title: Tracking interactions that stabilize the dimer structure of starch phosphorylase from Corynebacterium callunae. Roles of Arg234 and Arg242 revealed by sequence analysis and site-directed mutagenesis.
abstract: Glycogen phosphorylases (GPs) constitute a family of widely spread catabolic alpha1,4-glucosyltransferases that are active as dimers of two identical, pyridoxal 5'-phosphate-containing subunits. In GP from Corynebacterium callunae, physiological concentrations of phosphate are required to inhibit dissociation of protomers and cause a 100-fold increase in kinetic stability of the functional quarternary structure. To examine interactions involved in this large stabilization, we have cloned and sequenced the coding gene and have expressed fully active C. callunae GP in Escherichia coli. By comparing multiple sequence alignment to structure-function assignments for regulated and nonregulated GPs that are stable in the absence of phosphate, we have scrutinized the primary structure of C. callunae enzyme for sequence changes possibly related to phosphate-dependent dimer stability. Location of Arg234, Arg236, and Arg242 within the predicted subunit-to-subunit contact region made these residues primary candidates for site-directed mutagenesis. Individual Arg-->Ala mutants were purified and characterized using time-dependent denaturation assays in urea and at 45 degrees C. R234A and R242A are enzymatically active dimers and in the absence of added phosphate, they display a sixfold and fourfold greater kinetic stability of quarternary interactions than the wild-type, respectively. The stabilization by 10 mm of phosphate was, however, up to 20-fold greater in the wild-type than in the two mutants. The replacement of Arg236 by Ala was functionally silent under all conditions tested. Arg234 and Arg242 thus partially destabilize the C. callunae GP dimer structure, and phosphate binding causes a change of their tertiary or quartenary contacts, likely by an allosteric mechanism, which contributes to a reduced protomer dissociation rate.
authors.affiliation: Institute of Food Technology, University of Agricultural Sciences, Vienna, Austria.
authors.forename: Richard
authors.initials: R
authors.lastname: Griessler
authors.forename: Alexandra
authors.initials: A
authors.lastname: Schwarz
authors.forename: Jan
authors.initials: J
authors.lastname: Mucha
authors.forename: Bernd
authors.initials: B
authors.lastname: Nidetzky
mesh_terms: D000495:Allosteric Site; D000595:Amino Acid Sequence; D001120:Arginine / Q000737:chemistry; D001665:Binding Sites; D015139:Blotting, Southern; D002942:Circular Dichroism; D003001:Cloning, Molecular; D003352:Corynebacterium / Q000201:enzymology / Q000378:metabolism*; D004247:DNA / Q000378:metabolism; D019281:Dimerization; D004926:Escherichia coli / Q000378:metabolism; D015723:Gene Library; D005936:Glucans / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009841:Oligonucleotides / Q000494:pharmacology; D010957:Plasmids / Q000378:metabolism; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D024941:Starch Phosphorylase / Q000737:chemistry* / Q000378:metabolism; D013696:Temperature
chemical_list: D005936:Glucans; D009841:Oligonucleotides; D011994:Recombinant Proteins; D004247:DNA; D001120:Arginine; D024941:Starch Phosphorylase