Klein et al first suggested a water molecule assisted mechanism.
A single water molecule possibly held in place by hydrogen bonding with the carboxylate group of the persistent Asp144 residue transfers protons from the GAR-N to the THF-N. The nucleophilic nitrogen on the terminal amino group of GAR attacks the carbonyl carbon of the formyl group on THF pushing negative charge onto the oxygen.
Klein suggests that His108 stabilizes the transition state by hydrogen bonding with the negatively charged oxygen and that the reformation of the carbonyl double bond results in breaking the THF-N - formyl bond.
Calculations by Qiao et al suggest that the water assisted stepwise proton transfer from Gar-N to THF-N is 80-100 kj/mol more favorable than the concerted transfer suggested by Klein.
The mechanism shown is suggested by Qiao et al, whom admittedly did not consider surrounding residues in their calculations.
Much of the early active site mapping on GAR TFase was determined with the bacterial enzyme owing to the quantity available from its overexpression in E. coli.
 Using a bromoacetyl dideazafolate affinity analog James Inglese and colleagues first identified Asp144 as an active site residue likely involved in the formyl transfer mechanism.
Klein et al first suggested a water molecule assisted mechanism. A single water molecule possibly held in place by hydrogen bonding with the carboxylate group of the persistent Asp144 residue transfers protons from the GAR-N to the THF-N. The nucleophilic nitrogen on the terminal amino group of GAR attacks the carbonyl carbon of the formyl group on THF pushing negative charge onto the oxygen. Klein suggests that His108 stabilizes the transition state by hydrogen bonding with the negatively charged oxygen and that the reformation of the carbonyl double bond results in breaking the THF-N - formyl bond. Calculations by Qiao et al suggest that the water assisted stepwise proton transfer from Gar-N to THF-N is 80-100 kj/mol more favorable than the concerted transfer suggested by Klein. The mechanism shown is suggested by Qiao et al, whom admittedly did not consider surrounding residues in their calculations. Much of the early active site mapping on GAR TFase was determined with the bacterial enzyme owing to the quantity available from its overexpression in E. coli.  Using a bromoacetyl dideazafolate affinity analog James Inglese and colleagues first identified Asp144 as an active site residue likely involved in the formyl transfer mechanism.
Phosphoribosylglycinamide formyltransferase