I am intrigued with conservation and symmetry and general optimization properties as they pertain to biological systems. Well, perhaps simplicity also comes into play, in considerations of general aesthetics.
But there’s something profoundly beautiful about observations of the old biology tenet, “form follows function.” In an investigation of ion channels in biological membranes, particularly of neurons, one often notices a few things. First, a cell’s membrane is rather impermeable to ionic “stuff.” So in order for transport of ions, for instance, across a membrane, there have to be holes in the membrane.
As it turns out, there are many types of holes, including these ion channels, which are often selective for a particular ion. By and large, only that ion can go through its channel, and there are various ways of regulating this. Within the inside of the channels, called the pore (center of figure above), there are at least two factors involved in selectivity of ions – on one hand there is the physical size of the pore, and there is also what’s called a selectivity filter. A selectivity filter is essentially a binding site for the ion that enables it to continue along its pathway from one side of the membrane to the other. (There’s generally some sort of weak hydrogen-type bonding that occurs at that site, I think.)
So consider this – let’s say the ion, say a K+, is moving from the inside to the outside of the cell across a membrane. The selectivity would be such that you’d only want K+ leaving through the channel and not Ca2+ or Na+. But in the reverse direction – from out to in – the selectivity ought to be equally efficient.
This implies that the observed transverse symmetry in the protein channel is necessary for the bidirectional selectivity either by size of the ion or using a chemical segment as a filter! Quite fascinating, if you ask me!