We spend a lot of time thinking about proteins in this business - after all, they're the targets for almost every known drug. One of the puzzling things about them, though, is the question of just how orderly they are.
That's "order" as in "ordered structure". If you're used to seeing proteins in X-ray crystal structures, they appear quite orderly indeed, but that's an illusion. (In fact, to me, that's one of the biggest things to look out for when dealing with X-ray information - the need to remember that you're not seeing something that's built out of solid resin or metal bars. Those nice graphics are, even when they're right, just snapshots of something that can move around). Even in many X-ray studies, you can see some loops of proteins that just don't return useful electron density. They're "disordered". Sometimes, in the pictures, a structure will be put up in that region as a placeholder (and the crystallographers will tell you not to put much stock in it), and sometimes there will just be a blank region or some dotted lines. Either way, "disordered" means what it says - the protein in that region adopts and/or switches between a number of different conformations, with no clear preference for any of them.
And that makes sense for a big, floppy, loop that makes an excursion out from the ordered core of a protein. But how far can disorder extend? We have a tendency to think that the intrinsic state of a protein is a more or less orderly one, which we just refer to (if we do at all) as "folded". (You can divide that into two further classes - "properly folded" when the protein does what we want it to do, and "improperly folded" when it doesn't. There are a number of less polite synonyms for that latter state as well). Are all proteins so well folded, though?
It's becoming increasingly clear that the answer is no, they aren't. Here's a new paper in JACS that examines the crystallographic data and concludes that proteins cover the entire range, from almost completely ordered to almost completely disordered. When you consider that the more disordered ones are surely less likely to be represented in that data set, you have to conclude that there are probably a lot of them out there. Even the ones with relatively orderly regions can turn out to have important functions for their disordered parts. The study of these "intrinsically disordered proteins" (IDPs) has really taken off in the last few years. (Here's another paper on the subject that's also just out in JACS, to prove the point!)
So what's a disordered protein for? (Here's one of the key papers in the field that addresses this question). One such would have a number of conformations available to it inside a pretty small energy window, and this might permit it to have different functions, binding to rather different partners without having to do much energetically costly refolding. They could be useful for broad selectivity/low affinity situations and have faster on (or off) rates with their binding partners. (That second new JACS paper linked to above suggests that it's selection pressure on those rates that has given us so many disordered proteins in the first place). Interestingly, several of these IDPs have shown up with links to human disease, so we're going to have to deal with them somehow. Here's a recent attempt to come to grips with what structure they have; it's not an easy task. And it's not like figuring all this stuff out even for the ordered proteins is all that easy, either, but this is the world as we find it.