In the pattern above, the Pac-men are enzymes that use reactive cysteines in their active sites to carry out their jobs. The triangles are groups that latch onto cysteine if its thiol is available and sufficiently reactive, and these groups are tagged with tracers that are either heavy (red) or light (blue) so when used in parallel, changes in reactivity within the same enzyme upon treatment of some sort can be revealed. It is a clever method for making mass spectrometry, a technique that is not necessarily quantitative by nature, quantitative. If there was a way to do that for humans, maybe I would still be at the bench. Anyway, for the vast majority of enzymes that react with the probe, there will be equal amounts with heavy and light tracers, meaning there was no change in reactivity associated with the treatment. But, when you discover enzymes that have blue:red ratios other than 1, like in the overlaid trace above, then you may have just discovered a previously unknown function for an enzyme, a clue to a regulatory mechanism, or even a new drug target.
The inspiration for this comes from a plethora of papers from the Weerapana Lab at Boston College, which has pioneered the method for this purpose.
Check back March 1st for the next installment!