Danny Llewellyn

The cytotoxic plant protein, beta-puothionin, forms ion channels in lipid membranes

Thionins are small cysteine-containing, amphipathic plant proteins found in seeds and vegetative tissues of a number of plant genera. Many of them have been shown to be toxic to microorganisms such as fungi, yeast, and bacteria and also to mammalian cells. It has been suggested that thionins are present in seeds to protect them, and the germinating seedling, from attack by phytopathogenic microorganisms, but the mechanism by which they kill cells remains unclear. Using electrophysiological measurements, we have shown that β-purothionin from wheat flour can form cation-selective ion channels in artificial lipid bilayer membranes and in the plasmalemma of rat hippocampal neurons. We suggest that the generalized toxicity of thionins is due to their ability to generate ion channels in cell membranes, resulting in the dissipation of ion concentration gradients essential for the maintenance of cellular homeostasis.

An evaluation of active enzyme centrifugation as a zonal and boundary technique by the analysis of simulated data

Abstract An evaluation is presented of the zonal technique of active enzyme centrifugation which involved analyzing computer-simulated data generated from known centrifugal and kinetic models. The relative merits of the commonly used methods of measurement are discussed as are potential artifacts in data interpretation arising from the technique itself. In addition, an extension of the technique is proposed to make it applicable to interacting systems. This involves converting it from a zonal to a boundary technique and is achieved by layering sufficient enzyme on to the substrate solution to ensure a plateau region in which the enzyme concentration remains constant. A procedure for analyzing difference curves generated from the product distribution curves is presented and its suitability illustrated by application to a simulated system involving a monomeric and dimeric enzyme in a rapidly established equilibrium.

The relationship between molecular weight and quaternary structure of globular proteins

Abstract The relationship between the molecular weight and the number of subunits in oligomeric globular proteins consisting of identical subunits has been analyzed. It has been shown that the molecular weights of the subunits are distributed about a mean value of 48,000 and consequently that the molecular weights of the native oligomeric proteins are distributed in clearly distinguishable molecular weight regions. This observation allows the probability of a particular oligomeric structure to be predicted from a measurement of the oligomer molecular weight alone, which is useful in a number of types of study of protein structure, particularly comparative studies. Calculations have been performed which suggest that there is no thermodynamic limitation, in terms of the subunit interactions themselves, to the size of an oligomeric protein with a given number of subunits. Rather, an individual polypeptide chain itself has inherent size limitations, which consequently limits the molecular weight of the corresponding oligomer.