Structure of -lactalbumin and its fluctuation

Serological studies on heat-induced interactions of α-lactalbumin and milk proteins

The heat denaturation of α-lactalbumin (α-la) in NaCl and KCl solutions, milk ultrafiltrate and milk was studied using the method of micro complement fixation. It was established that this protein was very resistant to heat denaturation and that it was more stable in milk ultrafiltrate than in the other media studied at temperatures up to 70 °C. Of the various milk proteins added to α-la, only β-lactoglobulin (β-lg) formed a heat-induced complex with this protein. This complex was identical in milk ultrafiltrate or in milk and depended on the molar ratio between both proteins; it was not modified by any other milk proteins. The binding of a-la to β-lg changed the ability of the latter protein to bind κ-casein.

A calorimetric study of the thermal denaturation of whey proteins in simulated milk ultrafiltrate

Differential scanning calorimetry (DSC) was used to study thermal transitions of the following whey proteins and enzymes in milk ultrafiltrate solution: β-lactoglobulin, α-lactalbumin, serum albumin, γ-globulin, apo- and Fe-lactoferrin, lysozyme, ribonuclease, α-chymotrypsin and xanthine oxidase. Denaturation enthalpies (ΔHD), denaturation temperatures (TD) and the half width of the denaturation peaks in DSC thermograms (ΔT½D) were determined and the degree of renaturation was estimated by rescanning previously denatured samples. A fair correlation between the results obtained by DSC and other more classical methods was found in general. However, for some proteins (α-lactalbumin, lysozyme, ribonuclease and xanthine oxidase), which have so far been considered relatively thermostable, calorimetry reveals conformational changes starting at temperatures as low as about 45 °C. In these cases thermostability observed after heat treatment of milk should be interpreted in terms of renaturation and not of high temperatures of denaturation.

[Functional fluctuation]

The discovery of the double-helical structure of DNA, the elucidation of the genetic code, and the determination of the three-dimensional structure of several proteins are some of the outstanding achievements of biochemistry and life sciences in the latter half of the last century. Proteins play key roles in almost all the biological processes and the biological function of a protein depends on its conformation which is defined as the three-dimensional arrangement of the atoms of a molecule. The three-dimensional structure, however, is not rigid but fluctuated. Structural fluctuation plays an important role in bio-macromolecules. How about "functional fluctuation" in biological systems? The present review proposes that functional fluctuation is also very important for understanding the mechanism of supramolecules, biological processes in living cells, and the interaction between biological systems. This new theme is pretty well supported by our recent experiments for neuro-immune crosstalk, gene transfection with cationic liposomes, and cell signaling in embryonic stem cells.

Empirical studies of protein secondary structure by vibrational circular dichroism and related techniques. Alpha-lactalbumin and lysozyme as examples

Vibrational circular dichroism (VCD) has been shown to be sensitive to secondary structure in proteins and peptides and has been used as the basis for quantitative secondary-structure-prediction algorithms. However, the accuracy of these algorithms is not matched by the apparent qualitative sensitivity of the VCD spectra. This report provides examples of the use of VCD to follow structural change spectrally and to clarify the qualitative nature of the structural changes underlying the spectral variation. The VCD spectra and the complementary UV electronic CD (ECD) and FTIR spectra of alpha-lactalbumin (LA) have been studied as a function of pH, denaturation, Ca2+ ion and solvent conditions for several species. Spectral data for lysozyme were compared with those of LA because of their very similar crystal structures. In fact, these proteins in D2O-based pH 7 solution have quite different spectra using these optical techniques. Even for the LA proteins, the human differs from the bovine and goat species. Furthermore, under low pH conditions, where the LAs are in a reversibly denatured, molten globule form, the spectra are more similar, species variation is minimal and the spectral differences from lysozyme are in fact smaller. Our data are consistent with native, pH 7, alpha-lactalbumin having a less well organized structure than lysozyme, possibly in a dynamic sense. Conversely, in the low-pH, molten globule form of LA, tertiary structure is lost which could relax constraints that might distort the helical segments in the native form. The differences between the interpretation of our results and those from X-ray and NMR data may be due to motional sampling of various geometries in LA which all contribute to the spectral signatures seen in optical spectra but whose contributions are washed out in NMR or frozen out in the crystal structure. Part of this flexibility may relate to the rather large 3(10)-helical content in the LA protein structure. Fluctionality may have specific functional effects, perhaps allowing LA to bind better to beta-galactosyl transferase and form the biologically active lactose synthetase complex.

Hydrogen exchange of the tryptophan residues in bovine, goat, guinea pig, and human alpha-lactalbumin

Hydrogen exchange of the individual tryptophan residues of bovine, goat, guinea pig, and human alpha-lactalbumin has been studied by both ultraviolet and NMR spectra. The assignment of the slowly exchanging imino proton resonances to the tryptophan residues (Trp26 and Trp60) was obtained by comparison of the nuclear Overhauser effect difference spectra of bovine, guinea pig, and human alpha-lactalbumin. Taking account of the thermal unfolding of each alpha-lactalbumin, the hydrogen exchange rates of the individual tryptophan residues are analyzed. The temperature dependence of the exchange rates classified their exchange mechanisms into two exchange processes: the "low activation energy process" and the "high activation energy process" which is associated directly with the global thermal unfolding of the protein. Trp26 of alpha-lactalbumin exchanges through the high activation energy process. The exchange behavior of Trp26 of guinea pig alpha-lactalbumin suggests a difference of the globally unfolded state of the protein from the other species. The exchange mechanism of Trp60 of human alpha-lactalbumin is the low activation energy process in contrast with those of the bovine and goat proteins, although their global thermodynamic properties are similar to each other. Trp104 and Trp118 of alpha-lactalbumin exchange through the low activation energy process, and the reaction rates are affected by the local structural differences around the tryptophan residues among these proteins. The results presented in this paper indicate that the hydrogen exchange rate through the low activation energy process provides the information only about the local nature of a protein while that through the high activation energy process provides the information about the global nature of a protein.

Metal-ion binding and the molecular conformational properties of alpha lactalbumin

Mammary galactosyltransferase and alpha lactalbumin are the two protein components of lactose synthase which catalyze the transfer of galactose from UDP-gal to glucose in the presence of divalent cations. Recent studies suggest that alpha lactalbumin may have a broader function in modifying cell surface carbohydrates in cell-cell interactions and cell differentiation. Since the discovery that alpha lactalbumin, like galactosyltransferase, is a metalloprotein, there has been a great deal of interest in the metal-binding properties of this protein and how these relate to the metal-ion requirements of the lactose synthase reaction. The recent availability of an X-ray crystal structure of alpha lactalbumin has provided further impetus for establishing the molecular determinants of its biological activity. This review is directed toward critically examining and integrating our present knowledge of the properties of this protein, particularly the relationship between metal-ion binding and conformational state, and how these might relate to its biological function.

Comparison of the transient folding intermediates in lysozyme and alpha-lactalbumin

Refolding kinetics of two homologous proteins, lysozyme and alpha-lactalbumin, were studied by following the time-dependent changes in the circular dichroism spectra in the aromatic and the peptide regions. The refolding was initiated by 20-fold dilution of the protein solutions originally unfolded at 6 M guanidine hydrochloride, at pH 1.5 for lysozyme and pH 7.0 for alpha-lactalbumin at 4.5 degrees C. In the aromatic region, almost full changes in ellipticity that were expected from the equilibrium differences in the spectra between the native and unfolded proteins were observed kinetically. The major fast phase of lysozyme folding has a decay time of 15 s. The decay time of alpha-lactalbumin depends on the presence or absence of bound Ca2+: 10 s for the holoprotein and 100 s for the apoprotein. In the peptide region, however, most of the ellipticity changes of the two proteins occur within the dead time (less than 3 s) of the present measurements. This demonstrates existence of an early folding intermediate which is still unfolded when measured by the aromatic bands but has folded secondary structure as measured by the peptide bands. Extrapolation of the ellipticity changes to zero time at various wavelengths gives a spectrum of the folding intermediate. Curve fitting of the peptide spectra to estimate the secondary structure fractions has shown that the two proteins assume a similar structure at an early stage of folding and that the intermediate has a structure similar to that of partially unfolded species produced by heat and, for alpha-lactalbumin, also by acid and a moderate concentration of guanidine hydrochloride.(ABSTRACT TRUNCATED AT 250 WORDS)

Hydrogen-deuterium exchange studies on guanidinated pig heart lactate dehydrogenase

Pig heart lactate dehydrogenase becomes more thermostable on increasing the degree of guanidination (conversion of lysine to homoarginine) (Minotani, N., Sekiguchi, T., Bautista, J.G. and Nosoh, Y. (1979) Biochim. Biophys. Acta 581, 334-341). The conformational change of the protein on guanidination was then examined by hydrogen-deuterium (H-2H) exchange reactions. It ws found that (i) the fluctuation degrees of peptides and tyrosine and tryptophan residues in the protein decrease in that order, (ii) two H-2H exchangeable tryptophan residues per subunit are freely accessible to solvent and the fluctuation degrees of the residues does not change on guanidination, (iii) the H-2H exchange detectable tyrosine residues are not freely accessible to solvent and become less fluctuating when 15 lysine residues per subunit are guanidinated, and (iv) the peptides become much less fluctuating on increasing the degree of guanidination. The specific activity of the enzyme decreased on guanidination. The increased thermostability of the protein on guanidination may be related to the decrease in flexibility of the molecular structure by sacrificing the enzyme activity.

Hydrogen exchange and the dynamic structure of proteins

In native proteins, buried, labile protons undergo isotope exchange with solvent hydrogens, but the kinetics of exchange are markedly slower than in unfolded polypeptides. This indicates that, whereas buried protein atoms are shielded from solvent, the protein fluctuates around the time average structure and occasionally exposes buried sites to solvent. Generally, hydrogen exchange studies are designed to characterize the nature of the fluctuations between conformational substates, to monitor the shift in conformational equilibria among protein substates due to ligand binding or other factors, or to monitor the major cooperative denaturation transition. In this article, we review the recent reports of hydrogen exchange in proteins, focusing on recent advances in methodology, especially with regard to the implications of the results for the mechanism of hydrogen exchange in folded proteins.

Thermodynamics of alpha-lactalbumin unfolding

Thermodynamic investigations of alpha-lactalbumin have been performed by isothermal calorimetric guanidine hydrochloride titrations as well as by scanning calorimetric measurements in the presence and absence of guanidine hydrochloride. Compared with lysozyme, alpha-lactalbumin is less stable, and its changes of enthalpy and heat capacity at unfolding are lower. Thermal unfolding of alpha lactalbumin can be described to the first approximation by the two-state transition model even in the presence of guanidine hydrochloride.

Interaction of alpha-lactalbumin with dimyristoyl phosphatidylcholine vesicles. I. A microcalorimetric and fluorescence study

alpha-Lactalbumin and dimyristoyl phosphatidylcholine were used as a prototype to study the influence of a protein conformational change, induced by the pH, on the interaction between that protein and a phospholipid. The enthalpy changes associated with the interaction of alpha-lactalbumin with dimyristoyl phosphatidylcholine vesicles were measured as a function of the molar ratio of phospholipid to protein, pH and temperature. Gel-filtration, electron-microscopic and fluorescence data for the same experimental conditions were also obtained. At pH 4 and 5, the enthalphy changes (delta H) are not only larger than at physiological pH, but also show a maximum at aobut 23 degrees C in the delta H vs. temperature graph. At pH 6 and 7, on the contrary, delta H increases with decreasing temperature without a maximum in the curve. Gel-chromatographic and electron-microscopic data show that at pH 6 and 7, the morphological characteristics of the vesicles are unchanged upon addition of alpha-lactalbumin, while at pH 4 and 5 at 23 degrees C an extra peak appears in the gel-filtration graphs between the pure vesicles and alpha-lactalbumin. The new fraction contains lipid-protein complexes. Electron micrographs show that bar-shaped entities are formed. A red shift at 23 degrees C and a blue shift at 37 degrees C, both to 336 nm, are observed for lambda max of the fluorescence emission spectra at pH 4 when alpha-lactalbumin is brought into contact with the phospholipid. At the same time, a strong increase in the fluorescence intensity is observed. The chromatographic and fluorescence data indicate that a lipid-protein complex with a molar ratio of approx. 80 is formed. At pH 7 and different temperatures, the emission maximum remains at the wavelength of pure alpha-lactalbumin, the change in the fluorescence intensity, however, indicates that interaction with the lipid occurs. The results can be explained on the basis of an electrostatic interaction at pH 6 and 7, and a hydrophobic interaction at pH 4 and 5.

Equilibrium and kinetics of the thermal unfolding of alpha-lactalbumin. The relation to its folding mechanism

The thermal unfolding of alpha-lactalbumin has been studied by equilibrium measurements of aromatic difference spectra, and by kinetic measurements of the Joule heating temperature-jump. The unfolding at neutral pH is a reversible two-state transition. The equilibrium transition curves are analyzed by the nonlinear squares method, which gives correct values of thermodynamic parameters based on the data in a wide range of temperature. The results are discussed in relation to the previous studies on the unfolding by guanidine hydrochloride or by acid. The thermally unfolded state, a partially unfolded species, is shown to be thermodynamically similar to but not identical with the acid state. The folding pathway deduced from the kinetic results is essentially consistent with the folding model of alpha-lactalbumin proposed previously. Large decreases in entropy and in heat capacity during the reversed activation suggest the packing of the folded segments by hydrophobic interactions, while the forward activation shows a marked temperature dependence, probably caused by the disruption of specific long-range interactions.

Structure and fluctuation of a Streptomyces subtilisin inhibitor

The kinetics of the hydrogen-deuterium exchange reaction in a subtilisin inhibitor from Streptomyces albogriseolus has been examined by infrared absorption measurement in aqueous solutions at various pH values and temperatures. In the analysis of each piece of kinetic data, it was assumed that the total 104 peptide hydrogen atoms are classified into three kinetic classes A, B1, and B2, and that the sizes of these classes are 72, 15, and 17, respectively at every pH and at every temperature examined. On the basis of the peak position determined for the amide II band in each stage of the exchange reaction, an approximate assignment was suggested of the A, B1 and B2 respectively to an unordered structure, a beta-structure,and an alpha-helical structure in the molecule. This assignment was supported by infrared absorption measurement of a film of this protein and by circular dichroic study of the solutions. On the basis of the temperature effect on the hydrogen-exchange rate constants and on the basis of ultraviolet absorption study in the higher temperature region (40 to 90 degrees C), a discussion has been made on the nature of the fluctuation of the molecular structure of this protein.

Peptide hydrogen exchange rates in Streptomyces subtilisin inhibitor

The exchange reaction of the peptide NH protons of a microbial protease inhibitor (Streptomyces subtilisin inhibitor) with deuterium atoms in 2H2O (p2H 6.8) has been studied by proton magnetic resonance in the temperature range 56-71 degrees C. Both slowly and rapidly exchanging processes have been observed. The number of slowly exchanging protons is estimated to be 25 +/- 2 per subunit of the protein molecule. The decay of the slowly exchanging proton signals follows a single time-exponential function at each temperature. The observed first-order rate constants have been analyzed to give the denaturated fraction of the protein as a function of temperature with a consequent enthalpy (56 kcal/mol) and an entropy (137 cal/degree per mol) of denaturation. The results indicate the high conformational stability of this protein against heat denaturation.

Proton-magnetic-resonance spectroscopic study of the histidine residues of bovine alpha-lactalbumin

A study of the three histidine residues of bovine alpha-lactalbumin has been made using proton magnetic resonance (PMR) spectroscopy in order to obtain information on their environments in the protein and thereby to test in part the previously proposed structure. PMR titration curves are obtained for the H-4 resonances using difference spectroscopy and for the H-2 resonances and the 1-H-2-H exchange rates of the H-2 protons have been measured. The assignment of resonances to particular histidine residues is achieved by utilising their selective reaction with iodoacetate in conjunction with a PMR study of the carboxymethylation of alpha-N-acetyl-L-histidine. The H-2 and H-4 resonances labelled 1, 2 and 3 starting from the downfield end of the spectrum are assigned to histidine residues 107, 68 and 32 respectively. Their apparent pK values at low ionic strength and 20 degrees C are 5.78, 6.49 and 6.51 respectively. The experimental results on two histidine residues are consistent with the predictions of the proposed structure, which indicate that histidine-68 is an external residue and histidine-32 is partially buried and in the vicinity of aromatic residues. The experimental data on histidine 107 can also be rationalised with less certainty in terms of the proposed structure, which indicates a partially buried residue that may be involved in hydrogen bonding.