Detailed studies of enzymatic hydrolysis of the abundant but recalcitrant biopolymers cellulose and chitin has led to a revised understanding of enzymatic decomposition of polysaccharides. Copper-enzymes now classified as lytic polysaccharide monooxygenases (LPMOs) were first annotated as either glycosyl hydrolases or as binding domains without catalytic activity. It was shown that these enzymes are copper-dependent and cleaves glucosidic bonds when provided with plant derived cofactors or alternative electron donors. The use of LPMOs for industrial saccharification of lignocellulose has also been studied in detail and the importance of controlling the oxidative process was emphasised. A positive effect of adding catalase was attributed to prolonged enzyme half-life.
Lately, a combined study into two closely related AA9-family LPMOs from Lentinus similis (LsAA9A) and Collariella virescens (CvAA9A) was reported. LsAA9A and CvAA9A cleave a range of polysaccharides, including cellulose, xyloglucan, mixed-linkage glucan, and glucomannan. LsAA9A additionally cleaves isolated xylan substrates, the first LPMO to show such activity. Insights into the determinants of specificity come from the structures of CvAA9A and of LsAA9A bound to cellulosic and non-cellulosic oligosaccharides. EPR spectra further reveal differences in copper co-ordination upon the binding of xylan compared to glucans. LsAA9A activity is notably less sensitive to reducing agent potential on xylan when compared to other substrates, revealing a different mechanistic pathway for the cleavage of xylan. These data show that AA9 LPMOs can employ different oxidative mechanisms dependent upon both protein:carbohydrate interactions and electronic considerations at the copper active site.