The reactivity of arginine residues interacting with glucose 1-phosphate in glycogen phosphorylase. A comparison between pyridoxal-reconstituted phosphorylase and the native enzyme

The phosphate recognition site of Escherichia coli maltodextrin phosphorylase

The role of two positively charged amino acid residues located at the active site of Escherichia coli maltodextrin phosphorylase was investigated by site-directed mutagenesis. Substitution of Lys539 by an arginine caused a 600-fold reduction, substitution of Arg534 by a glutamine caused an even larger 7000-fold reduction of the catalytic rate while substrate binding remained essentially unaffected. Since the Arg534----Gln exchange reduces the catalytic rate near to inactivity and even the conservative Lys534----Arg exchange caused a marked decrease of activity, the central functional role of both positively charged residues in phosphorylase catalysis anticipated by the crystallographic analysis of the corresponding amino acid residues Arg569 and Lys574 in the catalytic site of phosphorylase b was confirmed.

Refined crystal structure of the phosphorylase-heptulose 2-phosphate-oligosaccharide-AMP complex

The crystal structure of phosphorylase b-heptulose 2-phosphate complex with oligosaccharide and AMP bound has been refined by molecular dynamics and crystallographic least-squares with the program XPLOR. Shifts in atomic positions of up to 4 A from the native enzyme structure were correctly determined by the program without manual intervention. The final crystallographic R value for data between 8 and 2.86 A resolution is 0.201, and the overall root-mean-square difference between the native and complexed structure is 0.58 A for all protein atoms. The results confirm the previous observation that there is a direct hydrogen bond between the phosphate of heptulose 2-phosphate and the pyridoxal phosphate 5'-phosphate group. The close proximity of the two phosphates is stabilized by an arginine residue, Arg569, which shifts from a site buried in the protein to a position where it can make contact with the product phosphate. There is a mutual interchange in position between the arginine and an acidic group, Asp283. These movements represent the first stage of the allosteric response which converts the catalytic site from a low to a high-affinity binding site. Communication of these changes to other sites is prevented in the crystal by the lattice forces, which also form the subunit interface. The constellation of groups in the phosphorylase transition state analogue complex provides a structural basis for understanding the catalytic mechanism in which the cofactor pyridoxal phosphate 5'-phosphate group functions as a general acid to promote attack by the substrate phosphate on the glycosidic bond when the reaction proceeds in the direction of glycogen degradation. In the direction of glycogen synthesis, stereoelectronic effects contribute to the cleavage of the C-1-O-1 bond. In both reactions the substrate phosphate plays a key role in transition state stabilization. The details of the oligosaccharide, maltoheptaose, interactions with the enzyme at the glycogen storage site are also described.

The allosteric transition of glycogen phosphorylase

The crystal structure of R-state glycogen phosphorylase b has been determined at 2.9 A resolution. A comparison of T-state and R-state structures of the enzyme explains its cooperative behaviour on ligand binding and the allosteric regulation of its activity. Communication between catalytic sites of the dimer is provided by a change in packing geometry of two helices linking each site with the subunit interface. Activation by AMP or by phosphorylation results in a quaternary conformational change that switches these two helices into the R-state conformation.