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Chemical reactions are usually studied under the assumption that bothsubstrates and catalysts are well mixed (WM) throughout the system. Althoughthis is often applicable to test-tube experimental conditions, it is notrealistic in cellular environments, where biomolecules can undergoliquid-liquid phase separation (LLPS) and form condensates, leading toimportant functional outcomes, including the modulation of catalytic action.Similar processes may also play a role in protocellular systems, like primitivecoacervates, or in membrane-assisted prebiotic pathways. Here we explorewhether the de-mixing of catalysts could lead to the formation ofmicro-environments that influence the kinetics of a linear (multi-step)reaction pathway, as compared to a WM system. We implemented a general latticemodel to simulate LLPS of an ensemble of different catalysts and extended it toinclude diffusion and a sequence of reactions of small substrates. We carriedout a quantitative analysis of how the phase separation of the catalystsaffects reaction times depending on the affinity between substrates andcatalysts, the length of the reaction pathway, the system size, and the degreeof homogeneity of the condensate. A key aspect underlying the differencesreported between the two scenarios is that the scale invariance observed in theWM system is broken by condensation processes. The main theoreticalimplications of our results for mean-field chemistry are drawn, extending themass action kinetics scheme to include substrate initial hitting times to reachthe catalysts condensate. We finally test this approach by considering opennon-linear conditions, where we successfully predict, through microscopicsimulations, that phase separation inhibits chemical oscillatory behaviour,providing a possible explanation for the marginal role that this complexdynamic behaviour plays in real metabolisms.

Modelling spatial constraints and scaling effects of catalyst phase separation on linear pathway kinetics