Metabolic Engineering of Vegetable Oils withShort and Medium-Chain Fatty Acids

Lauric acid (12:0) is employed extensively in the soaps and detergents industry. Although adequate supplies are available from coconut and palm oils, there has been interest in developing temperate sources of 12:0 feedstocks, and high laurate rapeseed canola was the first genetically engineered oil in commercial production. Oils enriched in shorter fatty acids such as capric acid (10:0) and caprylic acid (8:0) would also be useful in the manufacture of detergents, surfactants, and plasticizers.

Production of medium-chain fatty acids is initiated by the early release of acyl groups from ACP by medium-chain acyl-ACP thioesterases. Thioesterases specific for medium-chain fatty acids have been identified in seeds that produce high levels of 8:0, 10:0, 12:0, and 14:0 from plants as distantly related as California bay, Cuphea, coconut, elm, and camphor (Voelker et al., 1992, 1997). These novel thioesterases share significant amino acid sequence with FatB acyl-ACP thioesterases and have likely evolved from common predecessors. Comparison of these sequences has provided considerable insight into chain length specificity of acyl- ACP thioesterases, for example, Yuan et al. converted a 14:0-ACP thioesterase into a 12:0-ACP thioesterase by altering only three amino acids (Yuan et al., 1995).

Transformation of rapeseed with the California bay 12:0-ACP thioesterase gene has resulted in oils with lauric acid content exceeding 50% of the total fatty acid (Voelker et al., 1992). This represents one of the most significant technical achievements in the metabolic engineering of seed oil composition. Efforts to modify oil composition with thioesterases from 8:0- to 10:0-producing seeds have been less successful (Voelker and Kinney, 2001). However, cotransformation with genes for medium-chain thioesterase and KASIV, a 10:0 KAS from Cuphea, shifts fatty acid accumulation strongly toward shorter chain lengths (Leonard et al., 1998). The KASIV gene alone does not confer deposition of medium-chain fatty acids, perhaps because it is inhibited by short and medium-chain ACPs (Dehesh et al., 1998; Schu¨ tt et al., 2002).

Further improvement of medium-chain production in transgenic rapeseed has been achieved by transformation with a coconut lysophosphatidic acid acyltransferase (LPAAT) gene. While native rapeseed LPAAT excludes 12:0 from the sn-2 position, the transformants do not (Knutzon et al., 1999).