Assessment of the Active-Site Requirements of 5-Aminolevulinic Acid Dehydratase - Evaluation of Substrate and Product Analogs as Competitive Inhibitors

The enzyme 5-aminolaevulinic acid dehydratase (ALAD) is responsible for the synthesis of porphobilinogen (PBG) from two molecules of 5-aminolaevulinic acid (ALA). Porphobilinogen is an important committed intermediate in the biosynthesis of tetrapyrroles. The inhibition of ALAD from the purple bacterium Rhodopseudomonas sphaeroides was tested with various substrate and product analogues. Excellent inhibition was observed with the nitro analogue 5 (K(i) = 0.018 mM) of laevulinic acid (10) (K(i) = 1 mM), rac-2-hydroxy- (7) (K(i) = 0.43 mM), rac-3-hydroxy- (8) (K(i) = 1.2 mM), 5-hydroxy- (I 1) (K(i) = 0.25 mM), and 5-nitrilolaevulinic acid (12) (K(i) = 0.060 mM. The sulfonic acid 3 and the phosphonic acid analogue 4 did not inhibit the enzyme. The product analogues 15-18 only showed a moderate inhibition (K(i) = 10-15 mM) whereas the pyrazole 19, a close analogue of porphobilinogen, did not inhibit the enzyme at all (K(i) = 32 mM). Comparison of the K(i) values for the substrate analogues indicated the ALAD active site to be sensitive to the hybridization and charge at position 1, to be insensitive to polar and neutral substituents at position 5 unless they are negatively charged (14) or too bulky (13), and to require flexibility of the carbon chain of the substrate, since stiff molecules like beta-acetylacrylic acid 6 showed no affinity. The product analogues 15-19 indicated that the active site of ALAD was not inhibited by its direct product PBG.