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A protein is usually classified into one of the following four structural classes: all α, all β, (a + B) and α/β. In this paper, based on the maximum correlation-coefficient principle, a new formulation is proposed for predicting the structural class of a protein according to its amino acid composition. Calculations have been made for a development set of proteins from which the amino acid compositions for the standard structural classes were derived, and an independent set of proteins which are outside the development set. The former can test the self consistency of a method and the latter can test its extrapolating effectiveness. In both cases, the results showed that the new method gave a considerably higher rate of correct prediction than any of the previous methods, implying that a significant improvement has been achieved by implementing the maximum-correlation-coefficient principle in the new method. [ABSTRACT FROM AUTHOR]

The complete amino acid sequence of bovine phospholipase A2 (EC 3.1.1.4) was determined. This enzyme has a molecular weight of 13 782 and consists of a single polypeptide chain of 123 amino acids cross-linked by seven disulfide bridges. The main fragmentation of the polypeptide chain was accomplished by digesting the reduced and thialaminated derivative of the protein with trypsin, staphylococcal protease and cyanogen bromide. A number of chymotryptic peptides were used for alignment and to obtain overlaps of at least two residues. The sequence of the peptides was determined by Edman degradation by means of direct phenylthiohydantoin identification in combination with identification as dansyl amino acids. Although 71% of all residues of phospholipase A2 from bovine, porcine and equine sources are conserved, bovine phospholipase A2 differs from the others by the total number of residues and by substitutions at 20 (porcine) and 33 (equine) positions. [ABSTRACT FROM AUTHOR]

The S′-subsite specificity of bovine pancreatic α-chymotrypsin was investigated by acyl transfer reactions using a series of amino-acid- and peptide-derived nucleophiles. The nucleophilic efficiency covers a range of more than three orders of magnitude, reflecting the specificity of the acyl transfer process. Positively charged H-Arg-NH2 was the most efficient nucleophile of the series while peptides with free carboxyl groups show poor nucleophilic behaviour. This is explained by electrostatic interactions with the residues Asp35 and Asp64 of the enzyme. These negatively charged groups, which are localized near the appropriate S′ binding sites, repel carboxylate groups of the nucleophiles. There is a good correlation between the nucleophile efficiencies found for different acyl enzymes. An investigation of a series of 14 water-soluble acyl donor esters, differing both in the P1 residue and in the number of amino acids, revealed that the nature of the acyl group affected the acyl-enzyme partitioning between water and added nucleophile in the range of one order of magnitude. [ABSTRACT FROM AUTHOR]

Assignments in the ¹H NMR spectrum for more than 120 resonances arising from 38 of the 130 amino acid residues of human lysozyme are presented. Assignments have been achieved using a combination of one and two-dimensional NMR techniques. Two-dimensional double-quantum correlated spectroscopy and relayed coherence transfer spectroscopy were found to be particularly useful for the identification of spin systems in the aromatic and methyl regions of the spectrum. These spin systems were assigned to specific residues in human lysozyme with reference to the X-ray crystal structure using one-dimensional nuclear Overhauser enhancement (NOE) data and a computer-based search procedure. Unique assignments were found for resonances of 27 amino acid residues even when a distance constraint on NOE effects of 0.7 nm was used in the search procedure; for the remaining residues closer constraints or additional informational were required. The assignments include all but one of the resonances in the aromatic region of the spectrum and all the methyl group resonances in the region upfield of 0.6 ppm. The assignments presented here provide a basis for a comparison of the NMR spectra of human lysozyme and the more widely studied hen lysozyme. [ABSTRACT FROM AUTHOR]

The tetrameric enzyme D-glyceraldehyde-3-phosphate dehydrogenase from the moderate thermophile Bacillus stearothermophilus is more stable to thermal denaturation than its counterpart from lobster muscle [Harris et al. (1980) Eur. J. Biochem. 108, 535-547]. Extra buried ionic bonds between subunits of the thermophilic enzyme make an important contribution to thermal stabilisation. Further stabilisation of the tetrameric enzyme is derived from additional hydrophobic interactions between the S-loops at the core of the tetramer. In the enzyme from the extreme thermophile Thermus aquaticus, which is even more thermostable, intersubunit ion pairs must also play a role but changes in interactions at the surface appear to be equally important. Thus additional hydrophobic interactions at the edge of subunit interfaces would prevent access of water to the interior of the molecule. Furthermore, the arrangement of charged residues on the surface of the T. aquaticus enzyme would allow maximal surface ion pair formation. The presence of surface ion pairs in other proteins correlates well with thermal stability [Perutz, M. F. and Raidt, H. (1975) Nature (Lond.) 255, 256-258] and would provide a general stabilising influence on the subunit in this case. [ABSTRACT FROM AUTHOR]

The effect of urea on various ornithine cycle enzymes has been investigated. It was demonstrated that argininosuccinate lyase was the only ornithine cycle enzyme inhibited by urea in a competitive manner. Based on the data presented the possible role of urea in maintaining a physiological range of intracellular and extracellular urea concentration by controlling hepatic ureogenesis was discussed. [ABSTRACT FROM AUTHOR]

The binding of 10 isomeric α-amino-heptanoic acids, of two isomeric α-amino-octanoic acids, of isoleucinol and valinol, and of various α-hydroxy acids to isoleucyl-tRNA synthetase from Escherichia coli MRE 600 has been investigated by an ultracentrifuge method. It was found that the enzyme requires a primary amino group together with a not-too-small side chain as prerequisites for ligand recognition. Though the enzyme is absolutely specific for the L isomers, it is fairly tolerant against replacement of the carboxyl group of the natural substrate by more or less hydrophobic substituents. These findings can be explained in terms of Ogston's three-point-attachment model, if it is additionally assumed that there is no further space available in the binding region normally occupied by the α-hydrogen atom to accept other substituents which are as bulky as the carboxyl moiety. Similarly, the architecture of the binding region of the aliphatic side chain is discussed. Our measurements show that the free energy of binding strongly depends on the size and the structure of the remainder of the molecule. None of the isoleucine analogues employed is bound as tightly as the natural substrate itself, but there is a clear preference for side chains branched at the/3-carbon atom. The functioning of the side-chain recognition site is best understood by imagining a two-finger glove, of which one finger is tailored to a methyl and the other to an ethyl group. Both these fingers, together with the binding region for the glycine moiety and a steric barrier against a fourth substituent bulkier than hydrogen, are responsible for a high steric specificity towards the one side chain over its Cβ epimer. [ABSTRACT FROM AUTHOR]

Absorption spectroscopy and circular dichroism measurements have been used to elucidate thc mechanism of lysozyme inactivation on photo-oxidation of the methionyl residues to methionine sulphoxide. The monosulphoxide derivative, which is formed by irradiation of the protein in aqueous solution, displayed spectral properties very similar to those of native lysozyme and retained about 54% enzyme activity. This reduced catalytic efficiency appeared to be due to limited conformational changes of the protein molecule. Striking differences were found, on the contrary, between the optical parameters of native lysozyme and of the di-sulphoxide derivative, which is prepared by irradiation in acetic acid solution and is almost devoid of enzyme activity. Studies on the unirradiated lysozyme demonstrated that this extensive denaturation was caused by the action of the protic solvent used. Whereas the acid-induced denaturation was reversible in the case of unmodified lysozyme, the photo-oxidation of the two methionyl residues irreversibly blocked the molecule in a largely disordered form. These results are discussed with respect to the location of the two methionines in the known atomic model of lysozyme. [ABSTRACT FROM AUTHOR]