Histidine 282 in 5-Aminolevulinate Synthase Affects Substrate Binding and Catalysis

Abstract

5-Aminolevulinate synthase (ALAS), the first enzyme of the heme biosynthetic pathway in mammalian cells, is a member of the alpha-oxoamine synthase family of pyridoxal 5'-phosphate (PLP)-dependent enzymes. In all structures of the enzymes of the -oxoamine synthase family, a conserved histidine hydrogen bonds with the phenolic oxygen of the PLP cofactor and may be significant for substrate binding, PLP positioning, and maintenance of the pKa of the imine nitrogen. In ALAS, replacing the equivalent histidine, H282, with alanine reduces the catalytic efficiency for glycine 450-fold and decreases the slow phase rate for glycine binding by 85%. The distribution of the absorbing 420 and 330 nm species was altered with an A420/A330 ratio increased from 0.45 to 1.05. This shift in species distribution was mirrored in the cofactor fluorescence and 300-500 nm circular dichroic spectra and likely reflects variation in the tautomer distribution of the holoenzyme. The 300-500 nm circular dichroism spectra of ALAS and H282A diverged in the presence of either glycine or aminolevulinate, indicating that the reorientation of the PLP cofactor upon external aldimine formation is impeded in H282A. Alterations were also observed in the K(Gly)d value and spectroscopic and kinetic properties, while the K(PLP)d increased 9-fold. Altogether, the results imply that H282 coordinates the movement of the pyridine ring with the reorganization of the active site hydrogen bond network and acts as a hydrogen bond donor to the phenolic oxygen to maintain the protonated Schiff base and enhance the electron sink function of the PLP cofactor.