Carbon dioxide transport across membranes

Carbon dioxide (CO 2 ) movement across cellular membranes is passive and governed by Fick's law of diffusion. Until recently, we believed that gases cross biological membranes exclusively by dissolving in and then diffusing through membrane lipid. However, the observation that some membranes are CO 2 impermeable led to the discovery of a gas molecule moving through a channel; namely, CO 2 diffusion through aquaporin-1 (AQP1). Later work demonstrated CO 2 diffusion through rhesus (Rh) proteins and NH 3 diffusion through both AQPs and Rh proteins. The tetrameric AQPs exhibit differential selectivity for CO 2 versus NH 3 versus H 2 O, reflecting physico-chemical differences among the small molecules as well as among the hydrophilic monomeric pores and hydrophobic central pores of various AQPs. Preliminary work suggests that NH 3 moves through the monomeric pores of AQP1, whereas CO 2 moves through both monomeric and central pores. Initial work on AQP5 indicates that it is possible to create a metal-binding site on the central pore's extracellular face, thereby blocking CO 2 movement. The trimeric Rh proteins have monomers with hydrophilic pores surrounding a hydrophobic central pore. Preliminary work on the bacterial Rh homologue AmtB suggests that gas can diffuse through the central pore and three sets of interfacial clefts between monomers. Finally, initial work indicates that CO 2 diffuses through the electrogenic Na/HCO 3 cotransporter NBCe1. At least in some cells, CO 2 -permeable proteins could provide important pathways for transmembrane CO 2 movements. Such pathways could be amenable to cellular regulation and could become valuable drug targets.