Fig. 5: Schematic of the proposed poly-γ-glutamylation mechanism in CofE and its homologue FbiB.
Isoalloxazine and primary glutamate binding sites are depicted as bold, dotted lines, with glutamate or glutamyl residues colored green or blue by their order of addition. In STEP 1, F420-0 effects nucleophilic attack on the GTP γ-phosphate to form an acyl intermediate. The phosphate abstracts a proton from the free glutamate amino group and is eliminated from GTP. STEP 2 shows an additional proton abstraction, nucleophilic attack of the glutamate on the acyl phosphate, and the consequent and concerted loss of phosphate as H2PO4−. STEP 3 depicts the crystal structure model of F420-1 binding. STEP 4 and STEP 5 show a second cycle of phosphorylation/activation, proton abstraction, nucleophilic attack, and elimination of H2PO4–. STEP 6 depicts the crystal structure model of F420-2. STEPS 4–6 highlight the bulge in the growing poly-glutamyl chain. The schematic has been simplified for clarity and multiple binding, bond making, and bond breaking events have been combined – this does not imply a specific order of binding nor concerted reactions.
