Figure 2. The a-f nomenclature used to describe octahedral binding partners.

Inverting enzymes such as GalT1 (top) achieve nearly perfect octahedral geometry about the coordinated metal ion (displayed angles of 81° and 91° compared to ideal octahedral 90° bond angles) with subsequent “inline” (approaching 180°) placement of the acceptor nucleophile for classic inverting S_N2 backside attack. Retaining enzymes such as GTA (bottom), however, use an arrangement of acidic residues, often with acute bidentate Asp coordination, which severely skews metal geometry (displayed angles of 54° and 115°) and allots sufficient room between phosphate oxygens for orthogonal attack from the acceptor. U is uridine, C1 is donor galactose C1.