3'-axial CH2 OH substitution on glucopyranose does not increase glycogen phosphorylase inhibitory potency. QM/MM-PBSA calculations suggest why.

Glycogen phosphorylase is a molecular target for the design of potential hypoglycemic agents. Structure-based design pinpointed that the 3'-position of glucopyranose equipped with a suitable group has the potential to form interactions with enzyme's cofactor, pyridoxal 5'-phosphate (PLP), thus enhancing the inhibitory potency. Hence, we have investigated the binding of two ligands, 1-(ß-d-glucopyranosyl)5-fluorouracil (GlcFU) and its 3'-CH(2) OH glucopyranose derivative. Both ligands were found to be low micromolar inhibitors with K(i) values of 7.9 and 27.1 µm, respectively. X-ray crystallography revealed that the 3'-CH(2) OH glucopyranose substituent is indeed involved in additional molecular interactions with the PLP γ-phosphate compared with GlcFU. However, it is 3.4 times less potent. To elucidate this discovery, docking followed by postdocking Quantum Mechanics/Molecular Mechanics - Poisson-Boltzmann Surface Area (QM/MM-PBSA) binding affinity calculations were performed. While the docking predictions failed to reflect the kinetic results, the QM/MM-PBSA revealed that the desolvation energy cost for binding of the 3'-CH(2) OH-substituted glucopyranose derivative out-weigh the enthalpy gains from the extra contacts formed. The benefits of performing postdocking calculations employing a more accurate solvation model and the QM/MM-PBSA methodology in lead optimization are therefore highlighted, specifically when the role of a highly polar/charged binding interface is significant.

The σ-hole phenomenon of halogen atoms forms the structural basis of the strong inhibitory potency of C5 halogen substituted glucopyranosyl nucleosides towards glycogen phosphorylase b.

C5 halogen substituted glucopyranosyl nucleosides (1-(ß-D-glucopyranosyl)-5-X-uracil; X=Cl, Br, I) have been discovered as some of the most potent active site inhibitors of glycogen phosphorylase (GP), with respective K(i) values of 1.02, 3.27, and 1.94 µM. The ability of the halogen atom to form intermolecular electrostatic interactions through the σ-hole phenomenon rather than through steric effects alone forms the structural basis of their improved inhibitory potential relative to the unsubstituted 1-(ß-D-glucopyranosyl)uracil (K(i) =12.39 µM), as revealed by X-ray crystallography and modeling calculations exploiting quantum mechanics methods. Good agreement was obtained between kinetics results and relative binding affinities calculated by QM/MM-PBSA methodology for various substitutions at C5. Ex vivo experiments demonstrated that the most potent derivative (X=Cl) toward purified GP has no cytotoxicity and moderate inhibitory potency at the cellular level. In accordance, ADMET property predictions were performed, and suggest decreased polar surface areas as a potential means of improving activity in the cell.

Kinetics, in silico docking, molecular dynamics, and MM-GBSA binding studies on prototype indirubins, KT5720, and staurosporine as phosphorylase kinase ATP-binding site inhibitors: the role of water molecules examined.

With an aim toward glycogenolysis control in Type 2 diabetes, we have investigated via kinetic experiments and computation the potential of indirubin (IC50 > 50 µM), indirubin-3'-oxime (IC50 = 144 nM), KT5720 (K(i) = 18.4 nM) and staurosporine (K(i) = 0.37 nM) as phosphorylase kinase (PhKγtrnc) ATP-binding site inhibitors, with the latter two revealed as potent inhibitors in the low nM range. Because of lack of structural information, we have exploited information from homologous kinase complexes to direct in silico calculations (docking, molecular dynamics, and MMGBSA) to predict the binding characteristics of the four ligands. All inhibitors are predicted to bind in the same active site area as the ATP adenine ring, with binding dominated by hinge region hydrogen bonds to Asp104:O and Met106:O (all four ligands) and also Met106:NH (for the indirubins). The PhKγtrnc-staurosporine complex has the greatest number of receptor-ligand hydrogen bonds, while for the indirubin-3'-oxime and KT5720 complexes there is an important network of interchanging water molecules bridging inhibitor-enzyme contacts. The MM-GBSA results revealed the source of staurosporine's low nM potency to be favorable electrostatic interactions, while KT5720 has strong van der Waals contributions. KT5720 interacts with the greatest number of protein residues either by direct or 1-water bridged hydrogen bond interactions, and the potential for more selective PhK inhibition based on a KT5720 analogue has been established. Including receptor flexibility in Schrödinger induced-fit docking calculations in most cases correctly predicted the binding modes as compared with the molecular dynamics structures; the algorithm was less effective when there were key structural waters bridging receptor-ligand contacts.

1-(3-Deoxy-3-fluoro-beta-d-glucopyranosyl) pyrimidine derivatives as inhibitors of glycogen phosphorylase b: Kinetic, crystallographic and modelling studies.

Design of inhibitors of glycogen phosphorylase (GP) with pharmaceutical applications in improving glycaemic control in type 2 diabetes is a promising therapeutic strategy. The catalytic site of muscle glycogen phosphorylase b (GPb) has been probed with five deoxy-fluro-glucose derivatives. These inhibitors had fluorine instead of hydroxyl at the 3' position of the glucose moiety and a variety of pyrimidine derivatives at the 1' position. The best of this carbohydrate-based family of five inhibitors displays a K(i) value of 46muM. To elucidate the mechanism of inhibition for these compounds, the crystal structures of GPb in complex with each ligand were determined and refined to high resolution. The structures demonstrated that the inhibitors bind preferentially at the catalytic site and promote the less active T state conformation of the enzyme by making several favorable contacts with residues of the 280s loop. Fluorine is engaged in hydrogen bond interactions but does not improve glucose potency. The pyrimidine groups are located between residues 284-286 of the 280s loop, Ala383 of the 380s loop, and His341 of the beta-pocket. These interactions appear important in stabilizing the inactive quaternary T state of the enzyme. As a follow up to recent computations performed on beta-d-glucose pyrimidine derivatives, tautomeric forms of ligands 1-5 were considered as potential binding states. Using Glide-XP docking and QM/MM calculations, the ligands 2 and 5 are predicted to bind in different tautomeric states in their respective GPb complexes. Also, using alpha-d-glucose as a benchmark model, a series of substitutions for glucose -OH at the 3' (equatorial) position were investigated for their potential to improve the binding affinity of glucose-based GPb catalytic site inhibitors. Glide-XP and quantum mechanics polarized ligand (QPLD-SP/XP) docking calculations revealed favorable binding at this position to be dominated by hydrogen bond contributions; none of the substitutions (including fluorine) out-performed the native -OH substituent which can act both as hydrogen bond donor and acceptor. The structural analyses of these compounds can be exploited towards the development of better inhibitors.

The 1.76 A resolution crystal structure of glycogen phosphorylase B complexed with glucose, and CP320626, a potential antidiabetic drug.

CP320626, a potential antidiabetic drug, inhibits glycogen phosphorylase in synergism with glucose. To elucidate the structural basis of synergistic inhibition, we determined the structure of muscle glycogen phosphorylase b (MGPb) complexed with both glucose and CP320626 at 1.76 A resolution, and refined to a crystallographic R value of 0.211 (R(free)=0.235). CP320626 binds at a novel allosteric site, which is some 33 A from the catalytic site, where glucose binds. The high resolution structure allows unambiguous definition of the conformation of the 1-acetyl-4-hydroxy-piperidine ring supported by theoretical energy calculations. Both CP320626 and glucose promote the less active T-state, thereby explaining their synergistic inhibition. Structural comparison of MGPb--glucose--CP320626 complex with liver glycogen phosphorylase a (LGPa) complexed with a related compound (CP403700) show that the ligand binding site is conserved in LGPa.

Kinetic and crystallographic studies of glucopyranosylidene spirothiohydantoin binding to glycogen phosphorylase B.

Glucopyranosylidene spirothiohydantoin (TH) has been identified as a potential inhibitor of both muscle and liver glycogen phosphorylase b (GPb) and a (GPa) and shown to diminish liver GPa activity in vitro. Kinetic experiments reported here show that TH inhibits muscle GPb competitively with respect to both substrates phosphate (K(i)=2.3 microM) and glycogen (K(i)=2.8 microM). The structure of the GPb-TH complex has been determined at a resolution of 2.26 A and refined to a crystallographic R value of 0.193 (R(free)=0.211). The structure of GPb-TH complex reveals that the inhibitor can be accommodated in the catalytic site of T-state GPb with very little change of the tertiary structure, and provides a basis of understanding potency and specificity of the inhibitor. The glucopyranose moiety makes the standard hydrogen bonds and van der Waals contacts as observed in the glucose complex, while the rigid thiohydantoin group is in a favourable electrostatic environment and makes additional polar contacts to the protein.