Hydrophobic interaction of lysozyme and alpha-lactalbumin from equine milk whey.

From fluorescence measurements on mixtures of bis-ANS and equine lysozyme and from Ca(2+)-dependent hydrophobic interaction chromatography of equine lysozyme, it is demonstrated that Ca2+ binding induces a conformational change upon which hydrophobic regions in the protein become less accessible. Bis-ANS fluorescence titrations in the absence of Ca2+ and in 2 mM Ca2+ are also performed with equine alpha-lactalbumin variants B and C. These variants differ by an amino-acid exchange Asp----Ile at residue 95. The fluorescence titration curves indicate that the accessibility of the probe to the Ca2+ conformers is clearly influenced by the mutation. The Ca(2+)-dependent exclusion of a hydrophobic domain is used in a new and simplified method for preparing lysozyme and alpha-lactalbumins simultaneously from equine milk whey.     

Calcium-binding lysozymes

It was found that pigeon lysozyme binds one calcium ion, as does equine lysozyme. The protein was eluted with equimolar calcium ions from a Bio-Gel P-60 column. The binding constants of equine and pigeon lysozymes were determined to be 2 x 10(6) and 1.6 x 10(7) M-1, respectively, in 0.1 M KCl at pH 7.1 and 20 degrees C. During evolution the gene of calcium-binding lysozyme is deduced to be separated from that of non-calcium-binding lysozyme by gene duplication before splitting of avian and mammalian lineages, from their amino-acid sequences. It is assumed that the alpha-lactalbumin might have evolved from calcium-binding lysozyme.     

The complete primary structure of alpha-lactalbumin isolated from pig (Sus scrofa) milk

The complete amino-acid sequence of pig alpha-lactalbumin has been determined. It was obtained by microsequencing of the native protein and the peptides derived after tryptic or cyanogen bromide cleavage. The tryptic peptides were separated by a rapid microbore HPLC method. Pig alpha-lactalbumin is 122 amino acids long and differs from the bovine homologue by 26 exchanged residues. Of the two prolines present in bovine alpha-lactalbumin, one has been deleted in the pig structure. All previously sequenced alpha-lactalbumins have shown glutamic acid at position 49, which is known to be the active site in the homologous lysozyme c structure. This residue is replaced by phenylalanine in pig alpha-lactalbumin indicating that the pig protein is the first alpha-lactalbumin with complete loss of all lysozyme functional residues.     

Raman optical activity characterization of native and molten globule states of equine lysozyme: comparison with hen lysozyme and bovine alpha-lactalbumin

Vibrational Raman optical activity (ROA) spectra of the calcium-binding lysozyme from equine milk in native and nonnative states are measured and compared with those of the homologous proteins hen egg white lysozyme and bovine alpha-lactalbumin. The ROA spectrum of holo equine lysozyme at pH 4.6 and 22 degrees C closely resembles that of hen lysozyme in regions sensitive to backbone and side chain conformations, indicating similarity of the overall secondary and tertiary structures. However, the intensity of a strong positive ROA band at approximately 1340 cm(-1), which is assigned to a hydrated form of alpha helix, is more similar to that in the ROA spectrum of bovine alpha-lactalbumin than hen lysozyme and may be associated with the greater flexibility and calcium-binding ability of equine lysozyme and bovine alpha-lactalbumin compared with hen lysozyme. In place of a strong sharp positive ROA band at approximately 1300 cm(-1) in hen lysozyme that is assigned to an alpha helix in a more hydrophobic environment, equine lysozyme shows a broader band centered at approximately 1305 cm(-1), which may reflect greater heterogeneity in some alpha-helical sequences. The ROA spectrum of apo equine lysozyme at pH 4.6 and 22 degrees C is almost identical to that of the holo protein, which indicates that loss of calcium has little influence on the backbone and side chain conformations, including the calcium-binding loop. From the similarity of their ROA spectra, the A state at pH 1.9 and both 2 and 22 degrees C and the apo form at pH 4.5 and 48 degrees C, which are partially folded denatured (molten globule or state A) forms of equine lysozyme, have similar structures that the ROA suggests contain much hydrated alpha helix. The A state of equine lysozyme is shown by these results to be more highly ordered than that of bovine alpha-lactalbumin, the ROA spectrum of which has more features characteristic of disordered states. A positive tryptophan ROA band at approximately 1551 cm(-1) in the native holo protein disappears in the A state, which is probably due to the presence of nonnative conformations of the tryptophans associated with a previously identified cluster of hydrophobic residues.     

Protein dissection experiments reveal key differences in the equilibrium folding of alpha-lactalbumin and the calcium binding lysozymes

The alpha-lactalbumins and c-type lysozymes have virtually identical structure but exhibit very different folding behavior. All alpha-lactalbumins form a well populated molten globule state, while most of the lysozymes do not. alpha-Lactalbumin consists of two subdomains, and the alpha-subdomain is considerably more structured in the molten globule state than the beta-subdomain. Constructs derived from the alpha-subdomain of human alpha-lactalbumin containing the A, B, D, and 3(10) helices are known to form a molten globule state in the absence of the rest of the protein (Demarest, S. et al. (1999) J. Mol. Biol. 294, 213-221). Here we reported comparative studies of constructs derived from the same regions of canine and equine lysozymes. These proteins form two of the most stable molten globule states among all the lysozymes. A construct containing the A, B, D, and 3(10) helices of equine lysozyme is partially helical but is less structured than the corresponding human alpha-lactalbumin peptide. Addition of the C-helix leads to a construct that is still less structured and less stable than the alpha-lactalbumin construct. The corresponding construct from canine lysozyme is also less structured and less stable than the alpha-lactalbumin peptide. Thus, molten globule formation in human alpha-lactalbumin can be driven by the isolated alpha-subdomain, while more extensive interactions are required to generate a stable molten globule in the two lysozymes. The stability of the canine and equine lysozyme constructs is similar, indicating that the extraordinary stability of the canine lysozyme molten globule is not due to an unusually stable isolated alpha-subdomain.     

The primary structure of Bacillus cereus neutral proteinase and comparison with thermolysin and Bacillus subtilis neutral proteinase

The complete amino-acid sequence of a neutral proteinase, produced by Bacillus cereus, was determined by protein sequencing. The neutral proteinase consists of 317 amino-acid residues. The primary structure is 70% homologous to thermolysin, a thermostable neutral proteinase and 45% homologous to Bacillus subtilis neutral proteinase. The zinc-binding site and the hydrophobic pocket of the active site are highly similar in all three proteinases. B. cereus neutral proteinase which is 20 degrees C less thermostable (60 degrees C) than thermolysin (80 degrees C) shows only minor differences in calcium binding sites and salt bridges compared to thermolysin (known from its X-ray diffraction analysis), whereas B. subtilis neutral proteinase (50 degrees C) differs considerably. Therefore it was assumed that the difference in thermostability between B. cereus neutral proteinase and thermolysin is not caused by different metal binding properties, or differences in the active site, but by changes within the rest of the molecule. Calculation of secondary structure potentials according to Chou & Fasman, hydrophobicity and bulkiness of the different structural elements and preferred cold----hot amino-acid residue exchanges indicated, that the thermostability of thermolysin compared to B. cereus neutral proteinase is caused by small effects contributed by numerous amino-acid exchanges distributed over the whole molecule, resulting in increased hydrophobicity of beta-pleated sheet and higher bulkiness of alpha-helical regions.     

The molten globule state of a chimera of human alpha-lactalbumin and equine lysozyme.

The molten globule state of equine lysozyme is more stable than that of alpha-lactalbumin and is stabilized by non-specific hydrophobic interactions and native-like hydrophobic interactions. We constructed a chimeric protein which is produced by replacing the flexible loop (residues 105-110) in human alpha-lactalbumin with the helix D (residues 109-114) in equine lysozyme to investigate the possible role of the helix D for the high stability and native-like packing interaction in the molten globule state of equine lysozyme. The stability of the molten globule state formed by the chimeric protein to guanidine hydrochloride-induced unfolding is the same as that of equine lysozyme and is substantially greater than that of human alpha-lactalbumin, although only six residues come from equine lysozyme. Our results also suggest that the non-native interaction in the molten globule state of alpha-lactalbumin changes to the native-like packing interaction due to helix substitution. The solvent-accessibility of the Trp residues in the molten globule state of the chimeric protein is similar to that in the molten globule state of equine lysozyme in which packing interaction around the Trp residues in the native state is partially preserved. Therefore, the helix D in equine lysozyme is one of the contributing factors to the high stability and native-like packing interaction in the molten globule state of equine lysozyme. Our results indicate that the native-like packing interaction can stabilize the rudimentary intermediate which is stabilized by the non-specific hydrophobic interactions.     

The amino acid sequence of equine alpha-lactalbumin

The amino acid sequence of equine alpha-lactalbumin has been determined with the aid of an automatic sequencer. The protein chain consists of 123 amino acids and has a Mr of 14218. Elucidation of the structure involved sequence determination of native protein (residues 1-32), cyanogen bromide fragments, and tryptic, chymotryptic and S. aureus V8 proteolytic peptides. Approximately 67% of the residues are identical with corresponding residues of bovine alpha-lactalbumin B, and there is close homology with alpha-lactalbumin of other species.     

Separation and partial characterization of guinea-pig caseins.

1. Guinea-pig caseins A, B and C were purified free of each other by a combination of ion-exchange chromatography and gel filtration. 2. Determination of the amino acid composition showed all three caseins to contain a high proportion of proline and glutamic acid, but no cysteine. This apart, the amino acid composition of the three caseins was markedly different, though calculated divergence values suggest that some homology may exist between caseins A and B. Molecular-weight estimates based on amino acid composition were in good agreement with those based on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. 3. N-Terminal analysis showed lysine, methionine and lysine to be the N-terminal residues of caseins A, B and C respectively. 4. Two-dimensional separation of tryptic digests revealed a distinctive pattern for each casein. 5. All caseins were shown to be phosphoproteins. The casein C preparation also contained significant amounts of sialic acid, neutral and amino sugars. 6. The results suggest that each casein represents a separate gene product, and that the low-molecular-weight proteins are not the result of a post-translational cleavage of the largest. All were distinctly different from the whey protein alpha-lactalbumin.     

The expression of alpha D-chains in the hemoglobin of adult ostrich (Struthio camelus) and American rhea (Rhea americana). The different evolution of adult bird alpha A-, alpha D- and beta-chains

The hemoglobin of adult American rhea (Rhea americana) and ostrich (Struthio camelus) contains two components identified to be HbA (alpha 2A beta 2) and HbD (alpha 2D beta 2). The amino-acid sequence of alpha D-chains from HbD of adult American rhea and ostrich has been determined. The sequence was studied by Edman degradation of tryptic peptides and chemical cleavage products in a liquid phase sequencer. By homologous comparison with pheasant HbD (Phasianus colchicus colchicus), the alpha D-chains of American rhea differ by 28 amino-acid exchanges, the alpha D-chains of ostrich by 23 residues. These differences are higher than those observed for alpha A- as well as for beta-chains of HbA from the same species. The ratio of amino-acid exchanges for beta:alpha A:alpha D for American rhea and ostrich is found to be 1:5.5:6.5. At present the reason for the differences in evolution rates for the beta-, alpha A- and alpha D-chains of bird hemoglobins is still unclear.     

Structure and function of L-lactate dehydrogenases from thermophilic and mesophilic bacteria, XI. Engineering thermostability and activity of lactate dehydrogenases from bacilli

An extensive comparative structural analysis of lactate dehydrogenase (LDH) sequences from thermophilic, mesophilic and psychrophilic bacilli revealed characteristic primary structural differences. These specific amino-acid substitutions were found in the entire LDH molecule. However, in certain regions of the LDH an accumulation of these exchanges could be detected. These regions seem to be particularly important for the temperature adaptation of the enzyme. The influence of one of such regions at the N-terminus on stability and activity of LDHs was analysed by the construction of hybrid mutants between LDH sequences from thermophilic, mesophilic and psychrophilic bacilli and also by site-directed mutagenesis experiments at five different positions. The substitutions of Thr-29 or Ser-39 to Ala residues in the LDH from the mesophilic B. megaterium increased the thermostability of the enzyme drastically (15 degrees C). An increase of 20 degrees C could be observed when both amino-acid substitutions were introduced. These amino-acid substitutions resulted in an increase of Km for pyruvate and led to a three-fold reduction of the activity (kcat/Km) at 40 degrees C compared with the wild type enzyme. The influence of these amino-acid substitutions was also investigated in the LDHs from thermophilic and psychrophilic bacilli. The high heat resistance of the LDH from the thermophilic B. stearothermophilus was not altered by the Ala to Thr and Ser substitutions at positions 29 and 39, respectively. This indicates a cooperatively stabilized conformation of this LDH. However, in this mutant of the B. stearothermophilus LDH the activity (kcat/Km) was increased two-fold.     

Multiple forms of ubiquitin-protein ligase. Binding of activated ubiquitin to protein substrates

Enzyme activities that catalyzed the covalent attachment of ubiquitin to protein substrates (ubiquitin-protein ligase, UbL) were purified from the extracts of human red blood cells. These activities required the presence of ubiquitin-activating enzyme and ATP for activity. Four fractions (UbL A, B1, B2, and C) were resolved and showed different specificities toward added substrates [carboxymethylated bovine serum albumin (CM-BSA), G-actin, lysozyme, and alpha-lactalbumin]. The enzyme fractions gave different products with a given substrate. UbL A and UbL B1 were exclusively active with CM-BSA and alpha-lactalbumin, respectively. UbL B2 was most active toward CM-BSA with substantial activities to G-actin and alpha-lactalbumin and with no activity to lysozyme. UbL C showed significant activities with all four substrates, having a highest activity toward CM-BSA. There were many endogenous proteins present in the erythrocyte extract which were efficient substrates for ubiquitin conjugation. In particular, a pair of substrates were identified from erythrocyte extracts which were far more efficient substrates than the denatured proteins exogenously added.     

Effects of a helix substitution on the folding mechanism of bovine alpha-lactalbumin

The structure, stability, and unfolding-refolding kinetics of a chimeric protein, in which the amino acid sequence of the flexible loop region (residues 105-110) comes from equine lysozyme and the remainder of the sequence comes from bovine alpha-lactalbumin were studied by circular dichroism spectroscopy and stopped-flow measurements, and the results were compared with those of bovine alpha-lactalbumin. The substitution of the flexible loop in bovine alpha-lactalbumin with the helix D of equine lysozyme destabilizes the molten globule state, although the native state is significantly stabilized by substitution of the flexible loop region. The kinetic refolding and unfolding experiments showed that the chimeric protein refolds significantly faster and unfolds substantially slower than bovine alpha-lactalbumin. To characterize the transition state between the molten globule and the native states, we investigated the guanidine hydrochloride concentration dependence of the rate constants of refolding and unfolding. Despite the significant differences in the stabilities of both the molten globule and native states between the chimeric protein and bovine alpha-lactalbumin, the free energy level of the transition state is not affected by the amino acid substitution in the flexible loop region. Our results suggest that the destabilization in the molten globule state of the chimeric protein is caused by the disruption of the non-native interaction in the flexible loop region and that the disruption of the non-native interaction reduces the free energy barrier of refolding. We conclude that the non-native interaction in the molten globule state may act as a kinetic trap for the folding of alpha-lactalbumin.     

Guinea-pig milk-protein synthesis. Isolation and characterization of messenger ribonucleic acids from lactating mammary gland and identification of caseins and pre-alpha-lactalbumin as translation products in heterologous cell-free systems.

1. The major milk proteins synthesized by the lactating mammary gland of the guinea pig were identified and designated as caseins A, B and C and alpha-lactalbumin, with estimated mol.wts. of 28000, 25500, 20500 and 14500 respectively. 2. Antisera to the total casein fraction and to alpha-lactalbumin were prepared from rabbits. The milk proteins were also iodinated with either 131I or 125I. 3. A poly(A)-rich RNA fraction was isolated from lactating guinea-pig mammary glands. Isolation was by affinity chromatography on oligo(dT)-cellulose. 4. Examination of this RNA fraction by electrophoresis on polyacrylamide gels containing formamide indicated three major species 1, 2 and 3, with estimated wol.wts. of 5.4 X 10(5) and 3.3 X 10(5), and the apparent absence of rRNA species. 5. The poly(A)-rich RNA stimulated protein synthesis in heterologous cell-free systems based on wheat germ, Krebs II ascites-tumour cells, and the latter supplemented with an initiation factor-3 fraction from rabbit reticulocyte ribosomes. 6. Between 80 and 90% of the protein synthesis directed by the mRNA was for milk proteins. 7. Analysis of the proteins immunoprecipitated by the alpha-lactalbumin antiserum showed in the wheat-germ system that the product was a protein with a molecular weight greater than that of alpha-lactalbumin, whereas in the ascites-tumour-cell systems both this protein and alpha-lactalbumin were found. When the larger protein was treated with CNBr and the resulting peptides were examined, it was shown that the extra peptide was at the N-terminus. This and other evidence is adduced for the initial translation product of alpha-lactalbumin being a precursor with an addition of about ten amino acids at the N-terminus. 8. Similar analysis of the casein immlnospecific proteins produced under the direction of mRNA indicated that the products had a molecular weight that was apparently a littel smaller than that of the caseins synthesized in vivo. This was not consistent with higher-molecular weight casein precursors. 9. Possible explanations for the results obtained are discussed, especially in terms of the physiological significance of the pre-alpha-lactalbumin as a secretory protein.     

Structure and function of L-lactate dehydrogenases from thermophilic and mesophilic bacteria, IV. The primary structure of the mesophilic lactate dehydrogenase from Bacillus subtilis

The complete amino-acid sequence of lactate dehydrogenase from the mesophilic Bacillus subtilis (B. X1) was determined. Approximately 70% of the sequence was obtained by sequence analysis of intact protein (N-terminal sequence) and of four CNBr fragments (CNBr3, CNBr4, CNBr5 and CNBr6). Sequences overlapping the CNBr fragments were determined from polypeptide fragments obtained by cleavage using o-iodosobenzoic acid (cleavage at Trp) or clostripain (cleavage at Arg). The C-terminal amino-acid residue (Asn) was detected by carboxypeptidase Y-degradation. Lactate dehydrogenase from B. subtilis shows a 69% sequence homology to that from the thermophilic strain B. stearothermophilus, and a 34% sequence homology to those from higher organism. The homology of these enzymes is particularly high at the active site regions (the coenzyme and substrate binding sites). The relatively high sequence conservation of the lactate dehydrogenases from B. subtilis and B. stearothermophilus (and from other bacilli) allows a structural comparison of this temperature variants.     

Hydrogen exchange study of canine milk lysozyme: stabilization mechanism of the molten globule

The native state (1)H, (15)N resonance assignment of 123 of the 128 nonproline residues of canine milk lysozyme has enabled measurements of the amide hydrogen exchange of over 70 amide hydrogens in the molten globule state. To elucidate the mechanism of protein folding, the molten globule state has been studied as a model of the folding intermediate state. Lysozyme and alpha-lactalbumin are homologous to each other, but their equilibrium unfolding mechanisms differ. Generally, the folding mechanism of lysozyme obeys a two-state model, whereas that of alpha-lactalbumin follows a three-state model. Exceptions to this rule are equine and canine milk lysozymes, which exhibit a partially unfolded state during the equilibrium unfolding; this state resembles the molten globule state of alpha-lactalbumin but with extreme stability. Study of the molten globules of alpha-lactalbumin and equine milk lysozyme showed that the stabilities of their alpha-helices are similar, despite the differences in the thermodynamic stability of their molten globule states. On the other hand, our hydrogen exchange study of the molten globule of canine milk lysozyme showed that the alpha-helices are more stabilized than in alpha-lactalbumin or equine milk lysozyme and that this enhanced stability is caused by the strengthened cooperative interaction between secondary structure elements. Thus, our results underscore the importance of the cooperative interaction in the stability of the molten globule state.     

A comparative study of the alpha-subdomains of bovine and human alpha-lactalbumin reveals key differences that correlate with molten globule stability

The alpha-lactalbumins form stable molten globule states under a range of conditions, with the low pH form being the best characterized. The stability of the molten globule varies among different members of this family, but the origin of the stability difference is not clear. We compare the folding and stability of alpha-subdomain constructs of human and bovine alpha-lactalbumin. Previous studies have demonstrated that the isolated alpha-subdomain of human alpha-lactalbumin folds and forms a molten globule state. The minimum core construct has been defined to include the A, B, and D alpha-helices and the C-terminal 3(10) helix. A construct corresponding to the same region of bovine alpha-lactalbumin is much less structured and less stable than the human alpha-lactalbumin construct. Addition of the C-helix to generate a 75-residue bovine construct does not lead to a significant increase in structure or stability. This construct (AB-CD/3(10)) contains the entire alpha-subdomain of bovine alpha-lactalbumin. Thus molten globule formation in the human protein, but not in the bovine protein, can be rationalized on the basis of a stable alpha-subdomain. Interactions involving more of the protein chain are required to generate a well structured molten globule in the bovine protein. Comparison of AB-CD/3(10) to the molten globule formed by the intact protein and to the protein with the 6-120 disulfide reduced indicates that both the beta-subdomain and the 6-120 disulfide play a role in stabilizing the bovine alpha-lactalbumin molten globule.     

Uptake of valine and alanine by a neutral amino-acid carrier in sea urchin eggs: cyclic variations in the early cleavage stage

Summary The characteristics of valine and alanine uptake (respectively, preferential substrates of the L and A or ASC systems in mammalian cells) were studied in sea urchin eggs before and 40 min after fertilization. Substrate concentration dependence showed that in unfertilized eggs alanine is absorbed linearly up to an external concentration of 20mm, whereas valine uptake presented a saturable kinetic with aK _m of 6 m. Competition experiments showed that valine is absorbed by a carriermediated transport resembling the L system. Fertilization develops a new Na-dependent system, resembling the ASC system which is specific for neutral amino acids but does not discriminate between them. This system is superimposed on that of the unfertilized egg. In fertilized eggs, amino-acid transport displayed cyclic variations which have been previously associated with cell cleavage. We have found that eggs prevented from cleavage by treatment with antimitotic undergo a sequence of periodic amino-acid uptake timed with the mitotic cycle of untreated eggs. In addition, artificially activated eggs (A23187) which failed to divide showed a time course of amino-acid uptake similar to that observed in fertilized eggs. Furthermore, these variations are independent of protein synthesis. These results suggest to us that a biological clock exists in the cytoplasm or cortex of sea urchin eggs, which may be involved in timing the cell cycle.     

Pig beta-lactoglobulin I (Sus scrofa domestica, Artiodactyla). The primary structure of the major component

beta-Lactoglobulins from pooled milk (Sus scrofa domestica) are isolated and characterized. The complete primary structure of the major beta-lactoglobulin component I is presented. The amino-acid sequence was elucidated by automated Edman degradation of tryptic peptides and cyanogen bromide cleavage products in a liquid phase sequencer. The tryptic and cyanogen bromide peptides were separated by reverse-phase (RP-2) or size exclusion (TSK 2000 SW) high performance liquid chromatography. Pig beta-lactoglobulin is composed of only 159 amino acids in contrast to other beta-lactoglobulins which contain 162 or 166 amino acids. Sequence alignment with previously sequenced beta-lactoglobulins was obtained by introducing two gaps at positions 115 and 151-152. Thus bovine beta-lactoglobulin A reveals 62 amino-acid substitutions. The phylogenetic distance from horse beta-lactoglobulin I and II is indicated by 49.4% and 62% amino-acid exchanges, respectively. Pig beta-lactoglobulin is a mixture of two chains with Gln or Thr at position 119. The free thiol group is localized at position 59. The structural and functional aspects of beta-lactoglobulins and its role in vitamin A (retinol) transport are discussed.     

Sequence of the low activity equine erythrocyte carbonic anhydrase and delineation of the amino acid substitutions in various polymorphic forms

the sequence of the low activity form of equine erythrocyte carbonic anhydrase has been determined. The most common electrophoretic form, designated D, has been found to have five substitutions. Amino acid exchanges in the electrophoretic variants known as A1, A2, B, and T have been found at six other positions. The data do not permit calculation of the number of polymorphic forms of this enzyme. The equine D isozyme and the analogous human enzyme are quite homologous, 211 of their 260 residues, or 81%, being identical.