Ultraviolet-visible absorption spectrum for the complex phosphorylase b—AMP: study at 25°C

Abstract Absorption difference spectra of phosphorylase b when AMP binds to its high affinity site have been studied at 25°C and pH 6.9; the absorbance changes show linearity as a function of the amount of phophorylase b —AMP complex present in solution. The negative regions of these spectra have been interpreted by assigning the hypochromic effect in the absorption band of AMP to stacking of the adenine ring with an aromatic ring from some tyrosine and/or tryptophan residues. On the other hand, the positive region of the difference spectrum induced by binding of AMP to its high affinity site can be simulated assuming that six tyrosines and one or two tryptophans per monomer are embedded in a highly hydrophobic environment.