Role of Lytic Polysaccharide Monooxygenase Enzymes in Biomass Degradation

The Second generation of biofuels utilizes inedible lignocellulosic biomass, agricultural residues and different kinds of woods, for producing valuable materials such as bioethanol [1]. This is a very promising method in the field of renewable fuels generation. Enzymatic degradation is one of the most important phases of lignocellulosic materials conversion into fermentable sugars. Classically, three glycoside hydrolase enzymes including endoglucanases (EC 3.2.1.4), exoglucanases (EC 3.2.1.91) and β-glucosidases (EC 3.2.1.21), can degrade cellulose, the main substance of biomass, to glucose [2, 3]. Recently, this model of enzymatic decomposition of polysaccharides has been challenged by discovering of lytic polysaccharide monooxygenase (LPMO) enzymes. LPMOs are copper-dependent enzymes which cut glycoside bonds using of oxidative mechanism and an electron donor molecule [4]. They could act on different polysaccharides including cellulose [5], hemicellulose [6], chitin [7], and starch [8]. These enzymes are belonging to auxiliary activity families 9, 10, 11, 13, 14, and 15 [9-11]. The structure of LPMOs composite of two antiparallel beta strands that connect to each other by loops containing various numbers of α- helices. Active site has a flat shape and consisting of a copper center which coordinates with three nitrogen atoms (two atoms from His 1 and one atom from a conserved histidine) [12-14]. Some of LPMO enzymes have cellulose binding modules (CBMs) in addition to a catalytic domain. The CBMs are independent domains which form a planar surface with hydrophobic amino acids and play different roles such as polysaccharide recognition, substrate specificity and increasing enzyme efficiency [15, 16]. LPMO activities require molecular oxygen or hydrogen peroxide [17] and an electron donor [7, 12] and would generate oxidized carbon chain ends at C1 and/or C4 positions [18]. The main electron donors are included small reducing molecules, (gallic acid and ascorbic acid), soluble derivatives from biomass (phenols derived from lignin), dehydrogenase enzymes (cellobiose dehydrogenase and glucose dehydrogenase), and light harvesting pigments [7, 12, 19-21].Several studies indicate that LPMOs could synergistically increase cellulase enzymes activities during biomass decomposition [22, 23]. LPMO enzymes are considered as the key factors in new enzyme combinations designing for biofuels production.

Keywords:

Biofuels , Cellulose , Lytic Polysaccharide Monooxygenases.