Mitochondrial copper(I) transfer from Cox17 to Sco1 is coupled to electron transfer.

The human protein Cox17 contains three pairs of cysteines. In the mitochondrial intermembrane space (IMS) it exists in a partially oxidized form with two S-S bonds and two reduced cysteines (HCox17(2S-S)). HCox17(2S-S) is involved in copper transfer to the human cochaperones Sco1 and Cox11, which are implicated in the assembly of cytochrome c oxidase. We show here that Cu(I)HCox17(2S-S), i.e., the copper-loaded form of the protein, can transfer simultaneously copper(I) and two electrons to the human cochaperone Sco1 (HSco1) in the oxidized state, i.e., with its metal-binding cysteines forming a disulfide bond. The result is Cu(I)HSco1 and the fully oxidized apoHCox17(3S-S), which can be then reduced by glutathione to apoHCox17(2S-S). The HSco1/HCox17(2S-S) redox reaction is thermodynamically driven by copper transfer. These reactions may occur in vivo because HSco1 can be found in the partially oxidized state within the IMS, consistent with the variable redox properties of the latter compartment. The electron transfer-coupled metallation of HSco1 can be a mechanism within the IMS for an efficient specific transfer of the metal to proteins, where metal-binding thiols are oxidized. The same reaction of copper-electron-coupled transfer does not occur with the human homolog of Sco1, HSco2, for kinetic reasons that may be ascribed to the lack of a specific metal-bridged protein-protein complex, which is instead observed in the Cu(I)HCox17(2S-S)/HSco1 interaction.

Binding of oxalyl derivatives of beta-d-glucopyranosylamine to muscle glycogen phosphorylase b.

Five oxalyl derivatives of beta-d-glucopyranosylamine were synthesized as potential inhibitors of glycogen phosphorylase (GP). The compounds 1-4 were competitive inhibitors of rabbit muscle GPb (with respect to alpha-d-glucose-1-phosphate) with K(i) values of 0.2-1.4 mM, while compound 5 was not effective up to a concentration of 10 mM. In order to elucidate the structural basis of their inhibition, we analysed the structures of compounds 1-4 in complex with GPb at 1.93-1.96 Angstrom resolution. The complex structures reveal that the inhibitors can be accommodated at the catalytic site at approximately the same position as alpha-d-glucose and stabilize the T-state conformation of the 280 s loop by making several favourable contacts to Asp283 and Asn284 of this loop. Comparison with the lead compound N-acetyl-beta-d-glucopyranosylamine (6) shows that the hydrogen bonding interaction of the amide nitrogen with the main-chain carbonyl oxygen of His377 is not present in these complexes. The differences observed in the K(i) values of the four analogues can be interpreted in terms of subtle conformational changes of protein residues and shifts of water molecules in the vicinity of the catalytic site, variations in van der Waals interactions, conformational entropy and desolvation effects.

Crystallographic studies on N-azidoacetyl-beta-D-glucopyranosylamine, an inhibitor of glycogen phosphorylase: comparison with N-acetyl-beta-D-glucopyranosylamine.

N-acetyl-beta-D-glucopyranosylamine (NAG) is a potent inhibitor (Ki=32 microM) of glycogen phosphorylase b (GPb), and has been employed as a lead compound for the structure-based design of new analogues, in an effort to utilize its potential as a hypoglycaemic agent. Replacement of the acetamido group by azidoacetamido group resulted in an inhibitor, N-azidoacetyl-beta-D-glucopyranosylamine (azido-NAG), with a Ki value of 48.7 microM, in the direction of glycogen synthesis. In order to elucidate the mechanism of inhibition, we determined the ligand structure in complex with GPb at 2.03 A resolution, and the structure of the fully acetylated derivative in the free form. The molecular packing of the latter is stabilized by a number of bifurcated hydrogen bonds of which the one involving a bifurcated C-H...N...H-C type hydrogen bonding is rather unique in organic azides. Azido-NAG can be accommodated in the catalytic site of T-state GPb at approximately the same position as that of NAG and stabilizes the T-state conformation of the 280 s loop by making several favourable contacts to residues of this loop. The difference observed in the Ki values of the two analogues can be interpreted in terms of desolvation effects, subtle structural changes of protein residues and changes in water structure.

Crystallographic studies on two bioisosteric analogues, N-acetyl-beta-D-glucopyranosylamine and N-trifluoroacetyl-beta-D-glucopyranosylamine, potent inhibitors of muscle glycogen phosphorylase.

Structure-based inhibitor design has led to the discovery of a number of potent inhibitors of glycogen phosphorylase b (GPb), N-acyl derivatives of beta-D-glucopyranosylamine, that bind at the catalytic site of the enzyme. The first good inhibitor in this class of compounds, N-acetyl-beta-D-glucopyranosylamine (NAG) (K(i) = 32 microM), has been previously characterized by biochemical, biological and crystallographic experiments at 2.3 angstroms resolution. Bioisosteric replacement of the acetyl group by trifluoroacetyl group resulted in an inhibitor, N-trifluoroacetyl-beta-D-glucopyranosylamine (NFAG), with a K(i) = 75 microM. To elucidate the structural basis of its reduced potency, we determined the ligand structure in complex with GPb at 1.8 angstroms resolution. To compare the binding mode of N-trifluoroacetyl derivative with that of the lead molecule, we also determined the structure of GPb-NAG complex at a higher resolution (1.9 angstroms). NFAG can be accommodated in the catalytic site of T-state GPb at approximately the same position as that of NAG and stabilize the T-state conformation of the 280 s loop by making several favourable contacts to Asn284 of this loop. The difference observed in the K(i) values of the two analogues can be interpreted in terms of subtle conformational changes of protein residues and shifts of water molecules in the vicinity of the catalytic site, variations in van der Waals interaction, and desolvation effects.