Atomic-Scale Protein Filters

Summary

‍Cells are crowded and frenzied places. But their interiors still contain more order than their surroundings.Inside a cell, molecular machines convert raw inputs into highly ordered structures; DNA coils into chromosomes and proteins fold into precise three-dimensional shapes. Outside the cell, however, ions drift without gradients, while DNA and proteins exist mostly as scattered building blocks. Only a thin membrane separates the managed chaos inside from the high entropy outside.If a cell’s membrane is punctured, the fluid within — made mostly of water — drains away, and the cell swiftly dies. And yet, the membrane must let molecules in (food, nutrients, ions) and out (waste, toxins, messages) for the cell to survive. The membrane does so by means of hyperspecific proteins that allow only particular molecules to pass. It is thanks to these atomic-scale filters that life can exist at all.{{signup}}What makes some of these filters remarkable is that they work without any “added” energy. Separating water from protons, or potassium from sodium, seems like it ought to require a great deal of resources. And yet, the only energy a cell requires to do so is that used for building the proteins themselves. In other words, evolution has figured out how to perform (nearly) energy-free filtration at a scale that keeps cells alive in an environment that would otherwise overwhelm them.Two types of protein filters, aquaporin and the potassium ion channel, offer particularly elegant examples. Aquaporins let billions of water molecules pass in and out each second in a single file line, while blocking protons. Potassium channels conduct about 100 million K⁺ ions per second yet decisively reject Na⁺, a slightly smaller ion with the same charge. Both rely on geometry alone (namely, the precise placement of amino acids) to achieve their selectivity. And both demonstrate how seemingly complex problems are often solved in biology simply by positioning the correct atom, with the appropri...