Crystal structure of Mycobacterium tuberculosis 6-hydroxymethyl-7,8-dihydropteroate synthase in complex with pterin monophosphate: New insight into the enzymatic mechanism and sulfa-drug action

The enzyme 6-hydroxymethyl-7,8-dihydropteroate synthase (DHPS) catalyzes th e condensation of para-aminobenzoic acid (pABA) with 6-hydroxymethyl-7,8-di hydropterin-pyrophosphate to form 6-hydroxymethyl-7,8-dihydropteroate and p yrophosphate. DHPS is essential for the de novo synthesis of folate in prok aryotes, lower eukaryotes, and in plants, but is absent in mammals. Inhibit ion of this enzyme's activity by sulfonamide and sulfone drugs depletes the folate pool, resulting in growth inhibition and cell death. Here, we repor t the 1.7 Angstrom resolution crystal structure of the binary complex of 6- hydroxymethylpterin monophosphate (PtP) with DHPS from Mycobacterium tuberc ulosis (Mtb), a pathogen responsible for the death of millions of human bei ngs each year. Comparison to other DHPS structures reveals that the M. tube rculosis DHPS structure is in a unique conformation in which loop 1 closes over the active site. The Mtb DHPS structure hints at a mechanism in which both loops 1 and 2 play important roles in catalysis by shielding the activ e site from bulk solvent and allowing pyrophosphoryl transfer to occur. A b inding mode for pABA, sulfonamides and sulfones is suggested based on: (i) the new conformation of the closed loop 1; (ii) the distribution of dapsone and sulfonamide resistance mutations; (iii) the observed direction of the bond between the 6-methyl carbon atom and the bridging oxygen atom to the a lpha-phosphate group in the Mtb DHPS:PtP binary complex; and (iv) the confo rmation of loop 2 in the Escherichia coli DHPS structure. Finally, the Mtb DHPS structure reveals a highly conserved pterin binding pocket that may be exploited for the design of novel antimycobacterial agents. (C) 2000 Acade mic Press.