Structure and function of the hybrid cluster protein
Wilfred R. Hagen
Abstract
A hybrid cluster (HC) is a 4Fe cluster with both sulfur and oxygen bridges. Hybrid cluster proteins (Hcps) contain two 4Fe clusters, a one electron transferring iron-sulfur cluster and a hybrid cluster. The structural gene, hcp, is diffusely found in bacteria, archaea, and monocellular eukarya, and the HC binding motif involving amino acids H, E, C, C, C, C, E, (K) appears to be fully conserved. HC is the active site of the enzyme Hcp. Of several reported Hcp enzymatic activities the conversion of nitric oxide into nitrous oxide, NO reductase, has been established as physiologically relevant. Other activities, notably signal transduction by NO transfer to other proteins, are controversial. The HC undergoes a complex structural change associated with single-electron iron based redox chemistry as well as electron-pair redox chemistry of a persulfidocysteine sulfur atom. A mechanistic scheme is proposed for the HC encompassing its structural, magnetic, and enzymatic properties.