Proc. Natl. Acad. Sci. USA 108, 15079–15084 (2011)

Credit: LEILA LO LEGGIO

Degradation of cellulose-rich biomass is necessary for its conversion to a variety of commercially relevant end products. Previous work identified enzymes from glycoside hydrolase family GH61 as capable of assisting cellulases in breaking down cellulose's complex structure. However, the way in which the GH61 enzymes contributed to this process was unknown, as the available structures (which lack canonical active site residues and sugar-binding pockets) and their requirement for a metal are inconsistent with glycoside hydrolase activity. Quinlan et al. now shed light on this question in their exploration of a GH61 from Thermoascus aurantiacus (TaGH61). Hydrolysis assays confirmed that TaGH61's function is independent of and complementary to that of cellulases and requires a redox-active cofactor. Characterization of TaGH61's products further indicated that the enzyme oxidatively depolymerizes the cellulose chains, thus exposing new sites for hydrolysis by cellulases. The unknown metal was identified by ITC and EPR spectroscopy as copper, bound with a KD <1 pM and with tetragonal coordination geometry. Finally, an X-ray structure showed Cu(II) binding reminiscent of copper methane monooxygenase, suggesting that TaGH61 uses oxygen activation as a strong thermodynamic driving force for cellulose breakdown. Surprisingly, the structure also revealed a methyl group on one of the coordinating histidine ligands. As this modification is unprecedented for metalloenzymes, further investigation will be required to understand the impact of this methyl group on metal reactivity.