title: Substrate-cofactor interactions for glycogen phosphorylase b: a binding study in the crystal with heptenitol and heptulose 2-phosphate.
abstract: The structural relationships between substrate and pyridoxal phosphate in glycogen phosphorylase b (EC 2.4.1.1) have been studied by X-ray diffraction experiments at 3-A resolution. Recent work [Klein, H. W., Im, M. J., & Helmreich, E. J. M. (1984) in Chemical and Biological Aspects of Vitamin B6 Catalysis (Evangelopoulos, A. E., Ed.) pp 147-160, Liss, New York] has shown that phosphorylase in the presence of inorganic phosphate catalyzes the conversion of heptenitol to heptulose 2-phosphate. The latter compound is a dead-end product and a most potent inhibitor (Ki = 14 microM). The X-ray diffraction studies show that heptenitol binds at the catalytic site of phosphorylase in a position essentially identical with that observed for the glucopyranose moiety of glucose 1-phosphate. Incubation of a phosphorylase b crystal for 50 h in a solution containing the substrates heptenitol and inorganic phosphate and the activators AMP and maltohetaose resulted in the formation of a phosphorylated product bound at the active site. The structure of this product, as analyzed by a difference Fourier synthesis at 3 A, is consistent with that of heptulose 2-phosphate. Analysis of the surrounding soak solution by thin-layer chromatography showed that heptulose 2-phosphate was produced under these conditions. Heptulose 2-phosphate binds with its glucopyranose moiety in the same position as that for glucose 1-phosphate, but there is a marked difference in phosphate positions. The presence of the methyl group in the beta-configuration in heptulose 2-phosphate forces a change in the torsion angle O5-C1-O1-P from 117 degrees as observe in glucose 1-phosphate to -136 degrees in heptulose 2-phosphate. The "down" position of the phosphate (with respect to the crystallographic z axis) results in a change in the distance between the 5'-phosphorus atom of the pyridoxal phosphate and the phosphorus atom of the substrate from 6.8 (with glucose 1-phosphate) to 4.5 A (with heptulose 2-phosphate). The closest distance between the phosphate oxygen of the cofactor and a phosphate oxygen of heptulose 2-phosphate is 2.7 A, and it is assumed that there must be a hydrogen bond between them. These observations are consistent with the NMR experiments reported in the preceding paper in which sharing of a proton between heptulose 2-phosphate and pyridoxal 5'-phosphate is observed [Klein, H.W., Im, M. J., Palm, D., & Helmreich, E. J. M. (1984) Biochemistry (preceding paper in this issue)].(ABSTRACT TRUNCATED AT 400 WORDS)
mesh_terms: D000249:Adenosine Monophosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003460:Crystallization; D005936:Glucans / Q000378:metabolism; D005958:Glucosephosphates / Q000378:metabolism*; D009132:Muscles / Q000201:enzymology; D010763:Phosphorylase b / Q000378:metabolism*; D006005:Phosphorylases / Q000378:metabolism*; D011487:Protein Conformation; D011732:Pyridoxal Phosphate / Q000378:metabolism; D011817:Rabbits; D013402:Sugar Alcohols / Q000378:metabolism*; D013403:Sugar Phosphates; D014961:X-Ray Diffraction
chemical_list: D005936:Glucans; D005958:Glucosephosphates; D013402:Sugar Alcohols; D013403:Sugar Phosphates; C031146:maltoheptaose; D000249:Adenosine Monophosphate; D011732:Pyridoxal Phosphate; C026592:2,6-anhydro-1-deoxygluco-hept-1-enitol; C044547:1-deoxygluco-heptulose 2-phosphate; D010763:Phosphorylase b; D006005:Phosphorylases