Relationships between structure, function and stability for pyridoxal 5′-phosphate-dependent starch phosphorylase from Corynebacterium callunae as revealed by reversible cofactor dissociation studies

Using 0.4 m imidazole citrate buffer (pH 7.5) containing 0.1 mm l-cysteine, homodimeric starch phosphorylase from Corynebacterium calluane (CcStP) was dissociated into native-like folded subunits concomitant with release of pyridoxal 5′-phosphate and loss of activity. The inactivation rate of CcStP under resolution conditions at 30 °C was, respectively, four- and threefold reduced in two mutants, Arg234→Ala and Arg242→Ala, previously shown to cause thermostabilization of CcStP [Griessler, R., Schwarz, A., Mucha, J. & Nidetzky, B. (2003) Eur. J. Biochem.270, 2126–2136]. The proportion of original enzyme activity restored upon the reconstitution of wild-type and mutant apo-phosphorylases with pyridoxal 5′-phosphate was increased up to 4.5-fold by added phosphate. The effect on recovery of activity displayed a saturatable dependence on the phosphate concentration and results from interactions with the oxyanion that are specific to the quarternary state. Arg234→Ala and Arg242→Ala mutants showed, respectively, eight- and > 20-fold decreased apparent affinities for phosphate (K_app), compared to the wild-type (K_app≈ 6 mm). When reconstituted next to each other in solution, apo-protomers of CcStP and Escherichia coli maltodextrin phosphorylase did not detectably associate to hybrid dimers, indicating that structural complementarity among the different subunits was lacking. Pyridoxal-reconstituted CcStP was inactive but ≈ 60% and 5% of wild-type activity could be rescued at pH 7.5 by phosphate (3 mm) and phosphite (5 mm), respectively. pH effects on catalytic rates were different for the native enzyme and pyridoxal-phosphorylase bound to phosphate and could reflect the differences in pK_a values for the cofactor 5′-phosphate and the exogenous oxyanion.