Molecular Mechanism for Heme-Mediated Inhibition of 5-Aminolevulinic Acid Synthase 1

Mammalian 5-aminolevulinic acid synthase 1 (ALAS1), an isozyme expressed in all cell types, catalyzes the first reaction in the heme biosynthetic pathway in mitochondria. Heme regulates ALAS1 function at multiple levels including the regulation of transcription, translation, mitochondrial import, protein degradation, and enzyme activity to maintain intracellular heme concentrations in an appropriate range. In this study, we elucidated the molecular mechanism of heme-mediated regulation of enzymatic activity for rat ALAS1. ALAS1 has three putative heme regulatory motifs (HRMs), two of which were found to be the iron(III) heme binding sites in ALAS1. Electronic absorption and resonance Raman spectroscopy demonstrated that 110Cys and 527Cys were the axial ligand of the iron(III) hemes bound to ALAS1. The heme binding to the HRMs in ALAS1 was found not to be responsible for heme-mediated inhibition of ALAS1 activity. Protoporphyrin IX, a reaction intermediate of heme biogenesis, was found to inhibit ALAS1 activity more efficiently than heme, indicating the presence of multiple pathways for the feedback regulation of ALAS1 activity.