Crystal Structure of the Vitamin B12 Biosynthetic Cobaltochelatase, CbiXS, from Archaeoglobus Fulgidus
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The Archaeoglobus fulgidus gene af0721 encodes CbiXS, a small cobaltochelatase associated with the anaerobic biosynthesis of vitamin B12 (cobalamin). The protein was shown to have activity both in vivo and in vitro, catalyzing the insertion of Co2+ into sirohydrochlorin. The structure of CbiXS was determined in two different crystal forms and was shown to consist of a central mixed β-sheet flanked by four α-helices, one of which originates in the C-terminus of a neighboring molecule. CbiXS is about half the size of other Class II tetrapyrrole chelatases. The overall topography of CbiXS exhibits substantial resemblance to both the N- and C-terminal regions of several members of the Class II metal chelatases involved in tetrapyrrole biosynthesis. Two histidines (His10 and His74), are in similar positions as the catalytic histidine residues in the anaerobic cobaltochelatase CbiK (His145 and His207). In light of the hypothesis that suggests the larger chelatases evolved via gene duplication and fusion from a CbiXS-like enzyme, the structure of AF0721 may represent that of an “ancestral” precursor of class II metal chelatases.
KeywordsCobalamin (vitamin B12) biosynthesis Tetrapyrrole CbiK CbiX Cobaltochelatase Protein structure evolution
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Use of the Argonne National Laboratory Structural Biology Center beamlines (Beamline 19BM) at the Advanced Photon Source was supported by the US Department of Energy, Office of Energy Research, under Contract No. W-31-109-ENG-38. We wish to thank all members of the SBC at ANL for their help in conducting experiments. This work was supported by National Institutes of Health Grants GM62414-01, the Ontario Research and Development Challenge Fund, grants from the Canadian Institutes of Health Research (CIHR), and the Biotechnology and Biological Sciences Research Council (BBSRC). X-ray diffraction data for crystal form II were collected at the beamline 8.3.1 of the Advanced Light Source (ALS) at Lawrence Berkeley Lab, under an agreement with the Alberta Synchrotron Institute (ASI). The ALS is operated by the Department of Energy and supported by the National Institute of Health. Beamline 8.3.1 is funded by the National Science Foundation, the University of California and Henry Wheeler. The ASI synchrotron access program is supported by grants from the Alberta Science and Research Authority (ASRA), the Alberta Heritage Foundation for Medical Research (AHFMR) and Western Economic Diversification (WED) of the Canadian Government. The authors would like to thank Drs. Ernst Bergmann (ASI), Jonathan Parish (ASI), and James Holton (ALS) for their help in data collection. MNGJ holds a Canada Research Chair in Protein Structure and Function. JY is a recipient of the Izaak Walton Killam Memorial postdoctoral fellowship at the University of Alberta.