Amino-acid sequence of porcine pancreatic elastase and its homologies with other serine proteinases

Restrictions of sequence on the thickness of globular protein molecules

A model for globular protein molecules based on a linear and random sequence of polar and nonpolar amino acids was developed. Polar amino acids were assumed to be always in contact with the aqueous solvent, while nonpolar amino acids were assumed to have a tendency to be buried. Considerations of amino-acid dimensions indicated that only runs of four or more nonpolar amino acids in a row could allow some amino acids to be more than 1-nm (10-A) removed from the surface of the protein. The expected volume fraction of a protein molecule that could be more than 1-nm removed from the surface was obtained with the assumption of a random sequence. Calculation of this volume fraction for a number of simple geometric shapes indicated that some nonpolar amino acids must be exposed to solvent and that the maximum average thickness of globular proteins should be 3-4 nm. Good agreement with published protein dimensions was obtained.

Alkyl isocyanates as active site-specific inhibitors of chymotrypsin and elastase

Alkyl isocyanates react specifically with the two serine proteinases, chymotrypsin and elastase, to yield inactive enzyme derivatives containing 1 male of reagent per mole of enzyme. Octyl isocyanate inactivates chymotrypsin only, while butyl isocyanate inactivates both enzymes but shows greater efficiency toward elastase than toward chymotrypsin. These reagents may thus represent unique chemical "yardsticks" for the measurement of the relative dimensions of the active sites of the two very similar enzymes.

Pseudomonas putida tryptophan synthetase: partial sequence of the subunit

There is 50% identity in the sequences of the first 50 residues of the alpha chains of Escherichia coli and Pseudomonas putida. No deletions or additions of residues are found in this region, except for the N-terminal methionine residue which is missing in the polypeptide isolated from P. putida. Most of the residues which differ are chemically dissimilar, and half of them are specified by codons which differ by more than a single base. The two residues known by mutational analysis to be essential for catalysis in E. coli are conserved in P. putida. The potential taxonomic usefulness of information of this sort is analyzed.

Competitive labelling, a method for determining the reactivity of individual groups in proteins. The amino groups of porcine elastase

1. A method is described for determining the ionization constants and reactivities of individual amino groups in proteins. The principle is that in the presence of a trace amount of radioactive label, the various reactive groups in a protein molecule will compete for the label and the amount incorporated into any one group will be determined by its nucleophilicity, pK and micro-environment. The relative amounts of label incorporated into various groups will be proportional to their second-order rate constants and by comparing these rate constants with those expected on the basis of a linear free-energy relationship obtained with a series of standard compounds, the micro-environment can be defined for a particular amino group. 2. The method consists of treating a protein and an internal standard with a limiting amount of radioactive reagent and then with an excess of unlabelled reagent to yield a chemically homogeneous but heterogeneously labelled compound. After appropriate enzymic digestion peptides containing each labelled group are isolated and their rates of reaction, relative to the internal standard, are determined from their specific radioactivities. The entire procedure is repeated at several pH values. 3. When the method was applied to the amino groups of porcine elastase by using tritiated acetic anhydride as the labelling reagent, the N-terminus was found to have pK(a) 9.7 and a much lower than normal reactivity. Lysine-87 and lysine-224 were found to have pK(a) 10.3 and normal reactivities. At pH values greater than 10.5 there are discontinuities in all the titration curves, indicating that the entire molecule is undergoing a structural reorganization.

An active site peptide from pepsin C

Porcine pepsin C is inactivated rapidly and irreversibly by diazoacetyl-dl-norleucine methyl ester in the presence of cupric ions at pH values above 4.5. The inactivation is specific in that complete inactivation accompanies the incorporation of 1mol of inhibitor residue/mol of enzyme and evidence has been obtained to suggest that the reaction occurs with an active site residue. The site of reaction is the beta-carboxyl group of an aspartic acid residue in the sequence Ile-Val-Asp-Thr. This sequence is identical with the active-site sequence in pepsin and the significance of this in terms of the different activities of the two enzymes is discussed.

Evidence for an extended active center in elastase

Large increases in the esterase, amidase, and peptidase activities of elastase are observed on increasing the length of peptide substrates. The results suggest that at least five contiguous peptide units of the substrate (four N-terminal and one C-terminal to the scissile bond) can interact concurrently with the enzyme. When analyzed in terms of an acyl-enzyme mechanism, the increased effectiveness of the enzyme with larger substrates appears mainly in the acylation reaction and, to a lesser extent, in the binding and deacylation steps.

Strategy and tactics in protein chemistry

Images

Conformational similarity of ovine prolactin and bovine growth hormone

The behavior and properties of ovine prolactin have been evaluated by measurements of fluorescence, polarization of fluorescence, absorption, optical rotation, and circular dichroism. The helical content of the native molecule at pH 8 is 60 per cent as determined by circular dichroism. Three molecular transitions have been followed. The one in acid affects only 20 per cent of the helical residues. More profound conformational changes occur in urea solutions (pH 5.2 and 8.0) where most of the helical residues are randomized. There is a close parallel between the behavior of ovine prolactin and bovine growth hormone both in aqueous solutions between pH 2 and 11.5 and in urea solutions at pH 5.2 and 8.0. Based on the similarities in behavior it is proposed that the conformations of these two hormones are homologous.

The primary structure of alamethicin

Alamethicin, an antibiotic that can transport cations and induce action potentials in synthetic membranes, is shown to be a cyclic peptide with 18 residues including 7-alpha-aminoisobutyric acid residues, two glutamine residues and one free carboxyl group. The composition indicates microheterogeneity. Alamethicin itself and many peptides derived from it are immune to enzymic digestion, but specific partial acid cleavages have allowed determination of the complete sequence. Diborane reduction has shown that the alpha-carboxyl group of glutamine-18 is free, but the ring is formed by a peptide bond between the imino group of proline-1 and the gamma-carboxyl group of glutamic acid-17. The structure is contrasted with that of other cation-transporting antibiotics. Model building allows a structure that could stack to form a tunnel with a lipophilic exterior and hydrophilic interior and flexible internal arms formed by the pendant C-terminal glutamine residue.