Energetically optimal strategies in reactive-diffusive signaling

In biological processing networks, it is often necessary to move a signal between the output of one processing step to the input of the next. Many different physical schemes have evolved to send signals over physical distances for this purpose. We can compare different signaling schemes by computing their associated energetic costs, measured in Joules per bit, obtained by calculating the rate of information transfer and the corresponding energy dissipation. Perhaps the most common of these schemes in biology is diffusive signaling. In a diffusive signaling network, a sender molecule produces second messengers which travel through the cytoplasm where their concentration is measured by a receiver molecule. Here we present a general strategy of reactive-diffusive signaling that minimizes this cost per bit by colocalizing the enzyme responsible for deactivating messengers with the sender itself. Despite the increased dissipation caused by futile activation-deactivation cycles, the overall cost per bit decreases. While this is a general phenomena, we suggest that this explains the colocalization of the kinase CheA and phosphotase CheZ in the E. coli chemotactic cascade.