ABSTRACT
All the mutases show an apparent lengthening o f the axial Co-N bond length to
summarized in [54]. The following Co-N bond lengths (in A) are observed in proteinfree Co corrinoids: cyano-Cbl 2.01, MeCbl 2.19, AdoCbl 2.21, Co(II)-Cbl 2.16, and imidazolyl-B12, in which the nucleotide DMB has been replaced by imidazole, 1.97; cf. also MeCbl in the MetS fragment 2.2(2). By contrast, all four closed conformations of MMCM with Co(II)-Cbl reveal Co-N bond lengths of 2.4 or 2.5(1) and the two open conformations with AdoCbl 2.6(3) and 2.7(2); cf. also GluM with CN-Cbl 2.27 and 2.30 and with MeCbl 2.35 [66] and DiolD with CN-Cbl 2.50 [69]. The lengthening would suggest a positioning of the base by the protein to facilitate a reduction in coordination number either from six-to five-coordinate AdoCbl or from five-to fourcoordinate Co(II). The first would promote formation of a strained five-coordinate form of AdoCbl, which probably occurs as an intermediate (Sec. 4.1.3); lengthening of the Co-N bond per se is unlikely to promote bond fission since all of the evidence indicates that removal of the base from alkyl-Cbrs increases their kinetic and ther modynamic stability toward homolytic fission [72,73]. The second would serve to stabilize the normally four-coordinate Co(I) and promote the partial electron trans fer from the substrate radical, which appears to be involved in the rearrangement step (Sec. 4.3).
