INFLUENCES OF MACROMOLECULAR CROWDING ON BIOCHEMICAL SYSTEMS

细胞内的大分子浓度非常高，可以 可能会对热力学活动产生压倒性的影响 由于被排除的体积效应，细胞成分的数量减少。估计 对于这些影响的大小，我们做了一个实验研究 大肠埃希氏菌的细胞质。来自单元格和单元格的参数 萃取物用于计算近似的活度系数 细胞质条件。这些计算需要表示为 的大小、浓度和有效比体积 萃取物中的大分子。大分子大小表示法有 从两相分配和凝胶过滤分析中获得。 获得了大肠杆菌细胞质中的大分子浓度。 来自使用已知的细胞质不通透性的实验 音量到蔗糖。根据大分子比体积推算出 聚乙烯诱导密排沉淀物的组成 乙二醇。 几种与DNA结合的特征良好的蛋白质(Lac抑制物， RNA聚合酶)对盐浓度的变化极其敏感 在体外研究中，但在体内研究中不敏感。应用 本工作的活度系数表明，至少 这种差异的部分原因是排除的数量不同 在这些研究中。

The very high concentration of macromolecules within cells can potentially have an overwhelming effect on the thermodynamic activity of cellular components because of excluded volume effects. To estimate the magnitudes of these effects, we have made an experimental study of the cytoplasm of Escherichia coli. Parameters from cells and cell extracts are used to calculate approximate activity coefficients for cytoplasmic conditions. These calculations require a representation of the sizes, concentrations, and effective specific volumes of the macromolecules in the extracts. Macromolecule size representations are obtained from two-phase partition and gel filtration assays. Macromolecule concentrations in the cytoplasm of E.coli are obtained from experiments using the known impermeability of the cytoplasmic volume to sucrose. Macromolecule specific volumes are inferred from the composition of close-packed precipitates induced by polyethylene glycol. Several well-characterized proteins which bind to DNA (lac repressor, RNA polymerase) are extremely sensitive to changes in salt concentration in in vitro studies, but are insensitive in in vivo studies. Application of the activity coefficients from the present work indicates that at least part of this discrepancy arises from the difference in excluded volumes in these studies.