Binding thermodynamics of substituted diaminopyrimidine renin inhibitors.

Renin is an aspartyl protease involved in the production of angiotensin II, a potent vasoconstrictor. Renin inhibitors can prevent blood vessel constriction and therefore could be useful for the treatment of hypertension. High-throughput screening efforts identified a small molecule renin inhibitor with a core substituted diaminopyrimidine ring. Parallel medicinal chemistry efforts based on this lead resulted in compound 1. A complex of 1 bound to renin was crystallized, and structural data were obtained by X-ray diffraction. The structure indicated that there were adjacent unoccupied binding pockets. Synthetic efforts were initiated to extend functionality into these pockets so as to improve affinity and adjust pharmacokinetic parameters. Thermodynamics data for inhibitor binding to renin were also collected using isothermal titration calorimetry. These data were used to help guide inhibitor optimization by suggesting molecular alterations to improve binding affinity from both thermodynamic and structural perspectives. The addition of a methoxypropyl group extending into the S3 subpocket improved inhibitor affinity and resulted in greater binding enthalpy. Initial additions to the pyrimidine ring template that extended into the large hydrophobic S2 pocket did not improve affinity and dramatically altered the thermodynamic driving force for the binding interaction. Binding of the core template was enthalpically driven, whereas binding of initial inhibitors with S2 extensions was both enthalpically and entropically driven but lost significant binding enthalpy. Additional electrostatic interactions were then incorporated into the S2 extension to improve binding enthalpy while taking advantage of the favorable entropy.

X-ray crystal structure of Staphylococcus aureus FemA.

The latter stages of peptidoglycan biosynthesis in Staphylococci involve the synthesis of a pentaglycine bridge on the epsilon amino group of the pentapeptide lysine side chain. Genetic and biochemical evidence suggest that sequential addition of these glycines is catalyzed by three homologous enzymes, FemX (FmhB), FemA, and FemB. The first protein structure from this family, Staphylococcus aureus FemA, has been solved at 2.1 A resolution by X-ray crystallography. The FemA structure reveals a unique organization of several known protein folds involved in peptide and tRNA binding. The surface of the protein also reveals an L-shaped channel suitable for a peptidoglycan substrate. Analysis of the structural features of this enzyme provides clues to the mechanism of action of S. aureus FemA.