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M A Bothwell

Publications and source records attributed to M A Bothwell.

23 records · Page 2Linked to original sources
PubMed1977

Dissociation equilibrium constant of beta nerve growth factor.

The beta nerve growth factor (NGF) isolated from the high molecular weight 7 S NGF complex from mouse submaxillary gland is a protein of molecular weight 26,500 which contains two noncovalently associated peptide chains. A variety of techniques were employed to determine whether the NGF dimer dissociates at the low concentrations at which it is biologically active. No dissociation was detected in 48 h at low NGF concentrations using the techniques and minimum concentrations which follow: sedimentation equilibrium of NGF at 200 nM and of 125I-NGF at 2.5 nM; gel filtration chromatography of NGF at 5 nM, of 125I-NGF at 5 pM, and of 125I-NGF from which COOH-terminal arginyl residues, or NH2-terminal octapeptide had been cleaved, at 10 pM; and sucrose gradient centrifugation of 125I-NGF at 10 pM. Dissociation of heavily succinylated 125I-NGF was detected by gel filtration chromatography, and the equilibrium dissociation constant for this material is estimated to be 10 pM.

Animals↗
PubMed1977

Ligand-promoted weakening of intersubunit bonding domains in aspartate transcarbamolylase.

THE COOPERATIVITY AND FEEDBACK INHIBITION EXHIBITED BY THE REGULATORY ENZYME, ASPARTATE TRANSCARBAMOYLASE (CARBAMOYLPHOSPHATE: L-aspartate carbamoyltransferase; EC 2.1.3.2), from Escherichia coli are generally attributed to ligand-promoted conformational changes involving alterations in the subunit interactions. However, no quantitative estimates have been made of the effect of ligands on the strength of the intersubunit bonding domains. The native enzyme, composed of two catalytic trimers "crosslinked" by three regulatory dimers, shows little tendency to dissociate in neutral buffers at room temperature. In addition, very little exchange was observed in 2 hr between subunits within the intact enzyme and free subunits. Although exchange was enhanced in solutions of low ionic strength containing the bisubstrate analog, N-(phosphonacetyl)-L-aspartate, the rates of exchange were too small to permit reliable estimates of the weakening of the bonding domains caused by the ligand. Studies were conducted, therefore, on a less stable oligomeric complex which resembles the native enzyme in structure and allosteric behavior but lacks one regulatory subunit. These molecules, containing only four bonding domains between the catalytic and regulatory polypeptide chains (compared to six in the native enzyme), disproportionate to form the more stable native enzyme and free catalytic subunits. An electrophoretic technique is described for measuring the rate of disproportionation which is controlled by the rupture of the intersubunit bonding domains. This rate is enhanced about 300-fold upon the addition of the active-site ligand. Hence the ligand-promoted allosteric conversion of the enzyme-like complex from the constrained to the relaxed conformation involves a substantial weakening of the intersubunit interactions corresponding to about 1.7 kcal/mole (7.1 kJ/mole) per bonding domain between a catalytic and a regulatory chain.

Aspartate Carbamoyltransferase↗