Fig. 5. Homeostasis in a model of the effect of cytoplasmic concentration of translation and protein degradation.

a Plot of Eq. 2, which relates a bimolecular reaction rate to the relative cytoplasmic concentration, for various sizes of proteins. We assumed a = 0.018 nm^-1 (from Fig. 4f). b Calculated optimal relative cytoplasmic concentration for proteins of different assumed sizes, again assuming a = 0.018 nm^-1. c Fits of Eq. 2 to the experimental data for translation (green) and degradation (purple) as a function of cytoplasmic concentration, calculated assuming that both the substrate and enzyme varied with the cytoplasmic concentration. All of the data from Figs. 2d, h, and 3d, h were included in the fits. The $R^2$ values are 0.92 for the translation data and 0.95 for the degradation data. The fitted values for the size of the proteins involved are 104 ± 2 nm (mean ± S.E.) for translation and 14 ± 1 nm (mean ± S.E) for degradation. The fitted optimal cytoplasmic concentrations are 1.07 ± 0.02 for translation and 8.1 ± 0.8 for degradation (mean ± S.E.).