It is believed that all present-day organisms descended from a common cellular ancestor. Such a cell must have evolved from more primitive and simpler precursors, but neither their organization nor the route such evolution took are accessible to the molecular techniques available today. We propose a mechanism, based on functional properties of enzymes and the kinetics of growth, which allows us to reconstruct the general course of early enzyme evolution. A precursor cell containing very few multifunctional enzymes with low catalytic activities is shown to lead inevitably to descendants with a large number of differentiated monofunctional enzymes with high turnover numbers. Mutation and natural selection for faster growth are shown to be the only conditions necessary for such a change to have occurred.
SUMMARY. Approximately 30 % of the nitrogen of /c-casein was soluble at pH 4-7 after the protein had been treated with rennin at pH 7 while approximately 10 % was soluble in 12% trichloroacetic acid (TCA). The material soluble in 12% TCA appeared at a slower rate initially than did the nitrogen soluble at pH 4-7 but as the reaction proceeded it was released more rapidly.Treating K-casein with urea, or repeated precipitation of the protein at pH 4-7, caused the formation of material insoluble at pH 7, apparently para-/c-casein. Both treatments appeared to free the same soluble fraction as does rennin acting in low concentration or for a short time.Low concentrations of rennin (0-07 /*g/ml) released only part of the available soluble nitrogen from 2 % solutions of whole casein at pH 7. Heating the reaction mixture appeared to restore the casein complex, the restoration being less complete the longer the reaction had proceeded.It is suggested that (c-casein is not a single protein but a complex, and that the action of rennin is first to open the secondary bonds responsible for the stability of this complex.
SummaryWhen k-casein was treated with rennin at neutral pH all the sialic acid contained in the protein was found in the glycopeptide released by the enzyme. From the known molecular weight of the glycopeptide and its proportion in k -casein, a molecular weight of the order of 50000 was calculated for the protein
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