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 degrees 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) approximately 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 approximately 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.
Institute of Biotechnology and Biochemical Engineering, Graz University of Technology, Austria.
Richard
R
Griessler
Barbara
B
Psik
Alexandra
A
Schwarz
Bernd
B
Nidetzky
D001059:Apoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D001120:Arginine / Q000235:genetics / Q000378:metabolism; D002846:Chromatography, Affinity; D002942:Circular Dichroism; D003352:Corynebacterium / Q000201:enzymology* / Q000235:genetics; D019281:Dimerization; D004795:Enzyme Stability; D006207:Half-Life; D020035:Holoenzymes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009154:Mutation; D011134:Polysaccharides / Q000378:metabolism; D011732:Pyridoxal Phosphate / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D024941:Starch Phosphorylase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D013329:Structure-Activity Relationship
D001059:Apoproteins; D020035:Holoenzymes; D011134:Polysaccharides; D011732:Pyridoxal Phosphate; C008315:maltodextrin; D001120:Arginine; D024941:Starch Phosphorylase