Probing the urea dependence of residual structure in denatured human alpha-lactalbumin

α-Lactalbumin, Amazing Calcium-Binding Protein

α-Lactalbumin (α-LA) is a small (Mr 14,200), acidic (pI 4–5), Ca²⁺-binding protein. α-LA is a regulatory component of lactose synthase enzyme system functioning in the lactating mammary gland. The protein possesses a single strong Ca²⁺-binding site, which can also bind Mg²⁺, Mn²⁺, Na⁺, K⁺, and some other metal cations. It contains several distinct Zn²⁺-binding sites. Physical properties of α-LA strongly depend on the occupation of its metal binding sites by metal ions. In the absence of bound metal ions, α-LA is in the molten globule-like state. The binding of metal ions, and especially of Ca²⁺, increases stability of α-LA against the action of heat, various denaturing agents and proteases, while the binding of Zn²⁺ to the Ca²⁺-loaded protein decreases its stability and causes its aggregation. At pH 2, the protein is in the classical molten globule state. α-LA can associate with membranes at neutral or slightly acidic pH at physiological temperatures. Depending on external conditions, α-LA can form amyloid fibrils, amorphous aggregates, nanoparticles, and nanotubes. Some of these aggregated states of α-LA can be used in practical applications such as drug delivery to tissues and organs. α-LA and some of its fragments possess bactericidal and antiviral activities. Complexes of partially unfolded α-LA with oleic acid are cytotoxic to various tumor and bacterial cells. α-LA in the cytotoxic complexes plays a role of a delivery carrier of cytotoxic fatty acid molecules into tumor and bacterial cells across the cell membrane. Perhaps in the future the complexes of α-LA with oleic acid will be used for development of new anti-cancer drugs.

A Combined NMR and SAXS Analysis of the Partially Folded Cataract-Associated V75D γD-Crystallin

A cataract is a pathological condition characterized by the clouding of the normally clear eye lens brought about by deposition of crystallin proteins in the lens fiber cells. These protein aggregates reduce visual acuity by scattering or blocking incoming light. Chemical damage to proteins of the crystallin family, accumulated over a lifetime, leads to age-related cataract, whereas inherited mutations are associated with congenital or early-onset cataract. The V75D mutant of γD-crystallin is associated with congenital cataract in mice and was previously shown to un/fold via a partially folded intermediate. Here, we structurally characterized the stable equilibrium urea unfolding intermediate of V75D at the ensemble level using solution NMR and small-angle x-ray scattering. Our data show that, in the intermediate, the C-terminal domain retains a folded conformation that is similar to the native wild-type protein, whereas the N-terminal domain is unfolded and comprises an ensemble of random conformers, without any detectable residual structural propensities.

G-protein-coupled receptor structure, ligand binding and activation as studied by solid-state NMR spectroscopy

GPCRs (G-protein-coupled receptors) are versatile signalling molecules at the cell surface and make up the largest and most diverse family of membrane receptors in the human genome. They convert a large variety of extracellular stimuli into intracellular responses through the activation of heterotrimeric G-proteins, which make them key regulatory elements in a broad range of normal and pathological processes, and are therefore one of the most important targets for pharmaceutical drug discovery. Knowledge of a GPCR structure enables us to gain a mechanistic insight into its function and dynamics, and further aid rational drug design. Despite intensive research carried out over the last three decades, resolving the structural basis of GPCR function is still a major activity. The crystal structures obtained in the last 5 years provide the first opportunity to understand how protein structure dictates the unique functional properties of these complex signalling molecules. However, owing to the intrinsic hydrophobicity, flexibility and instability of membrane proteins, it is still a challenge to crystallize GPCRs, and, when this is possible, it is no longer in its native membrane environment and no longer without modification. Furthermore, the conformational change of the transmembrane α-helices associated with the structure activation increases the difficulty of capturing the activation state of a GPCR to a higher resolution by X-ray crystallography. On the other hand, solid-state NMR may offer a unique opportunity to study membrane protein structure, ligand binding and activation at atomic resolution in the native membrane environment, as well as described functionally significant dynamics. In the present review, we discuss some recent achievements of solid-state NMR for understanding GPCRs, the largest mammalian proteome at ~1% of the total expressed proteins. Structural information, details of determination, details of ligand conformations and the consequences of ligand binding to initiate activation can all be explored with solid-state NMR.

Structural insights into the Trp-cage folding intermediate formation

The 20 residue long Trp-cage is the smallest protein known, and thus has been the subject of several in vitro and in silico folding studies. Here, we report the multistate folding scenario of the miniprotein in atomic detail. We detected and characterized different intermediate states by temperature dependent NMR measurements of the (15)N and (13)C/(15)N labeled protein, both at neutral and acidic pH values. We developed a deconvolution technique to characterize the invisible--fully folded, unfolded and intermediate--fast exchanging states. Using nonlinear fitting methods we can obtain both the thermodynamic parameters (ΔH(F-I), T(m)(F-I), ΔC(p)(F-I) and ΔH(I-U), T(m)(I-U), ΔC(p)(I-U)) and the NMR chemical shifts of the conformers of the multistate unfolding process. During the unfolding of Trp-cage distinct intermediates evolve: a fast-exchanging intermediate is present under neutral conditions, whereas a slow-exchanging intermediate-pair emerges at acidic pH. The fast-exchanging intermediate has a native-like structure with a short α-helix in the G(11)-G(15) segment, whereas the slow-exchanging intermediate-pair presents elevated dynamics, with no detectable native-like residue contacts in which the G(11)-P(12) peptide bond has either cis or trans conformation. Heteronuclear relaxation studies combined with MD simulations revealed the source of backbone mobility and the nature of structural rearrangements during these transitions. The ability to detect structural and dynamic information about folding intermediates in vitro provides an excellent opportunity to gain new insights into the energetic aspects of the energy landscape of protein folding. Our new experimental data offer exceptional testing ground for further computational simulations.