Physical-Chemical Studies on the Pyridoxal Phosphate Binding Site in Sodium Borohydride-reduced and Native Phosphorylase

NaBH4-reduced phosphorylase b is a catalytically active species which retains many of the properties of the native enzyme. One property that is strikingly different, however, is the spectral behavior of bound pyridoxal phosphate. At pH 7.0, the pyridoxal phosphate shows essentially no absorption at 333 nm, where pyridoxal phsophate in the native enzyme absorbs; absorption at 333 nm returns upon lowering the pH. The 333 nm band at low pH in the reduced enzyme was found to be optically active, (ΔA/A) 333 nm ≅ 0.7 x 10-3. Circular dichroism of the reduced phosphorylase b at pH 7.0 indicated that no new band was formed above 300 nm; the ultraviolet spectrum also provided no evidence for a band above 300 nm. Guanidine·HCl (4.25 m) and sodium dodecyl sulfate (0.3%) caused the appearance of a band near 330 nm at pH 6.8. Difference spectra of reduced phosphorylase b (±sodium dodecyl sulfate; pH 5 versus pH 7) indicated that the hidden band must be below 300 nm. AMP (0.01 m) was shown to cause partial reversal of the hidden band to 333 nm conversion at pH 6.0; ATP was also effective. Substrates and AMP had no effect on the spectrum of the reduced phosphorylase b at pH 6.8. Ultracentrifugal studies showed that native and NaBH4-reduced enzyme have different conformations. The data were interpreted to mean that the 333 nm and hidden band forms of NaBH4-reduced phosphorylase represent different enzyme conformations. The hidden band form is thought to represent a form of pyridoxal phosphate in which the pyridinium nitrogen is unprotonated and the 3-hydroxyl group undissociated or strongly hydrogen bonded; the 333 nm species is believed to be a dipolar form of pyridoxal phosphate which is exposed to solvent. Spectral properties of enzyme-bound pyridoxal phosphate in native phosphorylase were compared to NaBH4-reduced enzyme, and it was suggested that pyridoxal phosphate exists in these two forms at pH 7.0 in the same microscopic environment. Circular dichroism of native phosphorylase b at 422 nm was observed. The presence of phosphate (0.01 m) and AMP (0.001 m) did not perturb the observed long wave length circular dichroism. Solvent perturbation studies showed that enzyme-bound pyridoxal phosphate in native phosphorylase is not available to ethylene glycol. From our results, evaluation of model compound data, and fluorescence results, a new structure was proposed for the 333 nm form for native phosphorylase. This species is thought to be a neutral tautomeric imine of pyridoxal phosphate.