page_title: Phosphoribosylglycinamide formyltransferase
section_title: Mechanism of purN GARTfase
hierarchical_section_title: Phosphoribosylglycinamide formyltransferase / Mechanism / Mechanism of purN GARTfase
caption_attribution_description: English: GAR transformylase mechanism suggested by Qiao et al
context_page_description: Phosphoribosylglycinamide formyltransferase is an enzyme with systematic name 10-formyltetrahydrofolate:5'-phosphoribosylglycinamide N-formyltransferase. This enzyme catalyses the following chemical reaction
  10-formyltetrahydrofolate + N¹-glycinamide tetrahydrofolate + N²-formyl-N¹-glycinamide
  This THF dependent enzyme catalyzes a nucleophilic acyl substitution of the formyl group from 10-formyltetrahydrofolate to N¹-glycinamide to form N²-formyl-N¹-glycinamide as shown above. This reaction plays an important role in the formation of purine through the de novo purine biosynthesis pathway. This pathway creates inosine monophosphate, a precursor to adenosine monophosphate and guanosine monophosphate. AMP is a building block for important energy carriers such as ATP, NAD⁺ and FAD, and signaling molecules such as cAMP. GARTfase's role in de novo purine biosynthesis makes it a target for anti-cancer drugs and its overexpression during postnatal development has been connected to Down syndrome. There are two known types of genes encoding GAR transformylase in E.coli: purN and purT, while only purN is found in humans.
context_section_description: 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.
caption: GAR transformylase water assisted mechanism suggested by Qiao et al
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