Xylanolytic Extremozymes Retrieved From Environmental Metagenomes: Characteristics, Genetic Engineering, and Applications

Abstract

Xylanolytic enzymes have extensive applications in paper, food, and feed, pharmaceutical, and biofuel industries. These industries demand xylanases that are functional under extreme conditions, such as high temperature, acidic/alkaline pH, and others, which are prevailing in bioprocessing industries. Despite the availability of several xylan-hydrolyzing enzymes from cultured microbes, there is a huge gap between what is available and what industries require. DNA manipulations as well as protein-engineering techniques are also not quite satisfactory in generating xylanhydrolyzing extremozymes. With a compound annual growth rate of 6.6% of xylanhydrolyzing enzymes in the global market, there is a need for xylanolytic extremozymes. Therefore, metagenomic approaches have been employed to uncover hidden xylanolytic genes that were earlier inaccessible in culture-dependent approaches. Appreciable success has been achieved in retrieving several unusual xylanolytic enzymes with novel and desirable characteristics from different extreme environments using functional and sequence-based metagenomic approaches. Moreover, the Carbohydrate Active Enzymes database includes approximately 400 GH-10 and GH-11 unclassified xylanases. This review discusses sources, characteristics, and applications of xylanolytic enzymes obtained through metagenomic approaches and their amelioration by genetic engineering techniques.

Extremophilic Oxidoreductases for the Industry: Five Successful Examples With Promising Projections

“…Further developments in sequencing, computation and bioinformatics might solve the problems associated with data mining for extremozymes and can help overcome the annotation difficulties in the near future. So far, we are currently seeing a boom of culture-independent methods for bioprospection of novel extremozymes from environmental metagenomes, which will help feed in the databases, opening the possibility to isolate and functionally characterize genes from unculturable extremophilic microorganisms (Khan and Sathya, 2018;Verma and Satyanarayana, 2020;Sysoev et al, 2021). On the other hand, there is a culture dependent functional approach for the discovery and development of novel extremozymes, which has been largely used over the years with proven successful results (Caceres-Moreno et al, 2019;Espina et al, 2020).…”

Extremophilic Prokaryotic Endoxylanases: Diversity, Applicability, and Molecular Insights

“…Archaea may be considered as a synonym of extremophiles due to their abundance in extreme habitats ( Cabrera and Blamey, 2018 ). This group of extreme prokaryotes has also been explored for recovering hemicellulases especially thermophilic endoxylanases ( Verma and Satyanarayana, 2020 ). Two strains of Thermophilum were the first to be reported from archaeal isolates for having putative hemicellulase activity ( Bragger et al, 1989 ).…”

“…Direct cloning of the community DNA and their analysis for recovering the product of interest made this technology a milestone invention in the field of microbiology. Several xylanolytic enzymes from mesophilic to extremophilic environments have been discovered by using metagenomic (functional and sequence-based) approaches ( Verma and Satyanarayana, 2020 ). The CAZy database shows the presence of approximately 500 candidates of GH-10 and GH-11 xylanolytic enzymes using this approach to date, where the majority of them are uncharacterized.…”

“…It indicates the wide distribution of GH-10 xylanases in the environment because these enzymes are less substrate-specific as compared to the GH-11 xylanases. The compost soil metagenome has come out with several hyperthermophilic endoxylanases that showed their optimum temperature beyond 65°C ( Verma and Satyanarayana, 2020 ). For example, metagenomic endoxylanases XYL38 ( Ellila et al, 2019 ), Mxyl ( Verma et al, 2013b ), and Xyn11 ( Kanokratana et al, 2014 ) recovered from compost metagenome showed the optimum temperature at 80°C.…”

“…The synergistic effect of these enzymes removes the xylan as well as lignin from the cellulosic fibers in a specific and refined manner without affecting the quality of the cellulose ( Sharma et al, 2007 ). In search of efficient enzymes, extremophilic microorganisms always come on the first choice as extremophiles are enough capable to perform this task due to the abundance of proteins or enzymes of extreme properties ( Verma and Satyanarayana, 2020 ). These astonishing microorganisms represent themselves as the pioneer organisms that were evolved during the origin of the earth ( Vieille and Zeikus, 2001 ).…”