Is the active form of pyridoxal-P in alpha-glucan phosphorylases a 5'-phosphate dianion?

The essential role of pyridoxal-5’-phosphate for the activity of all cY-glucan phosphorylases received strong support from recent studies with bacterial phosphorylases [ 11. The advantage of utilizing these enzymes for the study of the role of pyridoxal-P is obvious since they neither depend on allosteric effectors or covalent modification for the expression of activity [l-6]. The 5’-phosphate group of pyridoxalP is the sole phosphate moiety in catalytically active bacterial phosphorylases. Accordingly, ‘rP NMR spectra of Escherichia coli maltodextrin phosphorylase exhibit an exceptionally simple pattern. When compared with the 31P NMR spectra of rabbit skeletal muscle phosphorylase a and b [7] the resonances common to phosphorylase allow us to define the ionization state of the cofactor in the catalytically active enzyme.