Inhibition studies of porphobilinogen synthase from Escherichia coli differentiating between the two recognition sites

Porphobilinogen synthase condenses two molecules of 5-amino-levulinate in a n asymmetric way. This unusual transformation requires a selective recognit ion and differentiation between the :substrates ending up in the A site or in the P site of porphobilinogen synthase. Studies of inhibitors based on t he key intermediate first postulated by Jordan allowed differentiation of t he two recognition sites. The P site, whose structure is known from X-ray c rystallographic studies, tolerates ester functions well. The A site interac ts very strongly with nitro groups, but is not very tolerant to ester funct ions. This differentiation is a central factor in the asymmetric I handling of the two identical substrates. Finally, it could be shown nor the keto g roup of-the,Substrate bound at the A site is not Only essential for the rec ognition, but that an increase in electrophilicity of-the carbon atom also increases the inhibition potency considerably. This has important consequen ces for the recognition process at the A site, whose-exact structure is not yet known.