Overall reaction involves transfer of phosphoethanolamine to the 1' or 4' phosphate group of lipid A. This likely occurs via a ping-pong mechanism, involving two distinct steps: (B) First step of the proposed ping-pong reaction in a mono-zinc active site involves transfer of phosphoethanolamine (red, from a phosphatidylethanolamine donor) to Thr285. (C) Second step (proposed to require a dizinc active site) involves transfer of phosphoethanolamine to lipid A, and reprotonation of Thr285. Adapted from Suardíaz et al .