Lignocellulosic plant biomass is one of the most available and renewable materials on earth. It serves as a potent source of energy, being comprised of several polysaccharide sugars crosslinked to lignin. Sugars fermented into bioethanol can be used as a fuel source or additive. But removing the polysaccharides and breaking them down into smaller sugar subunits requires heat, chemical, and enzymatic treatment, and is energetically and financially expensive. Research on a wide variety of lignocellulose-degrading microbes, from archaea to bacteria to fungi, is therefore focused on finding a more efficient way to degrade these materials.
Enter Caldicellulosiruptor bescii. This organism is able to decompose lignocellulosic biomass anaerobically at high temperature, an ability researchers hope to harness to bypass costly thermochemical treatment. As a thermophile, the bacteria can survive at high temperature, a condition that already facilitates polysaccharide breakdown. Understanding this bacterial metabolic process could lead to more efficient biofuel production.
While analyzing the C. bescii genome, Israel Scott, working in Dr. Michael Adams’ lab, noticed an unusual finding: genes encoding a tungstate transporter. This was notable because utilization of the heavy metal tungsten is extremely rare in biological systems.
The researchers wanted to know if the genes were indeed importing tungsten, or if a similar, more commonly used element like molybdenum might be imported. The bacterium Pyrococcus furiosus has several characterized tungsten-utilizing, aldehyde-oxidoreductase enzymes (termed AOR). The P. furiosus enzyme was expressed in C. bescii, where it would only function if tungsten was being imported. After finding that the P. furiosus enzymes indeed functioned, the C. bescii homologs were identified and characterized.
AOR enzymes from P. furiosus have a range of aldehyde substrates (formaldehyde, propionaldehyde, crotonaldehyde, glutaraldehyde, isovaleraldehyde, benzaldehyde, and glyceraldehyde-3-phosphate), but the C. bescii strain was unable to oxidize any of these, leading the researchers to name the C. bescii homolog XOR, for “x”-oxidizing, and the divergence of XOR from the AOR family was confirmed through phylogenetic analyses.
The bacterium highly expresses XOR (it constitutes 2% of the cytoplasmic protein contents), which appears to be the only tungsten-containing enzyme in the genome. Attempts to make a gene deletion mutant were unsuccessful, suggesting that whatever substrate XOR degrades, it is an essential part of the bacterial metabolism.
Defining the enzymatic substrate will help scientists better utilize the unusual metabolism of C. bescii. AOR from P. furiosus can be utilized to generate alcohol from organic acids; maybe C. bescii can be similarly used. If we can better understand how C. bescii uses tungsten-containing enzyme to break down ligninocellulose, we can move closer to using its activity for bioengineered fuels.
-- Julie Wolf