Ether Hydrolysis, Ether Thiolysis and the Catalytic Power of Etherases in the Disassembly of Lignin.

The recycling of much of the carbon in Nature depends on the breakdown of polymers in woody matter, notably cellulose (a polyacetal) and lignin (a polyether). Here, we show that equilibrium favors ether hydrolysis in water, but that the rates of hydrolysis of ethers are too slow to measure in neutral solution except at temperatures approaching the critical point of water. Circumventing that obstacle, glutathione-dependent etherases from white-rot fungi are known to employ the thiolate group of glutathione to attack guaiacyl ethers. The present experiments at elevated temperatures indicate that thioglycolate attacks diethyl ether in water-in the absence of enzymes-with a rate constant of 6 x 10^-11 M^-1 s^-1 at 25 °, and that thiolysis is strongly favored thermodynamically, with a K(v)eq value of 2.5 x 10^6 (ΔG ~ -8.7 kcal/mol). Compared with the rate of nonenzymatic thiolysis, the lignin-degrading etherases LigE and LigF produce 10^15-fold rate enhancements, among the largest that have been observed for an enzyme acting on two substrates. As in other enzymatic displacement reactions, one function of these enzymes' active sites is presumably to remove the substrates from constraints on their reactivity imposed by solvent water.