CuAAC stabilization of an NMR mixed labeled dimer

Homo dimers are the most abundant type of enzyme in cells and as such, they represent the archetypal system for studying the remarkable phenomenon of allostery. In these systems, in which the allosteric features are manifest by the effect of the first binding event on the similar event at the second site, the most informative state is the asymmetric single bound (lig1) form, yet it tends to be elusive thermodynamically. Here we take significant steps towards obtaining milligram quantities of pure lig1 of the allosteric homodimer, chorismate mutase, in the form of a mixed isotopically labeled dimer stabilized by Cu(I)–catalyzed azide–alkyne cycloaddition (CuAAC) between the subunits. Below, we outline several critical steps required to generate high yields of both types of unnatural amino acid–containing proteins, and overcome multiple pitfalls intrinsic to CuAAC to obtain high yields of pure, fully intact, and active mixed labeled dimer. These data not only will make possible NMR–based investigations of allostery envisioned by us, but should also facilitate other structural applications where specific linkage of proteins is helpful.