  Biomolecular condensates are small droplets forming spontaneously in
biological cells via phase separation. They play a role in many cellular
processes, but it is unclear how cells control them. Cellular regulation often
relies on post-translational modifications of proteins. For biomolecular
condensates, such chemical modifications could alter the molecular interaction
of key condensate components. We here test this idea using a theoretical model
based on non-equilibrium thermodynamics. In particular, we describe the
chemical reactions using transition-state theory, which accounts for the
non-ideality of phase separation. We identify that fast control, like in cell
signaling, is only possible when external energy input drives the reaction out
of equilibrium. If this reaction differs inside and outside the droplet, it is
even possible to control droplet sizes. Such an imbalance in the reaction could
be created by enzymes localizing to the droplet. Since this situation is
typical inside cells, we speculate that our proposed mechanism is used to
stabilize multiple droplets with independently controlled size and count. Our
model provides a novel and thermodynamically consistent framework for
describing droplets subject to non-equilibrium chemical reactions.

Soft Condensed Matter