Hydrogen conversion in the presence of oxygen

Hydrogenase enzymes catalyse the reversible conversion of protons and electrons into molecular hydrogen (H2). Most of the microbial hydrogenase enzymes are very sensitive to oxygen (O2) and are irreversibly inhibited by it. A subclass of enzyme however, is O2-tolerant and retains its activity in the presence of oxygen. For some time it was thought that a particular narrow gas access channel to the active site was the reason for this tolerance or that modifications of the active site itself made the enzyme O2-tolerant. EXAFS investigations showed that the active site structures were identical between O2-sensitive and tolerant hydrogenases [1]. From a bioinformatics analysis, alterations in the vicinity of the proximal FeS cluster were suggested [2]. The FeS-cluster displayed coordination by two additional cysteine amino acid residues. By mutational experiments these additional cysteines were shown to alter the redox behaviour of the proximal FeS cluster and be responsible for the O2-tolerance [2]. Later, the crystal structure of the hydrogenase revealed a new type of FeS-cluster, a [4Fe-3S](6Cys) cluster with an unusual coordination geometry [3]. The electronic structure of this proximal FeS-cluster allowed an additional redox transition and protects the active site at 11 Å distance from oxidative damage. References 1 J. Fritsch, S. Löscher, O. Sanganas, E. Siebert, I. Zebger, M. Stein, M. Ludwig, A. L. de Lacey, H. Dau, B. Friedrich, O. Lenz, M. Haumann, Biochemistry, 2011, 50, 5858. 2 T. Goris, A.F. Wait, J. Fritsch, N. Heidary, M. Stein, I. Zebger, F. Lendzian, F.A. Armstrong, B. Friedrich, O. Lenz, Nature Chemical Biology, 7, 310. 3 J. Fritsch, P. Scheerer, S. Frielingsdorf, S. Kroschinsky, B. Friedrich, O. Lenz, C.M.T. Spahn, Nature 479, 249