ exists in equilibrium with the ammonium tautomer [Fe2(HadtH)(CO)2(dppv)2]+. Both [(term-H)Fe2(HadtH)(CO)2(dppv)2]2+ and [(term-H)Fe2(adtH)(CO)2(dppv)2]+ are highly active electrocatalysts for HER. Catalysis is initiated by reduction of the diferrous center, which induces coupling of the protic ammonium center and the hydride ligand. In contrast, the propanedithiolate [(term-H)Fe2(pdt)(CO)2(dppv)2]+ is a poor electrocatalyst for HER. Oxidation of H2 has been demonstrated, starting with models for the oxidized state (“Hox”), for example, [Fe2(adtH)-(CO)3(dppv)(PMe3)]+. Featuring a distorted Fe(II)Fe(I) center, this Hox model reacts slowly with high pressures of H2 to give [(μ-H)Fe2(adtH)(CO)3(dppv)(PMe3)]+. Highlighting the role of the proton relay, the propanedithiolate [Fe2(pdt)-(CO)3(dppv)(PMe3)]+ is unreactive toward H2. The Hox-model + H2 reaction is accelerated in the presence of ferrocenium salts, which simulate the role of the attached [4Fe–4S] cluster. The redox-complemented complex [Fe2(adtBn)(CO)3(dppv)-(FcP*)]n+ catalyzes both proton reduction and hydrogen oxidation (FcP* = (C5Me5)Fe(C5Me4CH2PEt2)).

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“…X-ray absorption and emission spectroscopy (XAS/XES) facilitates molecular fine structure and electronic configuration determination and is sensitive to all valence and spin states of nickel sites [7]. The pre-edge absorption region of the K-edge due to resonant core-level (1s) electron excitation into valence orbitals (core-to-valence transitions, ctv), and the Kß emission region, due to electronic 3p→1s decay (Kß main lines, Kß 1,3 and Kß´) or decay from valence levels in the Kß 2,5 region (valence-to-core transitions, vtc), bear valuable information on the electronic structure [8][9][10]. The ctv and vtc features can be calculated using density functional theory (DFT) [8][9][10].…”

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confidence: 99%