Detecting the Hydrogen Bond Cooperativity in a Protein β-Sheet by H/D Exchange

The hydrogen bond (H-bond) cooperativity in the β-sheet of GB3 is investigated by a NMR hydrogen/deuterium (H/D) exchange method. It is shown that the weakening of one backbone N-H…O=C H-bond between two β-strands, β1 and β2, due to the exchange of NH to ND of the H-bond donor in β1, perturbs the chemical shift of ¹³C_α, ¹³C_β, ¹H_α, ¹H_N, and ¹⁵N of the H-bond acceptor and its following residue in β2. Quantum mechanical calculations suggest that the -H-bond chemical shift isotope effect is caused by the structural reorganization in response to the H-bond weakening. This structural reorganization perturbs four neighboring H-bonds, with three being weaker and one being stronger, indicating that three H-bonds are cooperative and one is anticooperative with the perturbed H-bond. The sign of the cooperativity depends on the relative position of the H-bonds. This H-bond cooperativity, which contributes to β-sheet stability overall, can be important for conformational coupling across the β-sheet.

Mechanisms of Regulation of the Chemokine-Receptor Network

The interactions of chemokines with their G protein-coupled receptors promote the migration of leukocytes during normal immune function and as a key aspect of the inflammatory response to tissue injury or infection. This review summarizes the major cellular and biochemical mechanisms by which the interactions of chemokines with chemokine receptors are regulated, including: selective and competitive binding interactions; genetic polymorphisms; mRNA splice variation; variation of expression, degradation and localization; down-regulation by atypical (decoy) receptors; interactions with cell-surface glycosaminoglycans; post-translational modifications; oligomerization; alternative signaling responses; and binding to natural or pharmacological inhibitors.

Homogeneous sulfopeptides and sulfoproteins: synthetic approaches and applications to characterize the effects of tyrosine sulfation on biochemical function

Post-translational modification of proteins plays critical roles in regulating structure, stability, localization, and function. Sulfation of the phenolic side chain of tyrosine residues to form sulfotyrosine (sTyr) is a widespread modification of extracellular and integral membrane proteins, influencing the activities of these proteins in cellular adhesion, blood clotting, inflammatory responses, and pathogen infection. Tyrosine sulfation commonly occurs in sequences containing clusters of tyrosine residues and is incomplete at each site, resulting in heterogeneous mixtures of sulfoforms. Purification of individual sulfoforms is typically impractical. Therefore, the most promising approach to elucidate the influence of sulfation at each site is to prepare homogeneously sulfated proteins (or peptides) synthetically. This Account describes our recent progress in both development of such synthetic approaches and application of the resulting sulfopeptides and sulfoproteins to characterize the functional consequences of tyrosine sulfation. Initial synthetic studies used a cassette-based solid-phase peptide synthesis (SPPS) approach in which the side chain sulfate ester was protected to enable it to withstand Fmoc-based SPPS conditions. Subsequently, to address the need for efficient access to multiple sulfoforms of the same peptide, we developed a divergent solid-phase synthetic approach utilizing orthogonally side chain protected tyrosine residues. Using this methodology, we have carried out orthogonal deprotection and sulfation of up to three tyrosine residues within a given sequence, allowing access to all eight sulfoforms of a given target from a single solid-phase synthesis. With homogeneously sulfated peptides in hand, we have been able to probe the influence of tyrosine sulfation on biochemical function. Several of these studies focused on sulfated fragments of chemokine receptors, key mediators of leukocyte trafficking and inflammation. For the receptor CCR3, we showed that tyrosine sulfation enhances affinity and selectivity for binding to chemokine ligands, and we determined the structural basis of these affinity enhancements by NMR spectroscopy. Using a library of CCR5 sulfopeptides, we demonstrated the critical importance of sulfation at one specific site for supporting HIV-1 infection. Demonstrating the feasibility of producing homogeneously tyrosine-sulfated proteins, in addition to smaller peptides, we have used SPPS and native chemical ligation methods to synthesize the leech-derived antithrombotic protein hirudin P6, containing both tyrosine sulfation and glycosylation. Sulfation greatly enhanced inhibitory activity against thrombin, whereas addition of glycans to the sulfated protein decreased inhibition, indicating functional interplay between different post-translational modifications. In addition, the success of the ligation approach suggests that larger sulfoproteins could potentially be obtained by ligation of synthetic sulfopeptides to expressed proteins, using intein-based technology.

Site-selective solid-phase synthesis of a CCR5 sulfopeptide library to interrogate HIV binding and entry

Tyrosine (Tyr) sulfation is a common post-translational modification that is implicated in a variety of important biological processes, including the fusion and entry of human immunodeficiency virus type-1 (HIV-1). A number of sulfated Tyr (sTyr) residues on the N-terminus of the CCR5 chemokine receptor are involved in a crucial binding interaction with the gp120 HIV-1 envelope glycoprotein. Despite the established importance of these sTyr residues, the exact structural and functional role of this post-translational modification in HIV-1 infection is not fully understood. Detailed biological studies are hindered in part by the difficulty in accessing homogeneous sulfopeptides and sulfoproteins through biological expression and established synthetic techniques. Herein we describe an efficient approach to the synthesis of sulfopeptides bearing discrete sulfation patterns through the divergent, site-selective incorporation of sTyr residues on solid support. By employing three orthogonally protected Tyr building blocks and a solid-phase sulfation protocol, we demonstrate the synthesis of a library of target N-terminal CCR5(2-22) sulfoforms bearing discrete and differential sulfation at Tyr10, Tyr14, and Tyr15, from a single resin-bound intermediate. We demonstrate the importance of distinct sites of Tyr sulfation in binding gp120 through a competitive binding assay between the synthetic CCR5 sulfopeptides and an anti-gp120 monoclonal antibody. These studies revealed a critical role of sulfation at Tyr14 for binding and a possible additional role for sulfation at Tyr10. N-terminal CCR5 variants bearing a sTyr residue at position 14 were also found to complement viral entry into cells expressing an N-terminally truncated CCR5 receptor.