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In response to infection or injury, the body mounts an inflammatory immune response in order to neutralize pathogens and promote tissue repair. The key effector cells for these responses are the leukocytes (white blood cells), which are specifically recruited to the site of injury. However, dysregulation of the inflammatory response, characterized by the excessive migration of leukocytes to the affected tissues, can also lead to chronic inflammatory diseases. Leukocyte recruitment is regulated by inflammatory mediators, including an important family of small secreted chemokines and their corresponding G protein-coupled receptors expressed in leukocytes. Unsurprisingly, due to their central role in the leukocyte inflammatory response, chemokines and their receptors have been intensely investigated and represent attractive drug targets. Nonetheless, the full therapeutic potential of chemokine receptors has not been realized, largely due to the complexities in the chemokine system. The determination of chemokine-receptor structures in recent years has dramatically shaped our understanding of the molecular mechanisms that underpin chemokine signaling. In this review, we summarize the contemporary structural view of chemokine-receptor recognition, and describe the various binding modes of peptide and small-molecule ligands to chemokine receptors. We also provide some perspectives on the implications of these data for future research and therapeutic development. IMPORTANCE STATEMENT: Given their central role in the leukocyte inflammatory response, chemokines and their receptors are considered as important regulators of physiology and viable therapeutic targets. In this review, we provide a summary of the current understanding of chemokine: chemokine-receptor interactions that have been gained from structural studies, as well as their implications for future drug discovery efforts.

The mouse pheromones (+/-)-2-sec-butyl-4,5-dihydrothiazole (SBT) and 6-hydroxy-6-methyl-3-heptanone (HMH) bind into an occluded hydrophobic cavity in the mouse major urinary protein (MUP-1). Although the ligands are structurally unrelated, in both cases binding is accompanied by formation of a similar buried, water-mediated hydrogen bond network between the ligand and several backbone and side chain groups on the protein. To investigate the energetic contribution of this hydrogen bond network to ligand binding, we have applied isothermal titration calorimetry to measure the binding thermodynamics using several MUP mutants and ligand analogs. Mutation of Tyr-120 to Phe, which disrupts a hydrogen bond from the phenolic hydroxyl group of Tyr-120 to one of the bound water molecules, results in a substantial loss of favorable binding enthalpy, which is partially compensated by a favorable change in binding entropy. A similar thermodynamic effect was observed when the hydrogen bonded nitrogen atom of the heterocyclic ligand was replaced by a methyne group. Several other modifications of the protein or ligand had smaller effects on the binding thermodynamics. The data provide supporting evidence for the role of the hydrogen bond network in stabilizing the complex.

The contributions of backbone NH group dynamics to the conformational heat capacity of the B1 domain of Streptococcal protein G have been estimated from the temperature dependence of 15N NMR-derived order parameters. Longitudinal (R1) and transverse (R2) relaxation rates, transverse cross-relaxation rates (eta(xy)), and steady state [1H]-15N nuclear Overhauser effects were measured at temperatures of 0, 10, 20, 30, 40, and 50 degrees C for 89-100% of the backbone secondary amide nitrogen nuclei in the B1 domain. The ratio R2/eta(xy) was used to identify nuclei for which conformational exchange makes a significant contribution to R2. Relaxation data were fit to the extended model-free dynamics formalism, incorporating an axially symmetric molecular rotational diffusion tensor. The temperature dependence of the order parameter (S2) was used to calculate the contribution of each NH group to conformational heat capacity (Cp) and a characteristic temperature (T*), representing the density of conformational energy states accessible to each NH group. The heat capacities of the secondary structure regions of the B1 domain are significantly higher than those of comparable regions of other proteins, whereas the heat capacities of less structured regions are similar to those in other proteins. The higher local heat capacities are estimated to contribute up to approximately 0.8 kJ/mol K to the total heat capacity of the B1 domain, without which the denaturation temperature would be approximately 9 degrees C lower (78 degrees C rather than 87 degrees C). Thus, variation of backbone conformational heat capacity of native proteins may be a novel mechanism that contributes to high temperature stabilization of proteins.