Quite often, in science, you learn something that completely blows you away. Sometimes you learn something that continues to blow you away with how simply magnificent it is, every single time you think about it. For me, the human eye is one of those things. In particular, the way the human eye detects light. I will never stop finding this amazing.
Rhodopsin is a protein in your eye. Without it you could not see. There are many molecules that make up the cells of the human eye and all of them are intricate and beautiful in their mechanism. Rhodopsin though, is the actual light-sensing molecule, so it’s pretty key to the business of seeing. It lives in the membrane of light-detecting cells (photoreceptors) in the eye. It’s actually made up of two parts, a big bunch of protein that stretches across the membrane called opsin, and a teeny little molecule called retinal. Retinal is a chromophore, which means that it changes shape when it absorbs a particle of light.
In fact, just one bond of retinal changes position when it absorbs light, turning it from a slightly L-shaped molecule into a straight molecule. That’s it. One tiny little change, in the way one single chemical bond is arranged. And this is the key to sight.
You see, proteins are quite tightly packed molecules within themselves, so when retinal changes from an L-shape to a straight line, the opsin protein has to shift itself around slightly to accommodate this change in shape. It’s as if you’re standing on a really packed tube train, nestled into someone’s armpit with no room to move, and someone suddenly sticks their leg out into the space where your leg was… and your leg has to move. On a train, very bad etiquette and probably deserving of a highly meaningful tut, if not a muttered “honestly!”, but in a protein it’s fairly standard practise. A nearby ‘leg’, or in this case amino acid, shuffles out of the way of the retinal, possibly tutting slightly, and this change in conformation can be recognised by another protein, transducin.
Transducin’s job is effectively to sit very close by and watch rhodopsin until it moves. This might sound like watching paint dry, except rhodopsin moves every time it absorbs a photo of light, which is very very often when your eyes are open. Transducin probably gets a bit more bored at night… actually that raises an interesting question, when you ‘see’ in your dreams, are your eyes involved? Or are all your transducin molecules just sitting there sighing at the unmoving rhodopsin and hoping you wake up soon?
Hm, I wonder.
Anyway, transducin is a G-protein. As soon as transducin recognises the shape-change in rhodopsin, it becomes activated, and scurries off to contact another protein. This protein triggers some changes in the cell that cause a message to be sent to the next cell. This message is passed as an action potential along to a nerve cell, which shoots it off to the brain to let it know that you’ve seen light. Lots of and lots of these messages make up the overall picture that we get when we see images.
So that’s it… one tiny minute change in the position of one chemical bond can trigger a cascade of events that produce a message that is sent along the optic nerve to the brain. To me, that is beyond amazing. That such a small change can be so crucial to something as complex and wonderful as sight entirely captures the reason that I think molecular biology is so stunning. It demonstrates everything that I love about biochemistry.