Synthesis and application of a fluorescent substrate analogue to study ligand interactions for undecaprenyl pyrophosphate synthase

Farnesyl pyrophosphate (FPP) serves as a common substrate for many prenyltransferases involved in the biosynthesis of isoprenoid compounds. Undecaprenyl pyrophosphate synthase (UPPs) catalyzes the chain elongation of FPP to C-55 undecaprenyl pyrophosphate (UPP) which acts as a lipid carrier in bacterial peptidoglycan synthesis. In this study, 7-(2,6-dimethyl-8-diphospho-2,6-octadienyloxy)8-methyl-4-trifluoromethyl-chromen-2-one geranyl pyrophosphate, a fluorescent analogue of FPP, was prepared and utilized to study ligand interactions with E coli UPPs. This compound displays an absorbance maximum at 336 nm and emission maximum at 460 nm without interference from protein autofluorescence. It is a competitive inhibitor with respect to FPP (K-i = 0.57 uM) and also serves as an alternative substrate (K-m = 0.69 muM and k(cat) = 0.02 s(-1)), but mainly reacts with one isopentenyl pyrophosphate (IPP) probably due to unfavorable product translocation. Fluorescence intensity of this compound is reduced when bound to the enzyme (1:1 stoichiometry), and is recovered by FPP replacement. Using stopped-flow apparatus, the interaction of enzyme with the compound was measured (k(on) = 55.3 muM(-1) s(-1) and k(off) = 31.6 s(-1)). The product dissociation rate constant (0.5 s(-1)) determined from the competition experiments is consistent with our previous prediction from kinetic simulation. Unlike several other prenyltransferase reactions in which FPP dissociates slowly, UPPs binds FPP in a rapid equilibrium manner with a fast release rate constant of 30 s-1. The fluorescent analogue of FPP presented here may provide a tool to investigate the ligand interactions for a broad class of FPP-binding proteins.