Ethanolamine Ammonia-Lyase Has a “Base-On” Binding Mode for Coenzyme B12

Ethanolamine ammonia-lyase (EAL, EC 4.3.1.7) catalyzes a coenzyme B12-dependent deamination of vicinal amino alcohols. The mode of binding of coenzyme B12 to EAL has been investigated by electron paramagnetic resonance spectroscopy (EPR) using [15N]-dimethylbenzimidazole–coenzyme B12. EAL was incubated with either unlabeled or 15N-enriched coenzyme B12 and then either exposed to light or treated with ethanol to generate the cleaved form of the cofactor, cob(II)alamin (B12r) bound in the active site. The reaction mixtures were examined by EPR spectroscopy at 77 K. 15N superhyperfine splitting in the EPR signals of the low-spin Co2+ of B12r, bound in the active site of EAL, indicates that the dimethylbenzimidazole moiety of the cofactor contributes the lower axial ligand consistent with “base-on” binding of coenzyme B12 to EAL.