The blind watchmaker had a lot of foresight in choosing to use a rather funky looking amino acid (proline) resembling none of the others. A lot of kindness was also shown to structural molecular biologists by two of the watchmaker’s henchmen – Burkholderia gladioli and the common daisy.
All appear in a fascinating paper [ Cell vol. 176 pp. 435 – 447 ’19 ] in which the structure and better the mechanism of action of the mitochondrial ADP/ATP translocase, a molecule of some interest since our mitochondria make our body weight of ATP each day and need some way to get it out into the cytoplasm where it is used.
The molecule has quite a job to do, getting the rather large ATP molecule out to the intermembrane space (and thence out to the cytoplasm) without allowing protons to sneak out with it, since it is the proton gradient which is used to power ATP synthase the exquisite machine which makes ATP. This is quite a trick as no chemical moiety is as small as a proton.
The translocase has two states — one in which it is open to the mitochondrial matrix (called the m-state) and another in which it is open (eventually) to the cytoplasm — called the c-state. In the m-state the cytoplasmic portion is shut, and in the c-state the membrane portion is shut.
The rather wierd looking molecule bongkrekic acid made by Burkholderia gladioli https://en.wikipedia.org/wiki/Bongkrek_acid binds to the translocase fixing it in the m-state. Atractyloside, made by daisies binds to the molecule fixing it in the c-state. They made life much easier for the structural biologist and cryoEMographers who wrote the paper.
Proline comes in because when placed in an alpha helix, proline’s 5 membered ring structure fixes the alpha carbon so that it is essentially inflexible, meaning that it can’t get into the conformation that the other 19 amino acids can get into when an alpha helix is formed. Translation — proline is a helix breaker, forming a kink in the helix.
The translocase contains 3 modules of 100 amino acids each of which has 2 alpha helices, one of them containing a proline causing a kink in the helix. The prolines are in the middle of the helix. The ATP channel is formed by the 6 helices.
Essentially in the middle of the membrane, the kinked alpha helices form a pivot (fulcrum), so the helices rock back and forth, opening one side while simultaneously shutting the other, permitting ATP to bind near the fulcrum without letting anything else through, when the pivot shifts — out goes the ATP (without letting protons sneak past).
There is far more beautiful protein chemistry on display. There is a conserved signature motif Proline x Aspartic acid/Glutamic acid X X Lysine/Arginine at the carboxy terminal end of one of the helices of each other 3 modules — this forms a salt bridge shutting the channel on the matrix side. Glycine and other small amino acids (alanine) allow close packing of the helices on the cytoplasmic side.
It is unfortunate that the most of humanity doesn’t have the background to appreciate the elegance and beauty of Nature’s solution to the problem. I say Nature rather than God to be scientifically correct, but it’s elegant chemistry like this that makes it hard for me to accept that it arose by the machinations of a blind watchmaker.
Chemiotics II 