There was an intriguing paper published earlier this month from Manfred Reetz and co-workers at the Max Planck Institute. It's not only an interesting finding, but a good example of making lemonade from lemons.
They were looking at an enzyme called tHisF, a thermostable beast from a marine microorganism that's normally involved in histamine synthesis. It has an acid/base catalytic site, so Reetz's group, which has long been involved in pushing enzymes to do more than they usually do, was interested in seeing if this one would act as an esterase/hydrolase.
And so it did - not as efficiently as a real esterase, but not too shabby when given some generic nitrophenyl esters to chew on. There was some structure-activity trend at work: the larger the alkyl portion of the ester, the less the enzyme liked it. Given a racemic starting material, it did a good job of resolution, spitting out the R alcohol well over the S isomer. All just the sort of thing you'd expect from a normal enzyme.
Next, they used the crystal structure of the protein and previous work on the active site to see which amino acids were important for the esterase activity. And here's where the wheels came off. They did a series of amputations to all the active side chains, hacking aspartic acids and cysteines down to plain old alanine. And none of it did a thing. To what was no doubt a room full of shocked expressions, the enzyme kept rolling along exactly as before, even with what were supposed to be its key parts missing.
Further experiments confirmed that the active site actually seems to have nothing at all to do with the hydrolase activity. So what's doing it? They're not sure, but there must be some other non-obvious site that's capable of acting like a completely different enzyme. I'm sure that they're actively searching for it now, probably by doing a list of likely point mutations until they finally hit something that stops the thing.
So how often does this sort of thing happen? Are there other enzymes with "active sites" that no one's ever recognized? If so, do these have any physiological relevance? No one knows yet, but a whole new area of enzymology may have been opened up. I look forward to seeing more publications on this, and I'll enjoy them all the more knowing that they came from a series of frustrating, head-scratching "failed" experiments. Instead of pouring things into the waste can, Reetz and his co-workers stayed the course, and my hat's off to them.