The Fe-Only Hydrogenase

JW. Peters, WN. Lanzilotta, BJ. Lemon, LC. eefeldt (1998) X-ray crystal structure of the Fe-only hydrogenase (CpI) from Clostridium pasteurianum to 1.8 Angstrom resolution. Science, 282 1853-1858... 

Figure 3.12 Active site of a reduced form of the Fe-only hydrogenase from Desulphovibrio desul-phuricans. The Fe atom on the right is defined as the proximal Fe (relative to the neighbouring [Fe-S] cluster), Fep the Fe atom on the left is defined as the distal Fe, FeD. The arrow indicates the potential hydron-binding site on FeD that is occupied by either HzO or an extrinsic CO in the structure of Cp I. Also shown is a close contact between the bridgehead atom X of the exogenous dithiolate ligand and the S atom of cysteine-178. (Reprinted with permission from Parkin et al., 2006. Copyright (2005) American Chemical Society.)...

Figure 2.5 Evolutionary relationships amongst the Fe-only hydrogenases.Amino acid sequences of large subunits of Fe hydrogenases from Archaea, Eubacteria and Eukaryotes were used to construct the tree, as described for Fig. 2.3. 

The IR spectrum of the compound in the solid state reveals strong absorptions for the carbonyl ligands at 2,011 cm and l,957cm well within the range as is observed for the [Fe]-only hydrogenases. 

Bennett B, Lemon BJ, Peters JW. 2000. Reversible carbon monoxide binding and inhibition at the active site of the Fe-only hydrogenase. Biochemistry 39 7455-60. 

Ma Y, Balbuena PB (2007) Density functional theory approach for improving the catalytic activity of a biomimetic model based on the Fe-only hydrogenase active site. J Electroanal Chem (in press)... 

Nicolet Y, de Lacey AL, Vernede X, Fernandez VM, Hatchikian EC, Fontecilla-Camps JC. Crystallographic and FTIR spectroscopic evidence of changes in Fe coordination upon reduction of the active site of the Fe-only hydrogenase from Desulfovibrio desulfur-icans. J Am Chem Soc. 2001 123(8) 1596-601. 

FIGURE 4. The active site of hydrogenase. a. Structure of the [NiFe] cluster, b. Structure of the H cluster of the Fe-only hydrogenase. From (Nicolet et al., 1999). 

FIGURE 8. Proposed proton transfer pathway in the Fe-only hydrogenase. From (Nicolet... 

The crystal structures of the Fe-only hydrogenases from the bacteria Clostridium pasteurianum and Desulfovibrio desulficans have been determined. Although the major features of the active site have been elucidated, some... 

The Fe-only hydrogenases contain only Fe-S clusters, with the active site being a special Fe-S cluster called the H-cluster. The structure of this cluster has over many years been the topic of much speculation. At last this speculation has been resolved by crystallography. 

Figure 7. Proposed mechanism [32] for the action of the Fe-only hydrogenase.

This is also a feature of the mechanism in Figure 6, whereas in the mechanisms of Figures 4 and 5 cysteinate and cyano ligands are merely spectators. The DFT calculations on the Fe-only hydrogenase show that dihydrogen binds weakly to the iron remote from the Fe4S4 cluster. When two electron reduced, a mechanistically significant barrierless transfer of one H atom from Fe-Hj to form S-H occurs. 

We acknowledge financial support from the National Science Foundation (CHE-0111629) and contributions from the R. A. Welch Foundation. We would also like to thank two former group members, Drs Erica J. Lyon and Xuan Zhao, for their contributions to the model studies of the Fe-only hydrogenase active site. 

Figure 21 Hypothetical dihydrogen adduct structure at the active site of the Fe-only hydrogenase enzyme.

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