CYTOCHROME CD(1) STRUCTURE - UNUSUAL HEME ENVIRONMENTS IN A NITRITE REDUCTASE AND ANALYSIS OF FACTORS CONTRIBUTING TO BETA-PROPELLER FOLDS

The central tunnel of the eight-bladed beta-propeller domain of cytoch rome cd(1) (nitrite reductase) is seen, from a 1.28 Angstrom, resoluti on structure, to contain hydrogen donors and accepters that are satisf ied by interaction either with water or the d(1) haem. The d(1) haem, although bound by an extensive network of hydrogen bonds, is not disto rted in its binding pocket and is confirmed to have exactly the dioxoi sobacteriochlorin structure proposed from chemical studies. A biologic al rationale is advanced for the undistorted structure of the d(1) hae m and the large number of hydrogen bonds it makes. The beta-propeller domain can be closely superimposed on that of methanol dehydrogenase d espite the enzymes sharing no common sequence motifs and using a diffe rent set of interactions to ''Velcro'' close the propeller. The sequen ce and likely structural relationships between cytochrome cd(1) or met hanol dehydrogenase and other predicted eight-bladed beta-propeller do mains in proteins, such as the pyrolloquinoline quinone-dependent alco hol dehydrogenase, are discussed and compared with other propeller pro teins. From sequencing the nirS gene of Thiosphaera pantotropha, it is established that the amino acid sequence deduced previously in part f rom X-ray diffraction data at lower resolution was largely correct, as was the proposal that eight N-terminal amino acid residues were not s een in the structure. The unusual haem iron environments in both the c -type cytochrome domain, with His/His coordination, and the d(1)-type cytochrome domain with Tyr/His coordination are related to the functio ns of the redox centres. (C) 1997 Academic Press Limited.