Abstract
Plant polysaccharides continue to serve as a promising feedstock for bioproduct fermentation. However, the recalcitrant nature of plant biomass requires certain key enzymes, including cellobiohydrolases, for efficient solubilization of polysaccharides. Thermostable carbohydrate-active enzymes are sought for their stability and tolerance to other process parameters. Plant biomass degrading microbes found in biotopes like geothermally heated water sources, compost piles, and thermophilic digesters are a common source of thermostable enzymes. While traditional thermophilic enzyme discovery first focused on microbe isolation followed by functional characterization, metagenomic sequences are negating the initial need for species isolation. Here, we summarize the current state of knowledge about the extremely thermophilic genus Caldicellulosiruptor, including genomic and metagenomic analyses in addition to recent breakthroughs in enzymology and genetic manipulation of the genus. Ten years after completing the first Caldicellulosiruptor genome sequence, the tools required for systems biology of this non-model environmental microorganism are in place.
Highlights
Over 50 years have passed since Thomas Brock’s discovery that Yellowstone National Park’s hot springs were teeming with microbial life [1]
Possibilities for this observed phenomenon include the lack of crystalline cellulose in hyperthermophilic marine biotopes, or alternatively, that cellobiohydrolases evolved in the terrestrial extreme thermophiles and do not share a common ancestry with hyperthermophiles as a result
Based on earlier shotgun cloning efforts to identify genes located in the glucan degradation locus (GDL), it was already observed that C. bescii encoded for a number of cellulases and xylanases [18], and that at least one of these enzymes was a multifunctional, modular cellulase [19] that shared similarities to CelA encoded by C. saccharolyticus [20]
Summary
Over 50 years have passed since Thomas Brock’s discovery that Yellowstone National Park’s hot springs were teeming with microbial life [1]. Enzymes capable of hydrolyzing glycosidic bonds within cellulose chains (endo-acting) have been characterized from hyperthermophilic microoganisms [11,12,13], cellobiohydrolase enzymes (exo-acting) have been identified in microorganisms growing optimally up to a temperature range of 70 to 80 ◦C, which appears to be the thermal limit for true cellulase activity. Possibilities for this observed phenomenon include the lack of crystalline cellulose in hyperthermophilic marine biotopes, or alternatively, that cellobiohydrolases evolved in the terrestrial extreme thermophiles and do not share a common ancestry with hyperthermophiles as a result. Extremely thermophilic bacteria remain as the main source of the most thermostable primary cellulases on Earth
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