Ligand binding and self-association of phosphorylase b

Perturbation of molecular species distribution in steady states supported by a flow of energy. Models analogous to Ca2(+)-dependent ATPase and phosphorylase b

When an allosteric macromolecular system is capable of existing in two conformations, both of which may be converted into energy-storing forms by the binding of a substrate or by the absorption of radiant energy, then a kinetic process may occur, such as an enzymic conversion of the substrate into products, which liberates energy and selectively depletes one or more of the forms of the macromolecule. Upon a continuous supply of energy, a steady state, or pseudoequilibrium, is reached during which the selective depletion of molecular species results effectively in a directional flow of energy through the system. This perturbs the distribution of the various molecular species. This effect may simulate both positive and negative binding cooperativity, and mimic the presence of multiple binding sites with different affinities even in monomeric, monovalent systems. Specific model systems are presented analogous to the transport of Ca2+ by sarcoplasmic reticulum and the allosteric behavior of phosphorylase b.

Influence of pH on the removal of pyridoxal 5'-phosphate from phosphorylase b

A method to break the pyridoxal 5'-phosphate (PLP)-phosphorylase b bond using hydroxylamine and slightly acid pH is put forward and described in the present paper. This method does not involve drastic conditions or deforming reagents. The influence of pH and protein concentration on the removal of PLP from phosphorylase has also been studied, resulting in an order of -0.3 with respect to the enzyme, a value that implies a complex reaction. An additional conclusion is that an increase in the protein concentration entails better protection of the enzyme from attack by hydroxylamine.

Rate and equilibrium constants of the tetramerization process of phosphorylase b. Influence of AMP and Mg2+

The association-dissociation equilibrium of phosphorylase b at different enzymatic concentrations has been studied at 25 degrees C in this paper, pointing out how this equilibrium is affected by both AMP and Mg2+ concentrations. It has also been proved that association of phosphorylase b in the presence of AMP and Mg2+ follows second-order and first-order rate laws in the direction of tetramerization and dimerization, respectively. Rate constants have also been calculated and their dependence upon protein, AMP and Mg2+ concentrations studied thoroughly.

Ligand binding and self-association of proteins

This review deals with the quantitative analysis of protein self-association and ligand binding when there is a signficant mutual influence of the two processes. The particular points of interest are the evaluation of the pertinent equilibrium constants and the prediction and interpretation of concentration profiles in transport experiments, including gel elution and sedimentation velocity. The case of dimertetramer equilibrium with four binding sites for ligand is considered in detail. Three representative experimental studies are described which deal with hemoglobin, phosphorylase b, and tubulin.

Structural changes induced in glycogen phosphorylase b by the binding of glucose and caffeine

The allosteric inhibitors glucose and caffeine cause significant structural alterations in glycogen phosphorylase b (1,4-alpha-D-glucan:orthophosphate alpha-D-glucosyltransferase, EC 2.4.1.1). Both cause a masking of two sulfhydryl groups and a reduction of binding affinity for AMP. Caffeine produces an alteration in the microenvironment of the binding site for 1-anilin-naphthalene-8-sulfonate, resulting in a decrease of quantum yield of fluorescence and a change in spectral distribution. The binding of glucose is exothermic with an enthalpy of binding of -6.0 kcal/mol. Glucose causes a change in the molecular ellipticity in the pyridoxal-5'-phosphate region. The implications of these results are discussed.