Biochimica et Biophysica Acta (BBA) - Protein Structure最新文献

Porcine and equine colipases have been submitted to mild tryptic digestion. Proteolysis occurs at the Arg₅Gly₆ bond with the loss of the N-terminal pentapeptide. Studies of native and trypsin-treated colipases by circular dichroism and laser chemically induced dynamic nuclear polarization indicate that proteolysis induces conformational changes in the region of the tyrosine cluster. Experiments in the presence of phospholipid provide further evidence showing that these residues are in or close to the region of the protein interacting with aggregated lipids. Kinetic studies of the reaction of bile salt-inhibited lipase with emulsified triolein in the absence and in the presence of lecithin show that tryptic hydrolysis of the protein cofactor increases its affinity for the enzyme in the presence of lipid substrate. In both cases, it was found that the apparent dissociation constant of the lipase-colipase complex is decreased by one order of magnitude. Our results confirm that the biological activity of the lipase cofactor is enhanced by specific tryptic cleavage in the amino terminal region of the polypeptide and support the suggestion by Borgström et al. (Borgström, B., Wieloch, T., Erlanson-Albertsson (1981) FEBS. Lett. 108, 407–410) that the secreted form of colipase is a precursor.

A neurotoxin, designated toxin D, was isolated from the venom of Naja naja by gel-filtration on Sephadex G-50 followed by CM-cellulose chromatography. The amino acid sequence of toxin D was determined by analyzing tryptic and chymotryptic peptides of the reduced and $\text{S-}\text{carboxymethyl}$ derivative. The venom contains at least four long (toxins A, B, C and D) and two short neurotoxins (toxins I and II) that account for about 16% of the lyophilized crude venom by weight. Toxin D consists of 71 amino acid residues in a single peptide chain cross-linked by five intramolecular disulfide bridges. The lethal toxicity of toxin D is found to be about half those of toxins A, B and C in the venom. Analysis of the amino acid sequence reveals that toxin D differs from toxin A in having -Arg-31, -Glu-35 and -Lys-49 in place of -Ser-31, -Lys-35 and -Arg-49 residues.

From the seeds of Phaseolus vulgaris cv. ‘Pinto III’, previously regarded as a hemagglutinin-free bean, several glycoprotein lectins were purified by conventional methods, including solubility- and salt-fractionation, continuous high-voltage free-flow electrophoresis and molecular sieve chromatography. A preparation of similar lectins was also obtained by immunoaffinity chromatography on a Sepharose-4B column to which rabbit anti-Pinto seed lectin (conventionally purified) immunoglobulins had been attached. Both preparations gave one band of 28 000–29 000 subunit weight on SDS-polyacrylamide gel electrophoresis. They were however shown to contain components, with isoelectric points in the range of pH 4.7–5.0, by isoelectric focusing on polyacrylamide gels. The relative proportion of the individual lectin components was slightly different in the two preparations. Their sedimentation coefficient, 4.34 S, was the same; however, they had a slight difference in partial specific volume values; 0.691 ml/g for the conventional and 0.700 ml/g for the affinity preparation. The average molecular weight value ($\text{M}{}_{\mathrm{<mtext>r,av</mtext>}}{}^0$) of 52 200 for the conventional preparation was significantly lower ($\text{P > 0.1}$) than the value of 55 000 for the affinity-purified lectin. The ‘Pinto III’ seed lectin molecules contained two subunits only in place of the usual four subunits of the common P. vulgaris lectins. There was a slight immunochemical cross-reaction between the common P. vulgaris and the ‘Pinto III’ seed lectins. The ‘Pinto III’ seed lectins had low haemagglutinating activity when tested with rabbit erythrocytes, while their activity was high against pronase-treated rat cells. It is suggested that the low activity against rabbit cells might be the result of the lower affinity of the dimeric lectin for the exposed sugar structures on the red cell membrane.

The complete amino acid sequence of a tetrahaem protein, cytochrome $\text{c}{}_3$ isolated from Desulfovibrio desulfuricans (strain Norway 4), is presented. The protein molecule consists of a single polypeptide chain of 118 residues. The sequence elucidation together with the crystallographic determination at 2.5 Å resolution which has been published provide some answers about some essential properties of this molecule. Comparison of the amino acid sequence of cytochrome $\text{c}{}_3$ from D. desulfuricans Norway with other cytochrome $\text{c}{}_3$ shows the main characteristics of the structural features of the cytochrome $\text{c}{}_3$ group.

By utilizing homologous peptide mapping and amino acid analysis of recovered peptides, it has been possible to infer much of the sequence of human triosephosphate isomerase. This has been achieved with less than 5 mg of the human enzyme. The enzyme shows a great sequence homology with the rabbit enzyme. The methods used in this study can provide a rapid procedure for gaining structural information on human enzymes available only in microgram or milligram quantities.

This article reports the low-frequency regions of resonance Raman spectra of five- and six-coordinated ferroprotoporphyrin complexes in aqueous solution with or without detergent. For high-spin complexes having their iron atom monocoordinated to variously substituted imidazoles or to dimethylformamide, the frequency of a band observed between 194 and 237 cm⁻¹ (labelled band II) primarily depends on the $\text{p}\text{K}{}_{\mathrm{<mtext>a</mtext>}}$ of the axial ligand. In the absence of steric effects from the axial ligand, the lower is the $\text{p}\text{K}{}_{\mathrm{<mtext>a</mtext>}}$ of ligand, the higher the frequency of band II. We previously assigned band II to a mode essentially involving the Fe-N(pyrrole) bonds. The above $\text{p}\text{K}{}_{\mathrm{<mtext>a</mtext>}}$ dependence is readily explained, in the frame of this assignment, in terms of a decrease in the Fe-N(pyrrole) bond strength (and of an increase in bond length) when the basicity of the axial ligand increases. On the other hand, the alternative assignment of band II to a stretching mode of Fe-N(axial ligand) is inconsistent with the observed $\text{p}\text{K}{}_{\mathrm{<mtext>a</mtext>}}$ dependence. As far as hexacoordinated complexes are concerned, specific bands are observed at 203, 194 and 176 cm⁻¹ for imidazole, 1-methylimidazole and pyridine, respectively. These bands are assigned, on the basis of isotopic substitutions, to a summetric stretching mode of the axial ligands [ν(N-Fe-N)]. Band II is observed at 265 cm⁻¹ for these low-spin complexes, a frequency expected from the short Fe-N(pyrrole) bond lengths of nearly planar ferroporphyrins.

The two forms of $\text{α}{}_2\text{HS-glycoprotein}$ were purified from Cohn fraction VI of normal human plasma and characterized in terms of their major chemical and physicochemical properties. Separation of these two proteins was achieved by chromatography on DEAE-cellulose at pH 4.4 followed by gel filtration through Sephadex G-100. The isoelectric points of the disc gel electrophoretically and immunochemically homogeneous glycoproteins were found to be at 4.1 and 4.7 and their apparent molecular weights, as determined by SDS-polyacrylamide gel electrophoresis, were shown to be 51 000 and 56 000, respectively. The amino acid compositions of both proteins were very similar, although differences, particularly in the arginine and histidine contents, were noted. The amino- and carboxyl-terminal amino acids were found to be the same for both proteins and were threonine and alanine, and valine and leucine, respectively, suggesting that both forms of this protein consist of two polypeptide chains. The total carbohydrate moiety of the relatively basic form (14%) proved to be comparable to that of the relatively acidic form (13%). More important, however, the sialic acid content of the latter was higher than that of the former. These results suggest that the difference between the two forms of $\text{α}{}_2\text{HS-glycoprotein}$ resides both in its carbohydrate and polypeptide moieties.

The kinetics of the self-association of bovine liver glutamate dehydrogenase was studied at various phosphate buffer concentrations (pH 7.3) at 11.5°C by means of the temperature-jump technique with scattered light detection. The observed relaxation times were well explained by the random association model of Thusius et al. With increasing phosphate concentration, the association rate constant derived from the model decreased, while the dissociation rate constant was left almost constant. Relaxation amplitude was also dependent on the phosphate concentration. The changes in the rate constant and relaxation amplitude with phosphate concentration are well elucidated by assuming that glutamate dehydrogenase is protected from association by specific masking of the association site by phosphate ions.

The effect of sodium dodecyl sulfate (SDS) on the spectral properties of phenylalanine residues in bovine serum albumin was studied at neutral pH by second derivative spectrophotometry. It was found that phenylalanine residues in the interior of bovine serum albumin became almost completely exposed on the surface of the protein on formation of a so-called AD₁₂ complex. This conformational change began to be significant when 4 mol SDS bound to bovine serum albumin. At higher concentrations of SDS, when so-called AD_n and AD_2n complexes were formed, phenylalanine residues were transferred to the hydrophobic region again. This might be due to the involvement of phenylalanine residues in micelle-like clusters. Change in the conformation of bovine serum albumin involving tryptophan residues was also measured. These studies demonstrate the value of second derivative spectrophotometry in studies on conformational change of proteins.