On the role of the R configuration of the reaction-intermediate isostere in HIV-1 protease-inhibitor binding: X-ray structure at 2.0 A resolution.

Peptidomimetic inhibitors of human immunodeficiency virus-1 protease are successful lead substances for the development of virostatic drugs against HIV as the causative agent of acquired immunodeficiency syndrome (AIDS). The hydroxyethylamine isostere of the proteolytic cleavage intermediate provides a suitable replacement for the peptide bond. A series of acyclic pseudopeptide inhibitors with the hydroxyethylamine isostere varying in chiral carbon configuration and P'2 residue type were structurally analysed by single-crystal X-ray crystallography. The compounds inhibit HIV protease with subnanomolar inhibition constants and block viral replication in tissue cultures. Here, the structure of such a complex with the R configuration of the isosteric group (PDB code 1zsf) is presented together with newly available synchrotron data for a complex with the S stereoisomer of the inhibitor (PDB code 1zsr). Comparison of the structure and binding with other complexes of HIV-1 protease and similar inhibitors contributes to the understanding of how these molecules bind to the wild-type form of this enzyme. The hydroxy group of the R stereoisomer interacts with one of the catalytic aspartic acids by a short hydrogen bond with rather extreme geometry. The change of configuration of the chiral carbon bearing the hydroxyl from S to R does not influence the inhibition efficiency in this case.

Cold-active beta-galactosidase from Arthrobacter sp. C2-2 forms compact 660 kDa hexamers: crystal structure at 1.9A resolution.

The X-ray structure of cold-active beta-galactosidase (isoenzyme C-2-2-1) from an Antarctic bacterium Arthrobacter sp. C2-2 was solved at 1.9A resolution. The enzyme forms 660 kDa hexamers with active sites opened to the central cavity of the hexamer and connected by eight channels with exterior solvent. To our best knowledge, this is the first cold-active beta-galactosidase with known structure and also the first known beta-galactosidase structure in the form of compact hexamers. The hexamer organization regulates access of substrates and ligands to six active sites and this unique packing, present also in solution, raises questions about its purpose and function. This enzyme belongs to glycosyl hydrolase family 2, similarly to Escherichia coli beta-galactosidase, forming tetramers necessary for its enzymatic function. However, we discovered significant differences between these two enzymes affecting the ability of tetramer/hexamer formation and complementation of the active site. This structure reveals new insights into the cold-adaptation mechanisms of enzymatic pathways of extremophiles.

Modelling of carbohydrate-aromatic interactions: ab initio energetics and force field performance.

Aromatic amino acid residues are often present in carbohydrate-binding sites of proteins. These binding sites are characterized by a placement of a carbohydrate moiety in a stacking orientation to an aromatic ring. This arrangement is an example of CH/pi interactions. Ab initio interaction energies for 20 carbohydrate-aromatic complexes taken from 6 selected ultra-high resolution X-ray structures of glycosidases and carbohydrate-binding proteins were calculated. All interaction energies of a pyranose moiety with a side chain of an aromatic residue were calculated as attractive with interaction energy ranging from -2.8 to -12.3 kcal/mol as calculated at the MP2/6-311+G(d) level. Strong attractive interactions were observed for a wide range of orientations of carbohydrate and aromatic ring as present in selected X-ray structures. The most attractive interaction was associated with apparent combination of CH/pi interactions and classical H-bonds. The failure of Hartree-Fock method (interaction energies from +1.0 to -6.9 kcal/mol) can be explained by a dispersion nature of a majority of the studied complexes. We also present a comparison of interaction energies calculated at the MP2 level with those calculated using molecular mechanics force fields (OPLS, GROMOS, CSFF/CHARMM, CHEAT/CHARMM, Glycam/AMBER, MM2 and MM3). For a majority of force fields there was a strong correlation with MP2 values. RMSD between MP2 and force field values were 1.0 for CSFF/CHARMM, 1.2 for Glycam/AMBER, 1.2 for GROMOS, 1.3 for MM3, 1.4 for MM2, 1.5 for OPLS and to 2.3 for CHEAT/CHARMM (in kcal/mol). These results show that molecular mechanics approximates interaction energies very well and support an application of molecular mechanics methods in the area of glycochemistry and glycobiology.