Thermostable Enzymes as Biocatalysts in the Biofuel Industry

Abstract

Lignocellulose is the most abundant carbohydrate source in nature and represents an ideal renewable energy source. Thermostable enzymes that hydrolyze lignocellulose to its component sugars have significant advantages for improving the conversion rate of biomass over their mesophilic counterparts. We review here the recent literature on the development and use of thermostable enzymes for the depolymerization of lignocellulosic feedstocks for biofuel production. Furthermore, we discuss the protein structure, mechanisms of thermostability, and specific strategies that can be used to improve the thermal stability of lignocellulosic biocatalysts.

Biochemical and Structural Insights into Xylan Utilization by the Thermophilic Bacterium Caldanaerobius polysaccharolyticus

“…At the site of the first xylose residue (C-1 1 anomeric carbon), which harbors the reducing end of the xylotrisaccharide, O-3 1 is within hydrogen bonding distance to Arg-267, and O-1 1 is within hydrogen bonding distance to the backbone nitrogen of Gly-56. At site 2, both Gln-160 and the indole nitrogen of Trp-109 engage O-3 2 , and Arg-267 interacts with the hemiacetal oxygen O-5 2 . At site The inability of XBP1 to bind to glucose or cellotriose is a result of steric occlusion of the C-5 hydroxymethyl group within the tight confines of the ligand-binding pocket.…”

“…Xylan, the most common hemicellulose, is a heterogeneous polysaccharide composed mostly of linear chains of xylose with side chain substitutions. The backbone of xylan is composed of ␤-1,4-linked D-xylopyranosyl units and may be decorated with 4-O-methyl-D-glucuronyl, L-arabinofuranosyl, and acetyl substituents (2). The complete degradation of xylan requires the synergistic activity of several hemicellulolytic enzymes, such as ␤-1,4-endoxylanase, ␤-xylosidase, ␣-glucuronidase, ␣-L-arabinosidase, and acetylxylan esterase (1).…”

“…Thus, ␣-glucuronidases cleave the ␣-1,2-glycosidic bond between 4-O-methyl ␣-glucuronic acid and the xylopyranosyl unit of xylo-oligosaccharides (10). To date, most of the cloned microbial ␤-xylosidases fall within glycoside hydrolase (GH) 4 families 3, 30, 39, 43, 51, 52, and 54 (2,(11)(12)(13)(14), whereas characterized ␣-glucuronidases are assigned to either GH family 67 (6,15,16) or GH family 115 (17). With a number of advantages over mesophilic enzymes, thermostable enzymes are especially thought to improve hydrolytic performance and overall economy of the process of biofuel production from the plant cell wall.…”

Towards an Evolutionary Model of Animal-Associated Microbiomes

“…As an example, the depolymerization of cellulose and the hemicelluloses inherent in plant biomass within the GI tract represents an energetically expensive process requiring the synthesis and co-ordination of substantial enzymatic machinery [110]. Accounting for these costs and rates of hydrolysis, compared to the catabolism of the more simple sugar substrates, the deconstruction of these polymeric carbohydrates results in proportionately less energy profit per unit of time.…”

Discovery and characterization of thermophilic limonene‐1,2‐epoxide hydrolases from hot spring metagenomic libraries

“…the direct extraction and cloning of DNA from natural environments without culturing isolated microorganisms to discover novel enzymes, has been widely demonstrated in recent years [19][20][21][22]. Thermostable hydrolases with different activities, such as lipases, proteases and glycosidases, have already been identified from (hyper)thermophiles [23,24]; however, there is still no evidence for naturally thermostable EHs or the occurrence of this enzymatic activity in hot environments.…”