297 Hydrogenase maturation endopeptidase

This chapter elaborates the structural chemistry and the biological aspects of hydrogenase maturation endopeptidase. [NiFe]Hydrogenases consist of subunits of 30–35 kDa and of 60–65 kDa, depending on the organism. The large subunit carries the [NiFe]metallocenter, and it is synthesized in a precursor form and proteolytically processed to the form present in the mature enzyme. Hycl and HybD endopeptidases are monomeric proteins with molecular masses of 17 kDa and 17.5 kDa, respectively. There is no primary structure motifs characteristic of serine, cysteine or metalloproteases. Hycl lacks the N-terminal methionine. The crystal structure of HybD, which cleaves off 15 amino acid peptide from the C-terminus of the precursor of the large subunit of hydrogenase 2 from E. coli, has been solved at 2.2 Å resolutions. HybD crystallizes with a cadmium ion from the crystallization buffer whereby the metal is pentacoordinated by Glul6, Asp62 and His93, and a water molecule. These residues are strictly conserved amongst other members of the hydrogenase maturation endopeptidase family. Their replacement either abolishes processing activity completely or it impairs activity. The function of Hycl and HybD endopeptidases is the formation of active hydrogenase 3 and 2, respectively, in E. coli.