QUANTITATIVE DESCRIPTION OF THE ABSORPTION-SPECTRA OF THE COENZYME INGLYCOGEN PHOSPHORYLASES BASED ON LOG-NORMAL DISTRIBUTION CURVES

The absorption spectra of the coenzyme [pyridoxal 5'-phosphate (PLP)] in glycogen phosphorylase a (GPha), glycogen phosphorylase b (GPhb) an d of the latter bound to various effectors and substrates were analyse d on the basis of log-normal distribution curves. The results obtained showed that the ionization state of the PLP and GPha environment diff ers from that of GPhb. This divergence was interpreted in terms of tau tomeric equilibria between some forms of the Schiff base of PLP and en zymic Lys-679. The ionic forms are slightly more predominant in GPha t han they are in GPhb, so ionic and/or hydrogen-bonding interactions be tween the aromatic ring of PLP and GPha must be stronger than with GPh b. This confirms the purely structural role of the aromatic ring of th e coenzyme. Binding of GPhb to AMP and Mg2+ results in the coenzyme ad opting a similar state as in GPha. On the other hand, binding to IMP g ives rise to no detectable changes in the tautomeric equilibrium of th e coenzyme.