Inhibition of porphobilinogenase by porphyrins in Saccharomyces cerevisiae

The biosynthesis of uroporphyrinogen III, the precursor of hemes, chlorophylls, corrins and related structures, is catalyzed by the porphobilinogenase system (PBGase), a complex of two enzymes, PBG-Deaminase (PBG-D) and Isomerase. Although the separate enzymes have been studied in some detail less work has been performed on the properties of the complex.

In this study the kinetic behaviour of the enzyme PBGase in a normal yeast strain, D273-10B, and its derivative B231 has been investigated. Uroporphyrinogen formation was linear with time up to 2 hr at 37°C. The enzyme complex shows classical Michaelis-Menten kinetics. From the double reciprocal plots kinetic parameters were estimated for PBGase and PBG-D.

Porphyrins were found to be competitive inhibitors with respect to porphobilinogen (PBG) and these compounds appeared to act as inhibitors by forming dead-end complexes with the free enzyme. 5-Aminolevulinic acid (ALA) also inhibited PBGase and this inhibition was overcome by addition of levulinic acid (2μM). These results indicate that ALA, is not an inhibitor but acts through its conversion into porphyrins which are the true inhibitors.