Fig. 6.
CH3-H4folate and its interactions with conserved residues from the Fol barrel. The extended arrangement of the folate is determined by the interactions of the p-aminobenzoic acid (PABA) side chain: the ring stacks with Glu-321, N10 makes a through-water interaction with the conserved Asn-323, and Arg-516 binds the PABA carbonyl oxygen. In the saturated pyrazine ring of the pterin, the C6 side chain substituent is axial and the N5-methyl group is equatorial, pointing toward the reader. Hydrogen bonds are indicated for donor-acceptor pairs that are closer than 3.2 Å. The side chain amide of Asn-508 may reorient in the ternary complex to allow the oxygen to interact with a protonated pterin N5 (see text). The viewpoint is approximately along the expected approach of the corrin ring.