Deconstruction: Converting Lignocellulosic Biomass to Sugars
The simplest approach to overcoming the recalcitrance of biomass relies on pretreatment by mechanical or chemical methods. Pretreatment aims to decrease the crystallinity of cellulose and increase the accessibility of the biomass for subsequent hydrolysis. Biomass pretreatment by dilute acid hydrolyzes the hemicellulose component, whereas treatment with alkali removes part of the lignin. These approaches, however, are not economically optimal. Other pretreatment approaches include steam and alkaline explosive decompression, and hydrothermolysis. All of these pretreatment methods expose the cellulose fibers and make them more accessible to cellulase enzymes, which can then hydrolyze the cellulose to fermentable sugars. Enzymatic hydrolysis does not produce by-products and thus offers the possibility of improving the costs of biofuels production.
Cellulolytic microorganisms (fungi and bacteria) produce enzymes that act synergistically to hydrolyze plant cell wall materials. Fungi produce three types of cellulolytic enzymes, random-acting endoglucanases, which produce free ends from cellulose fibrils that can be degraded by exoglucanases, which produce the glucose dimer, cellobiose. The third type of enzyme, β-glucosidase hydrolyzes the released cellobiose to produce glucose. Bacteria typically employ a molecular “machine”, the cellulosome, to break down cellulose. Presently, our understanding of the fundamental mechanisms of enzymatic cellulose degradation is limited.
Hemicellulose is degraded by a class of enzymes known as hemicellulases, which are multi-domain enzymes containing structurally discrete catalytic and non-catalytic domains. Hemicellulases from different organisms are either classified generally as glycoside hydrolases or carbohydrate esterases, which hydrolyze acetate or ferulic acid side groups of the hemicellulose polymer, respectively. Compared to the research and development effort in the scientific community to understand and optimize cellulase enzymes, very little is known about the exact mechanisms of hemicellulases, and methods to engineer these enzymes are nascent. Lignin, the third major component of biomass, is the most recalcitrant component of biomass to enzymatic attack. Lignases generally consist of a family of enzymes, including phenol oxidase (laccase), peroxidases (lignin peroxidase) and manganese peroxidase.
Thus, the major objectives of JBEI’s Deconstruction Division are 1) to improve pretreatment methods with broad applicability to a range of feedstocks; 2) to explore new sources of lignocellulolytic enzymes from natural environments, relying on high-throughput protein production and directed evolution using on-chip technologies; 3) to examine microbial communities for new sources of cellulolytic and lignolytic enzymes; and 4) to develop lignin models and ligninase assays that enable the creation of modified ligninases for enhanced degradation and conversion of modified lignin.
