Figure 1.

Minimal mechanism of catalysis for coenzyme B₁₂-dependent ethanolamine ammonialyase (EAL) and structures of substrate, intermediates and product.⁸,⁹ The forward direction of reaction is indicated by arrows. The steps are as follows: (1) radical pair separation, (2) first hydrogen atom transfer (HT1), (3) radical rearrangement, (4) second hydrogen atom transfer (HT2), (5) radical pair recombination and (6) product release/substrate binding. Substrate-derived species are designated S-H (bound substrate), S^• (substrate radical), P^• (product radical; brackets indicate proposed structure), and PH (diamagnetic products). The brackets around the product radical indicate that the structure of this intermediate is not known, although the carbinolamine is favored. The 5′-deoxyadenosyl β-axial ligand is represented as Ad-CH₂- in the intact coenzyme, and as Ad-CH₂^• (5′-deoxyadenosyl radical) or Ad-CH₃ (5′-deoxyadenosine) following cobalt-carbon bond cleavage. The cobalt ion and its formal oxidation states are depicted, but the corrin ring and the dimethylbenzimidazole α-axial ligand of the coenzyme ⁴³,⁴⁴ are not shown for clarity.