New Frontiers in Hydrogenase Structure and Biosynthesis

The [NiFe]- and [FeFe]- hydrogenases have convergently evolved to efficiently catalyze the reversible oxidation of molecular H2. Extensive research efforts are currently aimed at using these enzymes to generate H2 for use as a renewable energy carrier, and at using these enzymes as a platinum substitute in fuel cells. Hydrogenases are found in taxonomically diverse microorganisms and function to either couple H2 oxidation to energy yielding processes or reduce protons as a mechanism to recycle reduced electron carriers that accumulate during fermentation. Microbial genome sequencing continues to demonstrate the ubiquitous occurrence and diversity of the hydrogenases. In recent years, significant strides have been made in elucidating the structure, catalytic properties, spectroscopic characteristics, and assembly of these intriguing enzymes. The [NiFe]- and [FeFe]- hydrogenases differ in active site composition and structural polypeptides but are unified by the presence of CN- and CO as ligands to the active site Fe atoms. These biologically unusual ligands are responsible for the unique electronic properties of the hydrogenase active site and are necessary to efficiently catalyze the reversible oxidation of H2. Although the active sites of the [NiFe]- and [FeFe]- enzymes contain similar ligands, the assembly proteins for each class of enzyme are unique. Since the term hydrogenase was first coined over seventy years ago, substantial progress has been made in characterizing several enzymes. These data, combined with recent genomics data, are used to compare the unique chemical properties of distinct hydrogenase enzymes, as well as the novel chemistries required for active site synthesis.