The state of proteins in water-protein systems was investigated qualitatively using viscosimetric technique and preparative ultracentrifugation. In the case of cooperative processes, 4 concentration intervals bounded by three critical concentrations have been found for each protein.The first value of critical concentration is bound up with the initiation of supermolecular formations (herein also referred to as associates) due to self-association of polypeptide chains. The second critical concentration is observed when the weight fraction of associate becomes predominant (80-85 %) and when the change of secondary and tertiary structures is completed.The third critical concentration is connected with the change in the nature of supermolecular formations. The thermodynamic stability of the water-protein system varies when the critical concentrations are passed through. The system is stable and, apparently, close to the true solution over the first concentration interval. As soon as the first critical concentration is reached, colloid particles appear in the system and become predominant after the second critical concentration is passed, and the solution should be considered as a colloid one.According to the generally accepted definition the solution is a uniphase system consisting of at least two components [I]. As far as water-protein mixtures are concerned, the term "solutions" is applied to systems which entirely differ in phase state and structure. Such is the case with slurries and dilute enzyme solutions, the latter being close to the true solutions.The purpose of our experimentation is to particularize the physical sense of the term "solutions" in application to water-protein systems and generalize the structural features and characteristics of water-soluble proteins in a wide range of temperatures and concentrations. The present paper is the first report on a series of our experiments wherein the relation between the volumetric processes in the system and the observed viscosity anomalies is discussed in the light of data obtained by other methods. Materials and methods Materials
Water systems formed by total protein of yeast Saccharomyces cerevisiae and model systems on the basis of unfractionated casein were studied by viscosimetry and electron microscopy. Methodologically, both methods offered a possibility of distinguishing several quantitatively different concentration intervals in protein solutions under different conditions. Viscosimetry gave the most informative results at the concentration intervals where the solutions approached the true ones, while electron microscopy did so when supermolecular formations prevailed. In comparing the data obtained by these two methods the following conclusions were drawn: At low temperatures (4-10 degrees C) yeast protein was in molecular-dissociated condition. Association in the system starts at 10 degrees C, and becomes enhanced with a rise in temperature. A temperature rise to 25-30 degrees C promotes a more compact packing of polypeptide chains; further heating leads to high-temperature denaturation. Lipids induce covering of polypeptide chains and prevent associations, thus leading to an increase in the system's thermostability.
Iane and identification of cross-linked proteins by diagonal polyacrylamide / sodium dodecyl sulfate gel electrophoresis//Meth. Enzymol.-1979.-59.-P. 539-650. 7. Crystallization of 70S ribosomes and 30S ribosomal subunits from Thermus thermophi
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