The previous blog post was about ammonia channels. Ammonia is a gas, and we can ask: under what conditions do cells use gas-conducting channels?
When ammonia (NH3) dissolves in water it is usually converted to ammonium ion (NH4+). However, our cells do not simply have ammonium ion channels. It appears that ammonium ions and potassium ions are very difficult for the selectivity filters of ion channel proteins to distinguish (for example, see “Competition between uptake of ammonium and potassium in barley and Arabidopsis roots: molecular mechanisms and physiological consequences“). Potassium ion channels are of fundamental importance, so the adopted evolutionary strategy seems to have been to keep ammonia levels low in the human body. If too much ammonia accumulates in the blood (hyperammonemia) then toxic effects are experienced. Ammonia is converted to urea, but the kidney does excrete some ammonia in the context of acid excretion. However, it is proposed that ammonia channels can function as molecular devices for dehydrating ammonium ions (Khademi, et al., 2004). In the channel model of Khademi, et al., only after ammonium is converted to ammonia can the ammonia molecules cross the cell membrane through the narrow central part of the ammonia transport channels (see the figure, above right). We can think of such an ammonia transport channel as being a “gas channel”.
Other biologically important gases include oxygen (O2), carbon dioxide (CO2) and nitric oxide (NO). There has been interest in the idea that some membrane transport proteins (such as aquaporin 1) might function in cells as carbon dioxide channels. One group of researchers concluded that aquaporin 1 makes a major contribution to the carbon dioxide permeability of red blood cells (1). However, it has been argued that carbon dioxide easily crosses lipid bilayer membranes and so biological membranes are not likely to act as a significant barrier to CO2 diffusion (2).
In a recent article about gas channels, it was proposed that cell membrane permeability to CO2 might be significantly lower than expected. While it is easy for CO2 to dissolve in the phospholipid of a cell membrane, it can be argued that some biological membranes might have lipid and protein compositions that greatly limit the ability of CO2 to move across the bilayer without help, thus making gas channels physiologically relevant for efficient carbon dioxide transport in some cells.
Related. Ammonia metabolism.
Image credit. I made this image based on of Figure 6 of “Mechanism of Ammonia Transport by Amt/MEP/Rh: Structure of AmtB at 1.35 Å“