More O’Ferrall–Jencks plot for the transition states of phosphoribosyltransferases. PfHGXPRT shows an almost-symmetrical leaving group (pyrophosphate) and attacking nucleophile (hypoxanthine). The PfHGXPRT transition state is closer to the DN + AN corner than other phosphoribosyltransferase transition states. Pauling bond order used a proportionality constant of 0.6 when the bond order is low (>2.0 Å) and 0.3 when the bond is <2.0 Å.29 Organisms: Hs, Homo sapiens; Pf, P. falciparum; St, S. typhimurium. Enzymes: HGXPRT, hypoxanthine-guanine-xanthine phosphoribosyltransferase; NAMPT, nicotinamide phosphoribosyltransferase; OPRT, orotate phosphoribosyltransferase.