In Science, a team of biochemists from Sweden and Indiana has presented the clearest models of aquaporins, at an astonishing 0.88 angstrom resolution. What are aquaporins, you ask? Let them describe these wonderful machines:

Aquaporins are membrane channels that facilitate the flow of water across biological membranes…. Aquaporins are water transport facilitators found in all kingdoms of life. They are primarily responsible for water homeostasis within living cells, although a subset of aquaporins also facilitates the flow of other small polar molecules, such as glycerol or urea. (Emphasis added.)

To begin with, we learn that the simplest, most “primitive” microbes have these things. The authors go on to describe how they consist of six proteins arranged in a hexagonal array, with a “selectivity filter” that permits water molecules to pass through, but rejects protons — even though they are much smaller. Aquaporins can even prevent water molecules from passing protons (H+ ions) between themselves by brief formation of hydronium (H3O+), the mechanism by which water conducts electricity. The authors wanted to investigate how these water gates prevent hydronium and proton interlopers.

They confirmed that the selectivity filter forces the water molecules to pass through in a specified orientation that disfavors their ability to pass protons. They also found that positive charge is concentrated on the nitrogen atom of a certain amino acid in the filter that inhibits protons from entering:

This concentration of positive charge on the nitrogen atom closest to the pore maximizes the electrostatic repulsive effect of Arg227 on hydronium ions and thereby helps inhibit their passage.

The selectivity filter (SF) forces water molecules to walk through in pairs. Though the channel could fit four H2O molecules, never are all four sites occupied. The configuration “is optimal for maximizing the water conduction rate,” they said.

In effect, the tiny, slippery molecules of water are forced to pass through a well-crafted channel, made of amino acids with precise locations of charge, that maximizes their speed of passage while minimizing their ability to slip protons through. Consider how complex and elegant this process is:

…water molecules move pair-wise through the SF in a highly correlated manner (Fig. 4B), and their connectivity to water molecules outside of the SF is weak. Correlated motions within the cytoplasmic half of the channel emerge from well-defined water-water H-bond interactions (Fig. 2C), whereas the correlations observed within the SF appear to be dictated by protein-water H-bond interactions. In addition to electrostatic effects, a disruption of the highly constrained SF water structure (Fig. 3) can explain why the mechanism of proton exclusion is sensitive to mutation of the conserved SF arginine and histidine residues.

Sounds like a great intelligent design victory, doesn’t it? But the Evolution Party members bugged the water gate paper with lines intended to throw the election to Darwin:

As with any membrane transport facilitator, aquaporins have evolved to be highly selective for their transported substrate without binding water so strongly that transport is inhibited….

These findings illustrate how evolution has fine-tuned the water channel geometry so as to optimize protein function, suppressing proton transport without compromising water flux.

It’s a scandal.

Image credit: Structure of aquaporin Z/Wikipedia.