Mechanistic dissection of increased enzymatic rate in a phase-separated compartment

Biomolecular condensates concentrate macromolecules into discrete cellular
foci without an encapsulating membrane.  Condensates are often
presumed to increase enzymatic reaction rates through increased
concentrations of enzymes and substrates (mass action), although this idea
has not been widely tested and other mechanisms of modulation are
possible.  Here we describe a synthetic system where the
SUMOylation enzyme cascade is recruited into engineered condensates
generated by liquid-liquid phase separation of multidomain scaffolding
proteins.  SUMOylation rates can be increased up to 36-fold in
these droplets compared to the surrounding bulk, depending on substrate
KM. This dependency produces substantial specificity among different
substrates.  Analyses of reactions above and below the phase
separation threshold lead to a quantitative model in which reactions in
condensates are accelerated by mass action and by changes in substrate KM,
likely due to scaffold-induced molecular organization. Thus, condensates
can modulate reaction rates both by concentrating molecules and by
physically organizing them.