January 22, 2016 at 06:36AM

Today I Learned: 1) The Mormon chruch has a board of living saints, who publish new scripture at least yearly, I think more often. It's kind of interesting having a religion whose fundamental writings include things being written *today*, for issues of today. I also wonder how modern Mormon scripture differs from, say, non-scriptural sermons of other Protestant faiths. 2) Laboratory refrigerators are expensive! Under-desk models run in the thousands of dollars. I'm sure they're better than the average refrigerator, but... an order of magnitude better? I don't think so. 3) Racemases are proteins which... actually, before I dive into this one, let me say a couple words about enantiomerism, or left- vs. right-handedness in molecules. If you know what an enantiomer is, feel free to skip the next two paragraphs. Two chemicals with the same component parts don't necessarily act at all the same. The arrangement of atoms in a molecule, as much as the types and numbers of those atoms, determines the properties of that molecule. This is generally pretty easy to recognize -- six carbon atoms linked in a straight chain aren't going to act the same as six carbon atoms arranged in a ring, for instance. A more subtle way molecules with the same atoms can differ is by their *handedness*. Consider a pair of gloves. Each glove is, in a way, substantively identical to the other. They both have four finger pockets and a thumb pocket arranged in roughly the same way... but one is a mirror image of the other. There's no way you can rotate a left-handed glove around to make it a right-handed glove. Some molecules can be the same way -- identical except that one is a mirror image of the other. When this is the case, we say that the left-handed version is one "enantiomer" (or "stereoisomer"), and the right handed version is another "enantiomer". Two different enantiomers of the same molecule have identical physical properties except that a) they interact with polarized light differently, and b) they interact differently depending on the enantiomer they interact with. As a metaphor, left-handed and right-handed gloves don't really have any different physical properties, but one only interacts properly with a left and and the other only interacts properly with the right hand. Enantiomers are really important any time biology gets involved with chemistry (or vice versa) because biological molecules generally only come in one enantiomer, or if they come in more than one, the different enantiomers typically have totally different biological properties and activities. The most famous example is thalidomide -- one stereoisomer is a very effective anti-nausea drug, but the other is a potent teratogen (it mutates babies). When thalidomide was deployed widely in hospitals in the 1950s, the drug was produced in both enantiomers, leading to thousands of cases of deadly mutations in newborns. Thus, chemists in a lot of contexts really, really care about making chemicals in one isomer over the other. Unfortunately, in general, it's really hard to selectively make or even isolate one enantiomer of a compound, because both isomers have identical physical properties! Pretty much all techniques for stereoselectivity involve using isomerically pure substances from natural sources. Back to racemases. Racemases are a class of enzyme that switch enantiomeric centers from one handedness to the other. They generally do so non-specifically, meaning that they will convert from either handedness to the other one (though usually one more efficiently than the other). To the cell, this is just another form of biochemial synthesis available to make new compounds. To a chemist, this is a wonder protein. "But wait," you might ask, "why would I want a molecule that turns pure solutions of an enantiomer into a mix of both enantiomers?" Good question. With some neat tricks, you can actually use racemases to do exactly the opposite, is the answer. Imagine you have a 50/50 mixture of two enantiomers of a molecule A, call them A_L and A_R. You want to convert the whole thing into A_L. Here's what you do: Add some racemase to the mixture. In the same pot, run a one-way reaction that converts A_L into something else (call it B_L) that's chemically inert in that setup (for example, by adding a protective group that blocks access by the racemase). Now B_L acts as a sink of A_L. As A_L is depleted, the racemase will start converting A_R into A_L to bring the two enantiomers into equilibrium. B_L can still pull A_L, though, so this set of reactions will suck all of A_R and A_L both into B_L. Once everything has been converted to B_L, you remove the racemase and run another reaction to turn B_L back into A_L (by, for example, removing the protecting group you added before). Amazingly, the racemase has helped you convert everything into one enantiomer! Thanks to Anders Knight for teaching me about these amazing proteins!