Investigations on [FeFe]-hydrogenase active site biosynthesis

Abstract

Hydrogenase enzymes, which catalyze the reversible oxidation of molecular hydrogen, occupy important roles as catalysts in microbial energy transfer and conservation. This seemingly simple reaction between protons and electrons necessitates the utilization of some of nature's most complicated organo-metallic cofactors. Remarkably, two evolutionarily independent types of enzymes capable of catalyzing this reaction exist - termed the [NiFe] and [FeFe]-hydrogenases. The biosynthesis of the cofactors harbored by these enzymes poses questions as to the assembly pathways involved in constructing hydrogen competent catalysts, and herein research as to the biosynthesis of the [FeFe]-hydrogenase active site is presented. Data as to the protein components involved in this process are presented which include the development of an E.coli based expression system for hydrogenase maturation protein factors, their isolation, and the first functional assignment of two of these proteins. The HydF protein is shown to be operative as an H-cluster intermediate bearing scaffold for [FeFe]-hydrogenase active site assembly, and the HydG protein is demonstrated to be responsible for the formation of cyanide and carbon monoxide from tyrosine. In addition, observations of a novel radical SAM enzyme is reported in conjunction with its putative involvement in the biosynthesis of the Hmd-hydrogenase found in methanogens. Together, these observations contribute to understanding biology's ability to construct complex organo-metallic cofactors, and lay a foundation for the consideration of the evolutionary events that led to the biological ability to assemble complex metallocofactors.