Aquaporins and CO 2 diffusion across biological membrane.

Despite the physiological significance of effective CO diffusion across biological membranes, the underlying mechanism behind this process is not yet resolved. Particularly debatable is the existence of CO-permeable aquaporins. The lipophilic characteristic of CO should, according to Overton's rule, result in a rapid flux across lipid bilayers. However, experimental evidence of limited membrane permeability poses a challenge to this idea of free diffusion. In this review, we summarized recent progress with regard to CO diffusion, and discussed the physiological effects of altered aquaporin expression, the molecular mechanisms of CO transport via aquaporins, and the function of sterols and other membrane proteins in CO permeability. In addition, we highlight the existing limits in measuring CO permeability and end up with perspectives on resolving such argument either by determining the atomic resolution structure of CO permeable aquaporins or by developing new methods for measuring permeability.