ATR FT-IR on gas-processing enzymes

We are investigating on the catalytic mechanism and biogenesis of gas-processing iron-sulfur proteins. Our research focus is on hydrogenases, ancient redox enzymes that catalyze uptake and release of molecular hydrogen, H2. Hydrogenases are ubiquitous in bacteria but have been found in yeast and algae as well. Depending on the composition of the bioinorganic transition metal cofactor, [NiFe]- and [FeFe]-hydrogenases are distinguished. Ligation of these bimetallic centers with carbon monoxide (CO) and cyanide (CN) makes hydrogenases an excellent target for IR studies. Our motivation is to understand the molecular mechanisms of hydrogen turnover.

Biogenesis of the [NiFe]-hydrogenase cofactor

The in-vivo synthesis of CO and CN demands tight enzymatic control. In E. coli, at least six accessory proteins are necessary to modify the iron ion of the [NiFe]-hydrogenase cofactor with one CO- and two CN-ligands. By means of ATR FT-IR we identified the Fe-(CN)2CO signature of the functional [NiFe]-hydrogenase cofactor on scaffold protein HypD. The CO2-binding activity of HypC, which is often found in complex with HypD, hints at CO2 as metabolic origin of CO. Isotope labeling revealed that CO is the first ligand to be attached to iron, that is, prior to CN. Careful analysis of the in-vitro synthesis of CN with HypE and HypF identified an isothiocyanate intermediate whose presence helps to explain the redox chemistry that drives in-vivo generation of the [NiFe]-hydrogenase cofactor.

The catalytic mechanisms of H2 uptake with [FeFe]-hydrogenases

[FeFe]-hydrogenases carry an active site cofactor that comprises a [4Fe4S]-cluster electronically coupled to a di-iron site. The later is modified with two CN and three CO ligands. Furthermore, a dimethylamine group bridges the sulfur atoms of the [2Fe2S]-site. Making use of ATR FT-IR we investigate on the effects of varying proton concentrations and gases like H2, O2, CO, and N2.

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