AA9

AA9 stands for Auxiliary Activity family 9, a group within the CAZy database of carbohydrate-active enzymes. AA9s are copper-dependent oxidative enzymes that catalyze the cleavage of glycosidic bonds in cellulose and related beta-1,4-linked polysaccharides. They were first recognized as a distinct class of cellulose-active enzymes and were historically labeled as GH61 and CBM33 before being reclassified under the CAZy system as AA9. They are produced by a variety of fungi and some bacteria and commonly function as accessory enzymes in biomass degradation, acting synergistically with hydrolytic cellulases.

The catalytic mechanism involves a copper ion in the enzyme’s active site, coordinated by a so-called histidine brace (two histidines and additional ligands). The enzyme abstracts electrons from a reductant, enabling a monooxygenation reaction that introduces oxidative cuts into the polysaccharide backbone, producing aldonic or ketoaldonic chain ends. Substrates include crystalline cellulose and soluble cello-oligosaccharides; activity can be stimulated by cellulase partners and electron donors such as cellobiose dehydrogenase.

In terms of structure, AA9 enzymes typically have a single catalytic domain, sometimes fused to carbohydrate-binding modules (notably CBM1) that help bind polysaccharide substrates. Phylogenetically, AA9s are divided into subfamilies with varying substrate preferences and regioselectivity (C1 vs C4 oxidation).

Applications and relevance: AA9 LPMOs are central to modern biomass conversion efforts, improving the efficiency of enzymatic saccharification of plant cell walls in biofuel production and other biotechnological uses. Ongoing research explores diversification, mechanism, and optimal partnering enzymes.