Roentgen-Struktur-Analyse von zwei loeslichen Proteinen mit Kofaktor: D-Aminosaeure Oxidase aus der Hefe Rhodotorula gracilis und dem Neun Haem Cytochrome c aus sulfatereduzierenden Bakterien Desulfovibrio desulfuricans Essex 6

Abstract

The flavin ring-system constitutes one of the most versatile redox cofactors in nature and is used by many enzymes to perform a multitude of chemical reactions. D-amino acid oxidase (DAAO), a member of the oxidase family, is regarded as a key enzyme for the understanding of the mechanism underlying flavin catalysis. The ultra-high resolution structure of yeast DAAO in complex with substrates allows the unambiguous identification of hydride transfer as the dehydrogenation mechanism. The hydride transfer mechanism proceeds without involvement of functional groups and points to orbital orientation as the major factor in catalysis. The results presented here are of general relevance for the mechanisms of flavoprotein oxidases and dehydrogenases and provide a unifying concept for flavin catalysis. The X-ray structure of nine heme cytochrome c from the sulfate reducing bacteria Desulfovibrio desulfuricans strain Essex 6 was solved by the multiple wavelength anomalous dispersion (MAD) phasing method. Nine heme cytochrome consists of two tetraheme cytochrome c3-like domains with an equivalent four heme arrangement. The two domains at the N- and C-terminus are connected by a ninth heme buried under the protein surface. It is held in position by loop extensions in the flanking domains. Detailed analysis of the three- dimensional structure allows to characterize their specialization in evolution for interacting in the most productive manner with their reaction partners. A positive charged patch at the surface of the C-terminal domain with a heme cofactor in its center is an optimal acceptor for electrons originating from hydrogenase. Two predominant loop extensions not found in tetraheme cytochrome c3 s in the N-terminal domain might enhance the contact to the membrane bound complex. This high molecular mass complex (hmc) is located in the cytoplasmic membrane and responsible for the transport of electrons from the periplasm to the