Intermolekulare Wechselwirkungen bei der Wärmeaggregation globulärer Proteine

Abstract

In order to determine the intermolecular forces in the process of the heat aggregation of globular proteins in solution, selected proteins with different amounts of disulfide- and thiolgroups were investigated by specific inhibition experiments and by degradation analysis, using lightscattering and ultracentrifugation methods. In accordance with the mechanism of the heat aggregation, which in general (SH —SS-proteins) may be characterized as a coupled coagulation- and exchange-reaction, auxiliary valences and covalent bonds take part in the aggregation process. Besides the pʜ-range of lanthionine-formation, the coagulation-mechanism by weak intermolecular forces exceeds the covalent type of aggregation. If only one of the sulphur functional groups is present in the protein molecules the aggregation is merely the result of the coagulation-mechanism, i. e. the degradation by urea, guanidine·HCl, variation of pʜ etc. leads back to the monomer. In the case of SH —SS-proteins the degradation rate depends on the temperature and duration of aggregation: In the range of predenaturation and under isoelectric conditions the native monomer is restored while increasing net charge leads more and more to covalently bound aggregates which are due to disulfide- and lanthionine-groups. High alkalinity promotes the formation of lanthionine. Regarding the weak intermolecular bonds the application of specific criteria in degradation and inhibition experiments proves that Η-bonds and hydrophobic interactions participate in the aggregation process while ion pair bonds may be excluded. The hydrophobic interactions do not become apparent, until partial denaturation of the aggregating protein takes place. The proportion of the total aggregation at extreme pʜ-values which is produced by the coagulation mechanism may be explained in a tentative way by assuming specific electrostatic short range interactions between the partially dehydrated molecules, leading to fibrillar associates.