Roles for Cationic Residues at the Quinolinic Acid Binding Site of Quinolinate Phosphoribosyltransferase

Quinolinic acid phosphoribosyltransferase (QAPRTase, EC 2.4.2.19) forms nicotinate mononucleotide (NAMN) from quinolinic acid (QA) and 5-phosphoribosyl 1-pyrophosphate (PRPP). Previously determined crystal structures of QAPRTase·QA and QAPRTase·PA·PRPP complexes show positively charged residues (Arg118, Arg152, Arg175, Lys185, and His188) lining the QA binding site. To assess the roles of these residues in the Salmonella typhimurium QAPRTase reaction, they were individually mutated to alanine and the recombinant proteins overexpressed and purified from a recombineered Escherichia coli strain that lacks the QAPRTase gene. Gel filtration indicated that the mutations did not affect the dimeric aggregation state of the enzymes. Arg175 is critical for the QAPRTase reaction, and its mutation to alanine produced an inactive enzyme. The k cat values for R152A and K185A were reduced by 33-fold and 625-fold, and binding affinity of PRPP and QA to the enzymes decreased. R152A and K185A mutants displayed 116-fold and 83-fold increases in activity toward the normally inactive QA analogue, nicotinic acid (NA), indicating roles for these residues in defining the substrate specificity of QAPRTase. Moreover, K185A QAPRTase displayed a 300-fold higher k cat/K m for NA over the natural substrate QA. Pre-steady-state analysis of K185A with QA revealed a burst of nucleotide formation followed by a slower steady-state rate, unlike the linear kinetics of WT. Intriguingly, pre-steady-state analysis of K185A with NA produced a rapid but linear rate for NAMN formation. The result implies a critical role for Lys185 in the chemistry of the QAPRTase intermediate. Arg118 is an essential residue that reaches across the dimer interface. Mutation of Arg118 to alanine resulted in 5000-fold decrease in k cat value and a decrease in the binding affinity of QA and PRPP to R152A. Equimolar mixtures of R118A with inactive or virtually inactive mutants produced approximately 50% of the enzymatic activity of WT, establishing an interfacial role for Arg118 during catalysis.