How does cytoplasmic crowding affect reaction rates?

The cytoplasm of eukaryotic cells is crowded with macromolecules. In principle, this crowding could have either a positive or a negative effect on the rates of biochemical reactions. Here, we review two commonly invoked theories to account for these possible effects then survey recent experimental work in cells and extracts that measures the effects. The evidence so far suggests that the effective second-order rate constants (a measure of the speed of a reaction for a given concentration of reactants) for reactions in vivo generally go down when crowding increases due to the slowing of diffusion. If the evidence presented so far proves to be general, it would have important implications for how we view the trade-offs that determine the biochemical dynamics of the cytoplasm.