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The xanthine oxidase (XO) family comprises molybdenum-dependent enzymes that usually form homodimers (or dimers of heterodimers/trimers) organized in three domains that harbor two [2Fe-2S] clusters, one FAD, and a Mo cofactor. In this work, we crystallized an unusual member of the family, the periplasmic aldehyde oxidoreductase PaoABC from Escherichia coli. This is the first example of an E. coli

The class of [NiFe]-hydrogenases comprises oxygen-sensitive periplasmic (PH) and oxygen-tolerant membrane-bound (MBH) enzymes. For three PHs and four MBHs from six bacterial species, structural features of the nickel-iron active site of hydrogen turnover and of the iron-sulfur clusters functioning in electron transfer were determined using X-ray absorption spectroscopy (XAS). Fe-XAS indicated surp

[FeFe]-hydrogenase from green algae (HydA1) is the most efficient hydrogen (H2) producing enzyme in nature and of prime interest for (bio)technology. Its active site is a unique six-iron center (H-cluster) composed of a cubane cluster, [4Fe4S]H, cysteine-linked to a diiron unit, [2Fe]H, which carries unusual carbon monoxide (CO) and cyanide ligands and a bridging azadithiolate group. We have probe

The [FeFe]-hydrogenase (HydA1) from green algae is the minimal enzyme for efficient biological hydrogen (H2) production. Its active-site six-iron center (H-cluster) consists of a cubane, [4Fe4S]H, cysteine-linked to a diiron site, [2Fe]H. We utilized the spin-polarization of the iron Kβ X-ray fluorescence emission to perform site-selective X-ray absorption experiments for spectral discrimination o

Sulfite oxidase (SO) is an essential molybdoenzyme for humans, catalyzing the final step in the degradation of sulfur-containing amino acids and lipids, which is the oxidation of sulfite to sulfate. The catalytic site of SO consists of a molybdenum ion bound to the dithiolene sulfurs of one molybdopterin (MPT) molecule, carrying two oxygen ligands, and is further coordinated by the thiol sulfur of

The molybdenum cofactor is an important cofactor, and its biosynthesis is essential for many organisms, including humans. Its basic form comprises a single molybdopterin (MPT) unit, which binds a molybdenum ion bearing three oxygen ligands via a dithiolene function, thus forming Mo-MPT. In bacteria, this form is modified to form the bis-MPT guanine dinucleotide cofactor with two MPT units coordina

Two crystallized [FeFe] hydrogenase model complexes, 1 = (μ-pdt)[Fe(CO)2(PMe3)]2 (pdt = SC1H 2C2H2C3H2S), and their bridging-hydride (Hy) derivative, [1Hy]+ = [(μ-H)(μ-pdt)[Fe(CO)2 (PMe 3)]2]+ (BF4-), were studied by Fe K-edge X-ray absorption and emission spectroscopy, supported by density functional theory. Structural changes in [1Hy]+ compared to 1 involved small bond elongations (<0.03 Å) and

High-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection, supported by density functional theory calculations (XAES-DFT), was used to study a model complex, ([Fe2(μ-adt)(CO) 4(PMe3)2] (1, adt = S-CH2-(NCH 2Ph)-CH2-S), of the [FeFe] hydrogenase active site. For 1 in powder material (1powder), in MeCN solution (1′), and in its three protonated states (1H, 1Hy, 1HHy; H

The active site for hydrogen production in [FeFe] hydrogenase comprises a diiron unit. Bioinorganic chemistry has modeled important features of this center, aiming at mechanistic understanding and the development of novel catalysts. However, new assays are required for analyzing the effects of ligand variations at the metal ions. By high-resolution X-ray absorption spectroscopy with narrow-band X-

Cryogenic illumination of Photosystem II (PSII) can lead to the trapping of the metastable radical Y Z •, the radical form of the redox-active tyrosine residue D1-Tyr161 (known as Y Z). Magnetic interaction between this radical and the CaMn 4 cluster of PSII gives rise to so-called split electron paramagnetic resonance (EPR) signals with characteristics that are dependent on the S state. We report

Ribonucleotide reductases (RNRs) are essential for DNA synthesis in most organisms. In class-Ic RNR from Chlamydia trachomatis (Ct), a MnFe cofactor in subunit R2 forms the site required for enzyme activity, instead of an FeFe cofactor plus a redox-active tyrosine in class-Ia RNRs, for example in mouse (Mus musculus, Mm). For R2 proteins from Ct and Mm, either grown in the presence of, or reconsti

Irreversible inhibition by molecular oxygen (O 2) complicates the use of [FeFe]-hydrogenases (HydA) for biotechnological hydrogen (H 2) production. Modification by O 2 of the active site six-iron complex denoted as the H-cluster ([4Fe4S]-2Fe H) of HydA1 from the green alga Chlamydomonas reinhardtii was characterized by x-ray absorption spectroscopy at the iron K-edge. In a time-resolved approach,

Ten square-based pyramidal molybdenum complexes with different sulfur donor ligands, that is, a variety of dithiolenes and sulfides, were prepared, which mimic coordination motifs of the molybdenum cofactors of molybdenum-dependent oxidoreductases. The model compounds were investigated by Mo K-edge X-ray absorption spectroscopy (XAS) and (with one exception) their molecular structures were analyze

YedY from Escherichia coli is a new member of the sulfite oxidase family of molybdenum cofactor (Moco)-containing oxidoreductases. We investigated the atomic structure of the molybdenum site in YedY by X-ray absorption spectroscopy, in comparison to human sulfite oxidase (hSO) and to a MoIV model complex. The K-edge energy was indicative of MoV in YedY, in agreement with X-and Q-band electron para

Metalloradical EPR signals have been found in intact Photosystem II at cryogenic temperatures. They reflect the light-driven formation of the tyrosine Z radical (YZ) in magnetic interaction with the CaMn4 cluster in a particular S state. These so-called split EPR signals, induced at cryogenic temperatures, provide means to study the otherwise transient YZ and to probe the S states with EPR spectro

The electrons extracted from the CaMn4 cluster during water oxidation in photosystem II are transferred to P680+ via the redox-active tyrosine D1-Tyr161 (YZ). Upon YZ oxidation a proton moves in a hydrogen bond toward D1-His190 (HisZ). The deprotonation and reprotonation mechanism of YZ-OH/Y Z-O is of key importance for the catalytic turnover of photosystem II. By light illumination at liquid heli

The redox-active tyrosine residue (YZ) plays a crucial role in the mechanism of the water oxidation. Metalloradical electron paramagnetic resonance (EPR) signals reflecting the light-induced YZ· in magnetic interaction with the CaMn4-cluster in the particular S-state, YZ·SX intermediates, have been found in intact photosystem II. These so-called split EPR signals are induced by illumination at cry

Detailed optical and EPR analyses of states induced in dark-adapted PS II membranes by cryogenic illumination permit characterization and quantification of all pigment derived donors and acceptors, as well as optically silent (in the visible, near infrared) species which are EPR active. Near complete turnover formation of QA- is seen in all centers, but with variable efficiency, depending on the d

EPR spectroscopy is very useful in studies of the oxygen evolving cycle in Photosystem II and EPR signals from the CaMn4 cluster are known in all S states except S4. Many signals are insufficiently understood and the S0, S1, and S3 states have not yet been quantifiable through their EPR signals. Recently, split EPR signals, induced by illumination at liquid helium temperatures, have been reported

The interaction EPR split signals from photosystem II (PSII) have been reported from the S0, S1, and S3 states. The signals are induced by illumination at cryogenic temperatures and are proposed to reflect the magnetic interaction between YZ• and the Mn 4Ca cluster. We have investigated the formation spectra of these split EPR signals induced in PSII enriched membranes at 5 K using monochromatic l