The ionization states of the 5'-phosphate group in the various coenzyme forms bound to mitochondrial aspartate aminotransferase

We have carried out a Fourier transform infrared spectroscopic study of mitochondrial aspartate aminotransferase in the spectral region where phosphate monoesters give rise to absorption. Infrared spectra in the above-mentioned region are dominated by protein absorption. Yet, below 1020 cm-1 protein interferences are minor, permitting the detection of the band arising from the symmetric stretching of dianionic phosphate monoesters T. Shimanouchi, M. Tsuboi, and Y. Kyogoku (1964) Adv. Chem. Phys. 8, 435-498]. The integrated intensity of this band in several enzyme forms (pyridoxal phosphate, pyridoxamine phosphate, and sodium borohydride-reduced, pyridoxyl phosphate form) does not change with pH in the range 5-9. This behavior contrasts that of free pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP) in solution, where the dependence of the same infrared band intensity with pH can be correlated to the known pK values for the 5'-phosphate ester in solution. The integrated intensity value of this infrared band for the PLP enzyme form before and after reduction with sodium borohydride is close to that given by free PLP at pH 8-9. These results are taken as evidence that in the active site of mitochondrial aspartate aminotransferase the 5'-phosphate group of PLP remains mostly dianionic even at a pH near 5. Thus, it is suggested that the chemical shift changes associated with pH titrations of various PLP forms reported in a previous 31P NMR study of this enzyme M. E. Mattingly, J. R. Mattingly, and M. Martinez-Carrion (1982) J. Biol. Chem. 257, 8872] are due to the fact that the phosphorus chemical shift senses the O-P-O bond distortions induced by the ionization of a nearby residue. Since no chemical shift changes were observed in pH titrations of the PMP forms (lacking an ionizable internal aldimine) of this isozyme, the Schiff base between PLP and Lys-258 at the active site is the most likely candidate for the ionizing group influencing the phosphorus chemical shift in this enzyme.