Figure 1.

(A) E. coli DHFR kinetic cycle adapted from Fierke et al.²¹ Closed states of the enzyme are highlighted in blue, occluded states in red. An extra step (within the hatched box) has been added between the Michaelis complex (E:DHF:NADPH) and the first product complex (E:THF:NADP⁺) to represent relaxation of the closed excited state of the product ternary complex, formed immediately following hydride transfer, into its occluded ground state conformation.⁶¹ (B) Closed conformation of the Met20 loop (blue) with bound ligands, the nicotinamide moiety occupies the active site. (C) Occluded conformation of the Met20 loop (red) with bound ligands, the nicotinamide moiety hangs outside of the active site.