Characterization of the Hydrogen-Deuterium Exchange Activities of the Energy-Transducing HupSL Hydrogenase and H2-Signaling HupUV Hydrogenase in Rhodobacter capsulatus

摘要

Rhodobacter capsulatus synthesizes two homologous protein complexes capable of activating molecular H2, a membrane-bound [NiFe] hydrogenase (HupSL) linked to the respiratory chain, and an H2 sensor encoded by the hupUV genes. The activities of hydrogen-deuterium (H-D) exchange catalyzed by the hupSL-encoded and the hupUV-encoded enzymes in the presence of D2 and H2O were studied comparatively. Whereas HupSL is in the membranes, HupUV activity was localized in the soluble cytoplasmic fraction. Since the hydrogenase gene cluster of R. capsulatus contains a gene homologous to hoxH, which encodes the large subunit of NAD-linked tetrameric soluble hydrogenases, the chromosomal hoxH gene was inactivated and hoxH mutants were used to demonstrate the H-D exchange activity of the cytoplasmic HupUV protein complex. The H-D exchange reaction catalyzed by HupSL hydrogenase was maximal at pH 4.5 and inhibited by acetylene and oxygen, whereas the H-D exchange catalyzed by the HupUV protein complex was insensitive to acetylene and oxygen and did not vary significantly between pH 4 and pH 11. Based on these properties, the product of the accessory hypD gene was shown to be necessary for the synthesis of active HupUV enzyme. The kinetics of HD and H2 formed in exchange with D2 by HupUV point to a restricted access of protons and gasses to the active site. Measurement of concentration changes in D2, HD, and H2 by mass spectrometry showed that, besides the H-D exchange reaction, HupUV oxidized H2 with benzyl viologen, produced H2 with reduced methyl viologen, and demonstrated true hydrogenase activity. Therefore, not only with respect to its H2 signaling function in the cell, but also to its catalytic properties, the HupUV enzyme represents a distinct class of hydrogenases.