Abstract
Potato and yeast phosphorylase were modified with a substrate analog, the 2', 3'-dialdehyde derivative of γ- and α-cyclodextrin (dial-γ- or α-CD), respectively. Both native and dial-CD showed competitive inhibition for the two phosphorylases, though the dial-CD gave much smaller Ki values of less than 1 mM. Among the three CD, γ- and α-forms of the native and dial-CD gave the smallest Ki values for the yeast and potato enzymes, respectively. The dial-CD modification was accelerated by higher pH and higher concentrations of dial-CD. Both potato and yeast enzymes were stoichiometrically modified with the dial-CD and lost the affinity for the substrate α-glucans. Moreover, the modification was retarded by the glucan substrates, which suggested that the dial-CD might bind at the glucan binding site of phosphorylases. In contrast with the case of glucose 1-phosphate, which showed no significant change in Km values of native and modified enzymes, Km values of modified enzymes for the glucan substrates clearly increased. These results indicated that the dial-CD binding might occur at the so-called glycogen storage site of yeast enzyme or cyclodextrin binding site for the potato enzyme rather than at the active site.
