Researchers have identified a novel molecular mechanism for the biosynthesis of hydrocarbons in the algae Botryococcus braunii race B. (Earlier post.) The discovery, published in the Proceedings of the National Academy of Sciences (PNAS) , could aid the direct production of biofuels from engineered organisms.
B. braunii can grow in both freshwater and brackish environments. The algae synthesize long-chain liquid hydrocarbon compounds and sequester them for buoyancy. (Typical hydrocarbon content of the organism is approximately 30-40% of the dry weight of the cells.)
Three phenotypically distinct isolates, or “races,” of B. braunii have been reported (races A, B, and L), which are identified by the type of oil produced and accumulated by the organism. Of these, the oils produced by race B, a family of isoprenoid compounds termed botryococcenes, hold the most promise as an alternative energy source. Botryococcenes have been converted to fuel suitable for internal combustion engines through caustic hydrolysis, and geochemical analysis has shown that botryococcenes, presumably from ancient B. braunii communities, also compose a portion of the hydrocarbon masses in several modern-day petroleum and coal deposits.
The US Department of Energy Joint Genome Institute (JGI) began sequencing B. braunii in 2008, targeting the identification of specific metabolic pathways responsible for hydrocarbon synthesis. Co-authors of the PNAS paper Joseph Chappell (Univ. of Kentucky); Shigeru Okada (Tokyo Univ.); and Timothy P. Devarenne (Texas A&M Univ.) were among the principal investigators in the JGI sequencing effort.
Botryococcene biosynthesis is thought to resemble that of squalene, a metabolite essential for sterol metabolism in all eukaryotes. Squalene arises from an initial condensation of two molecules of farnesyl diphosphate (FPP) to form presqualene diphosphate (PSPP), which then undergoes a reductive rearrangement to form squalene. In principle, botryococcene could arise from an alternative rearrangement of the presqualene intermediate.
Because of these proposed similarities, we predicted that a botryococcene synthase would resemble squalene synthase and hence isolated squalene synthase-like genes from Botryococcus braunii race B. While B. braunii does harbor at least one typical squalene synthase, none of the other three squalene synthase-like (SSL) genes encodes for botryococcene biosynthesis directly. SSL-1 catalyzes the biosynthesis of PSPP and SSL-2 the biosynthesis of bisfarnesyl ether, while SSL-3 does not appear able to directly utilize FPP as a substrate.
However, when combinations of the synthase-like enzymes were mixed together, in vivo and in vitro, robust botryococcene (SSL-1+SSL-3) or squalene biosynthesis (SSL1+SSL-2) was observed. These findings were unexpected because squalene synthase, an ancient and likely progenitor to the other Botryococcus triterpene synthases, catalyzes a two-step reaction within a single enzyme unit without intermediate release, yet in B. braunii, these activities appear to have separated and evolved interdependently for specialized triterpene oil production greater than 500 MYA. Coexpression of the SSL-1 and SSL-3 genes in different configurations, as independent genes, as gene fusions, or targeted to intracellular membranes, also demonstrate the potential for engineering even greater efficiencies of botryococcene biosynthesis.—Niehuas et al.
Tom D. Niehaus, Shigeru Okada, Timothy P. Devarenne, David S. Watt, Vitaliy Sviripa, and Joe Chappell (2011) Identification of unique mechanisms for triterpene biosynthesis in Botryococcus braunii, PNAS doi: 10.1073/pnas.1106222108