AbbreviationsIAA, iodoacetamide, FDH, formate dehydrogenase, Moco, molybdenum cofactor, MGD, molybdopterin guanine dinucleotide, MV, methyl viologen, SeCys, selenocysteine, XAS, X-ray absorption spectroscopy 3 AbstractFormate dehydrogenases (FDHs) are capable to perform the reversible oxidation of formate and are enzymes of high interest for fuel cell applications and for the production of reduced carbon compounds as energy source from CO 2 . Metalcontaining FDHs in general contain a highly conserved active site, comprising a molybdenum (or tungsten) center coordinated by two molybdopterin guanine dinucleotide molecules, a sulfido and a (seleno-)cysteine ligand, in addition to a histidine and arginine residue in the second coordination sphere. So far, the role of these amino acids for catalysis has not been studied in detail, due to the lack of suitable expression systems and the lability or oxygen sensitivity of the enzymes.Here, the roles of these active-site residues is revealed using the Mo-containing FDH from Rhodobacter capsulatus. Our results show that the cysteine ligand at the Mo ion is displaced by the formate substrate during the reaction, the arginine has a direct role in substrate binding and stabilization and the histidine elevates the pK a of the active-site cysteine. We further found that in addition to reversible formate oxidation, the enzyme is further capable to reduce nitrate to nitrite. We propose a mechanistic scheme, which combines both functionalities and provides important insights into the distinct mechanisms of C-H bond cleavage and oxygen atom transfer catalyzed by formate dehydrogenase. 4A specific enzyme system of increasing interest is formate dehydrogenase (FDH), being involved in the reversible conversion of CO 2 in biological systems (1,2 Desulfovibrio gigas (FdhAB) have shown that the metal ion in the oxidized enzyme is coordinated by four sulfur ligands of two dithiolene groups of the bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor, a selenocycteine (SeCys), and by a sixth ligand, which is now established as a sulfido group (5-9) ( Fig. 1). After reduction with formate, the structure of E. coli FdhF showed that the SeCys ligand was displaced from the Mo ion (5). In the second coordination sphere, a highly conserved histidine and arginine are present in all FDH enzymes described so far. Experimental evidence in which these residues are replaced by other amino acids is lacking, due 5 to the high oxygen sensitivity of the E. coli and D. gigas enzymes and the lack of suitable overexpression systems.Closely related to FDHs are periplasmic nitrate reductases (e.g. Cupriavidus necator NapA), which similarly belong to the DMSO reductase family of molybdoenzymes.Periplasmic nitrate reductases comprise an identical Mo coordination sphere containing six sulfur ligands in the oxidized state (including a Cys) and show a remarkable structural homology to FDHs (10-12) (Fig. 1). A conserved threonine and methionine are found close to the Mo ion in periplasmic nitrate reductases ( Fi...

Background: R2-like ligand-binding oxidases (R2lox) can assemble a Mn/Fe or diiron cofactor. Results: The metal centers are structurally similar and activate oxygen, resulting in redox-coupled structural changes. Conclusion: Oxygen activation likely proceeds via similar mechanisms at Mn/Fe and diiron clusters, while their redox state controls oxygen and substrate access. Significance: R2lox proteins could provide novel catalysts for oxidative chemistry.

Formate dehydrogenase (FDH) enzymes are attractive catalysts for potential carbon dioxide conversion applications. The FDH from Rhodobacter capsulatus (RcFDH) binds a bis-molybdopterin-guanine-dinucleotide (bis-MGD) cofactor, facilitating reversible formate (HCOO(-)) to CO2 oxidation. We characterized the molecular structure of the active site of wildtype RcFDH and protein variants using X-ray absorption spectroscopy (XAS) at the Mo K-edge. This approach has revealed concomitant binding of a sulfido ligand (Mo=S) and a conserved cysteine residue (S(Cys386)) to Mo(VI) in the active oxidized molybdenum cofactor (Moco), retention of such a coordination motif at Mo(V) in a chemically reduced enzyme, and replacement of only the S(Cys386) ligand by an oxygen of formate upon Mo(IV) formation. The lack of a Mo=S bond in RcFDH expressed in the absence of FdsC implies specific metal sulfuration by this bis-MGD binding chaperone. This process still functioned in the Cys386Ser variant, showing no Mo-S(Cys386) ligand, but retaining a Mo=S bond. The C386S variant and the protein expressed without FdsC were inactive in formate oxidation, supporting that both Mo-ligands are essential for catalysis. Low-pH inhibition of RcFDH was attributed to protonation at the conserved His387, supported by the enhanced activity of the His387Met variant at low pH, whereas inactive cofactor species showed sulfido-to-oxo group exchange at the Mo ion. Our results support that the sulfido and S(Cys386) ligands at Mo and a hydrogen-bonded network including His387 are crucial for positioning, deprotonation, and oxidation of formate during the reaction cycle of RcFDH.

Formate dehydrogenase (FDH) enzymes are versatile catalysts for CO2 conversion. The FDH from Rhodobacter capsulatus contains a molybdenum cofactor with the dithiolene functions of two pyranopterin guanine dinucleotide molecules, a conserved cysteine, and a sulfido group bound at Mo­(VI). In this study, we focused on metal oxidation state and coordination changes in response to exposure to O2, inhibitory anions, and redox agents using X-ray absorption spectroscopy (XAS) at the Mo K-edge. Differences in the oxidative modification of the bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor relative to samples prepared aerobically without inhibitor, such as variations in the relative numbers of sulfido (MoS) and oxo (MoO) bonds, were observed in the presence of azide (N3 –) or cyanate (OCN–). Azide provided best protection against O2, resulting in a quantitatively sulfurated cofactor with a displaced cysteine ligand and optimized formate oxidation activity. Replacement of the cysteine ligand by a formate (HCO2 –) ligand at the molybdenum in active enzyme is compatible with our XAS data. Cyanide (CN–) inactivated the enzyme by replacing the sulfido ligand at Mo­(VI) with an oxo ligand. Evidence that the sulfido group may become protonated upon molybdenum reduction was obtained. Our results emphasize the role of coordination flexibility at the molybdenum center during inhibitory and catalytic processes of FDH enzymes.

A cobalamin (Cbl) cofactor in corrinoid iron-sulfur protein (CoFeSP) is the primary methyl group donor and acceptor in biological carbon oxide conversion along the reductive acetyl-CoA pathway. Changes of the axial coordination of the cobalt ion within the corrin macrocycle upon redox transitions in aqua-, methyl-, and cyano-Cbl bound to CoFeSP or in solution were studied using X-ray absorption spectroscopy (XAS) at the Co K-edge in combination with density functional theory (DFT) calculations, supported by metal content and cobalt redox level quantification with further spectroscopic methods. Calculation of the highly variable pre-edge X-ray absorption features due to core-to-valence (ctv) electronic transitions, XANES shape analysis, and cobalt-ligand bond lengths determination from EXAFS has yielded models for the molecular and electronic structures of the cobalt sites. This suggested the absence of a ligand at cobalt in CoFeSP in α-position where the dimethylbenzimidazole (dmb) base of the cofactor is bound in Cbl in solution. As main species, (dmb)CoIII(OH2), (dmb)CoII(OH2), and (dmb)CoIII(CH3) sites for solution Cbl and CoIII(OH2), CoII(OH2), and CoIII(CH3) sites in CoFeSP-Cbl were identified. Our data support binding of a serine residue from the reductive-activator protein (RACo) of CoFeSP to the cobalt ion in the CoFeSP-RACo protein complex that stabilizes Co(II). The absence of an α-ligand at cobalt not only tunes the redox potential of the cobalamin cofactor into the physiological range, but is also important for CoFeSP reactivation.

Annotated hemoglobin genes in Chlamydomonas reinhardtii form functional globins, despite unusual architectures. Spectral characteristics show subtle biochemical differences. Multiple globins might help the alga to cope with its versatile environment. The unicellular green alga C. reinhardtii is a photosynthetic, often soil-dwelling organism, subjected to a changeable environment in nature. The alga contains 12 genes encoding so-called truncated hemoglobins that feature a two-on-two helical fold instead of the three-on-three helix arrangement of the long-studied vertebrate globins or plant symbiotic and non-symbiotic hemoglobins. In plants, non-symbiotic hemoglobins often play a role in acclimation to stress, and we could show recently that one of the C. reinhardtii globin genes is vital for anoxic growth. Here, three further globin encoding transcripts (Cre16.g661000.t1.1, Cre16.g661300.t2.1 and Cre16.g662750.t1.2) were heterologously expressed along with the recently studied THB1. UV-Vis and X-ray absorption spectroscopy analyses show that the sequences indeed encode functional hemoglobins, despite their uncommon primary sequences, which include long C-termini without any predictable function, or a split heme-binding domain. The proteins show some variations regarding the coordination of the heme iron or the interaction with diatomic ligands, indicating different functionalities. The respective transcripts are not responsive to the nitrogen source, in contrast to results reported for THB1, but they accumulate in darkness. This work advances experimental data on the very large globin family in general, and, more specifically, on hemoglobins in photosynthetic organisms.

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The oxidoreductase YdhV in Escherichia coli has been predicted to belong to the family of molybdenum/tungsten cofactor (Moco/Wco) containing enzymes. In this study, we characterized the YdhV protein in detail, which shares amino acid sequence homologies to a tungsten-containing benzoyl-CoA reductase binding the bis-W-MPT (for metal-binding pterin) cofactor. Our studies showed that YdhV has a preference for molybdenum over tungsten as metal to be inserted into the MPT backbone. The cofactor was identified to be of a bis-Mo-MPT type, which represents a novel form of Moco that has not been found earlier in any molybdoenzyme. In-depth characterization of YdhV by X-ray absorption and EPR spectroscopy revealed that the bis-Mo-MPT cofactor in YdhV is redox active, while a bis-W-MPT cofactor is redox inactive. The bis-Mo-MPT and bis-W-MPT cofactors include metal sites with the metal centers binding the four sulfurs from the two dithiolene groups in addition to a cysteine and likely a sulfido ligand. The unexpected presence of a bis-Mo-MPT cofactor opens an additional route for cofactor biosynthesis in E. coli and expands the canon of the structurally highly versatile molybdenum and tungsten cofactors. 10). Accordingly, E. coli is able to assemble not only molybdenum-, but also tungstencontaining cofactors. The presence of a Mo/W containing bis-MPT cofactor in E. coli opens a new route of Moco biosynthesis and expands the canon of the versatile molybdenum and tungsten enzymes. AUTHOR INFORMATION Corresponding Authors