Four 2 µs long ARBD simulations were performed to calculate the rate for a CO2 molecule to bind to a rubisco active site (k b ) or to exit the shell (ke), with the animation for a single simulation given in SI Animation 8. The atomic resolution offered by ARBD allows individual CO2 molecules to be tracked relative to the well-conserved (SI Fig. ) rubisco active site , in our case K201, H294, and H327 shown to be involved in catalysis . In analyzing the ARBD trajectories, we define that a CO2 molecule is bound once the molecule comes within 10 Å of the Cα atoms in this catalytic triad. The larger cutoff accounts for sticky spots on the van der Waals grid for rubisco once it has been discretized, the size of the residues, as well as conformational changes that might take place within rubisco that ARBD cannot capture. A CO2 can also hit the bounding surface that is assumed to represent the carboxysome shell proteins. The ratio between these two events is related to the kinetic rate at which exiting and binding occur, as given by the simple Markov model derived in the Supporting Information section D.