Kinetics of folding and unfolding of goat 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.

Sequential four-state folding/unfolding of goat α-lactalbumin and its N-terminal variants

Equilibria and kinetics of folding/unfolding of α-lactalbumin and its two N-terminal variants were studied by circular dichroism spectroscopy. The two variants were wild-type recombinant and Glu1-deletion (E1M) variants expressed in Escherichia coli. The presence of an extra methionine at the N terminus in recombinant α-lactalbumin destabilized the protein by 2 kcal/mol, while the stability was recovered in the E1M variant in which Glu1 was replaced by Met1. Kinetic folding/unfolding reactions of the proteins, induced by stopped-flow concentration jumps of guanidine hydrochloride, indicated the presence of a burst-phase in refolding, and gave chevron plots with significant curvatures in both the folding and unfolding limbs. The folding-limb curvature was interpreted in terms of accumulation of the burst-phase intermediate. However, there was no burst phase observed in the unfolding kinetics to interpret the unfolding-limb curvature. We thus assumed a sequential four-state mechanism, in which the folding from the burst-phase intermediate takes place via two transition states separated by a high-energy intermediate. We estimated changes in the free energies of the burst-phase intermediate and two transition states, caused by the N-terminal variations and also by the presence of stabilizing calcium ions. The Φ values at the N terminus and at the Ca(2+)-binding site thus obtained increased successively during folding, demonstrating the validity of the sequential mechanism. The stability and the folding behavior of the E1M variant were essentially identical to those of the authentic protein, allowing us to use this variant as a pseudo-wild-type α-lactalbumin in future studies.

Can misfolded proteins be beneficial? The HAMLET case

By changing the three-dimensional structure, a protein can attain new functions, distinct from those of the native protein. Amyloid-forming proteins are one example, in which conformational change may lead to fibril formation and, in many cases, neurodegenerative disease. We have proposed that partial unfolding provides a mechanism to generate new and useful functional variants from a given polypeptide chain. Here we present HAMLET (Human Alpha-lactalbumin Made LEthal to Tumor cells) as an example where partial unfolding and the incorporation of cofactor create a complex with new, beneficial properties. Native alpha-lactalbumin functions as a substrate specifier in lactose synthesis, but when partially unfolded the protein binds oleic acid and forms the tumoricidal HAMLET complex. When the properties of HAMLET were first described they were surprising, as protein folding intermediates and especially amyloid-forming protein intermediates had been regarded as toxic conformations, but since then structural studies have supported functional diversity arising from a change in fold. The properties of HAMLET suggest a mechanism of structure-function variation, which might help the limited number of human protein genes to generate sufficient structural diversity to meet the diverse functional demands of complex organisms.

NMR studies on the equilibrium unfolding of ketosteroid isomerase by urea

Multidimensional NMR was employed to investigate the structural changes in the urea-induced equilibrium unfolding of the dimeric ketosteroid isomerase (KSI) from Pseudomonas putida biotype B. Sequence specific backbone assignments for the native KSI and the protein with 3.5 M urea were carried out using various 3D NMR experiments. Hydrogen exchange measurements indicated that the secondary structures of KSI were not affected significantly by urea up to 3.5 M. However, the chemical shift analysis of 1H-(15)N HSQC spectra at various urea concentrations revealed that the residues in the dimeric interface region, particularly around the beta5-strand, were significantly perturbed by urea at low concentrations, while the line-width analysis indicated the possibility of conformational exchange at the interface region around the beta6-strand. The results thus suggest that the interface region primarily around the beta5- and beta6-strands could play an important role as the starting positions in the unfolding process of KSI.

α-Lactalbumin species variation, HAMLET formation, and tumor cell death

HAMLET (human alpha-lactalbumin made lethal to tumor cells) is a tumoricidal complex of apo alpha-lactalbumin and oleic acid, formed in casein after low pH treatment of human milk. This study examined if HAMLET-like complexes are present in casein from different species and if isolated alpha-lactalbumin from those species can form such complexes with oleic acid. Casein from human, bovine, equine, and porcine milk was separated by ion exchange chromatography and active complexes were only found in human casein. This was not explained by alpha-lactalbumin sequence variation, as purified bovine, equine, porcine, and caprine alpha-lactalbumins formed complexes with oleic acid with biological activity similar to HAMLET. We conclude that structural variation of alpha-lactalbumins does not preclude the formation of HAMLET-like complexes and that natural HAMLET formation in casein was unique to human milk, which also showed the highest oleic acid content.

Characterization and inhibitor discovery of one novel malonyl-CoA: Acyl carrier protein transacylase (MCAT) fromHelicobacter pylori

Type II fatty acid synthesis (FAS II) is an essential process for bacteria survival, and malonyl-CoA:acyl carrier protein transacylase (MCAT) is a key enzyme in FAS II pathway, which is responsible for transferring the malonyl group from malonyl-CoA to the holo-ACP by forming malonyl-ACP. In this work, we described the cloning, characterization and enzymatic inhibition of a new MCAT from Helicobacter pylori strain SS1 (HpMCAT), and the gene sequence of HpfabD was deposited in the GenBank database (Accession No. AY738332 ). Enzymatic characterization of HpMCAT showed that the K(m) value for malonyl-CoA was 21.01+/-2.3 microM, and the thermal- and guanidinium hydrochloride-induced unfolding processes for HpMCAT were quantitatively investigated by circular dichroism spectral analyses. Moreover, a natural product, corytuberine, was discovered to demonstrate inhibitory activity against HpMCAT with IC(50) value at 33.1+/-3.29 microM. Further enzymatic assay results indicated that corytuberine inhibits HpMCAT in an uncompetitive manner. To our knowledge, this is the firstly reported MCAT inhibitor to date. This current work is hoped to supply useful information for better understanding the MCAT features of H. pylori strain, and corytuberine might be used as a potential lead compound in the discovery of the antibacterial agents using HpMCAT as target.

A novel folding intermediate state for apolipoprotein A-I: role of the amino and carboxy termini

Intramolecular interactions between the amino and carboxy termini of apolipoprotein A-I (apoAI) are believed to stabilize the helix bundle conformation of the protein. During lipid assembly the protein undergoes conformational changes that result in an exposure of the carboxy terminus and its insertion into the lipid phase. To determine the role of the two termini in the energetics of unfolding, we studied the guanidine-hydrochloride-induced unfolding and refolding of apoAI as well as its N-terminal deletion (del[1-43]), C-terminal deletion (del[186-243]), and the double deletion containing only the central residues 44-185. Thermodynamic analysis of the equilibrium unfolding measured by fluorescence spectroscopy revealed the presence of an intermediate unfolded state (I(equil)) in addition to the native (N) and unfolded states. Refolding kinetics of apoAI, measured by stopped-flow circular dichroism, revealed two kinetic intermediates, I(burst) and I(recovery). Computer modeling suggested that the first resembles the partially unfolded protein, whereas the second overlaps with the native state of the protein. The free energy changes for the N --> I(equil) transition of the N-terminal and double deletions were lower then that of the full-length form, whereas that for the C-terminal deletion was higher. Our findings suggest that the N-terminus of apoAI stabilizes the native state of the protein by increasing the Eyring energy barrier for the N --> I(equil) unfolding transition; whereas the carboxyl terminus destabilizes that state.

Tryptophan to phenylalanine substitutions allow differentiation of short- and long-range conformational changes during denaturation of goat α-lactalbumin

To test the occurrence of local particularities during the unfolding of Ca2+-loaded goat alpha-lactalbumin (GLA) we replaced Trp60 and -118, either one or both, by Phe. In contrast with alternative studies, our recombinant alpha-lactalbumins are expressed in Pichia pastoris and do not contain the extra N-terminal methionine. The substitution of Trp60 leads to a reduction of the global stability. The effect of the Trp118Phe substitution on the conformation and stability of the mutant, however, is negligible. Comparison of the fluorescence spectra of these mutants makes clear that Trp60 and -118 are strongly quenched in the native state. They both contribute to the quenching of Trp26 and -104 emission. By the interplay of these quenching effects, the fluorescence intensity changes upon thermal unfolding of the mutants behave very differently. This is the reason for a discrepancy of the apparent transition temperatures derived from the shift of the emission maxima (Tm,Fl lambda) and those derived from DSC (Tm,DSC). However, the transition temperatures derived from fluorescence intensity (Tm,Fl int) and from DSC (Tm,DSC), respectively, are quite similar, and thus, no local rearrangements are observed upon heat-induced unfolding. At room temperature, the occurrence of specific local rearrangements upon GdnHCl-induced denaturation of the different mutants is deduced from the apparent free energies of their transition state obtained from stopped-flow fluorescence measurements. By phi-value analysis it appears that, while the surroundings of Trp118 are exposed in the kinetic transition state, the surroundings of Trp60 remain native.