Structure-function studies of 5-aminolevulinic acid (ALA) synthases

The essential metabolite 5-aminolevulinic acid (ALA) is a precursor in the production of tetrapyrroles and 2-amino-3-hydroxycyclopent-2-en-1-one (C5N unit), a substructure of C5Ncontaining polyketides. In non-plant eukaryotes and ..-proteobacteria, the pyridoxal 5'-phosphate (PLP)-dependent ALA synthase enzyme catalyzes the synthesis of ALA. While an understanding as to the distinct roles of multiple ALA synthases in animals is well developed, much less is known about the presence of more than one enzyme in bacteria. The role of HemA and HemT ALA synthase isoenzymes in Rhodobacter sphaeroides was investigated by comparing the enzymatic properties of three HemAs and two HemTs from three strains: one strain has hemA and hemT genes present and both are expressed, another has both genes but hemT is not expressed, and the third strain has the hemA gene only. Although all five enzymes had similar kinetic properties, HemA enzymes were more sensitive to hemin with a 13-fold difference in Ki value compared to HemT. HemT was found to be sensitive to oxidation suggesting that the hemT gene encodes an enzyme that is more active anaerobically, which agrees well with the maximal anaerobic-dark transcription of hemT in strain 2.4.9 (Coulianos N, Kaganjo J, and JH Zeilstra-Ryalls. 2017. Mauscript in preparation). These properties indicate that the role of HemT is to supply ALA when inhibitory heme levels are elevated in the cell, which is thought to occur when cells undergo a transition from aerobic to anaerobic-dark respiration.