Anthranilate phosphoribosyltransferase: Binding determinants for 5'-phospho-alpha-d-ribosyl-1'-pyrophosphate (PRPP)...

Phosphoribosyltransferases (PRTs) bind 5'-phospho-α-d-ribosyl-1'-pyrophosphate (PRPP) and transfer its phosphoribosyl group (PRib) to specific nucleophiles. Anthranilate PRT (AnPRT) is a promiscuous PRT that can phosphoribosylate both anthranilate and alternative substrates, and is the only example of a type III PRT. Comparison of the PRPP binding mode in type I, II and III PRTs indicates that AnPRT does not bind PRPP, or nearby metals, in the same conformation as other PRTs. A structure with a stereoisomer of PRPP bound to AnPRT from Mycobacterium tuberculosis (Mtb) suggests a catalytic or post-catalytic state that links PRib movement to metal movement. Crystal structures of Mtb-AnPRT in complex with PRPP and with varying occupancies of the two metal binding sites, complemented by activity assay data, indicate that this type III PRT binds a single metal-coordinated species of PRPP, while an adjacent second metal site can be occupied due to a separate binding event. A series of compounds were synthesized that included a phosphonate group to probe PRPP binding site. Compounds containing a "bianthranilate"-like moiety are inhibitors with IC₅₀ values of 10-60μM, and K_i values of 1.3-15μM. Structures of Mtb-AnPRT in complex with these compounds indicate that their phosphonate moieties are unable to mimic the binding modes of the PRib or pyrophosphate moieties of PRPP. The AnPRT structures presented herein indicated that PRPP binds a surface cleft and becomes enclosed due to re-positioning of two mobile loops.