Δ⁶ Hexadecenoic acid is synthesized by the activity of a soluble Δ⁶ palmitoyl-acyl carrier protein desaturase in Thunbergia alata endosperm

Δ⁶ Hexadecenoic acid (16:1Δ⁶) composes more than 80% of the seed oil of Thunbergia alata. Studies were conducted to determine the biosynthetic origin of the double bond of this unusual fatty acid. Assays of fractions of developing T. alata seed endosperm with [1-¹⁴C]palmitoyl (16:0)-acyl carrier protein (ACP) revealed the presence of a soluble Δ⁶ desaturase activity. This activity was greatest when 16:0-ACP was provided as a substrate, whereas no desaturation of the coenzyme A ester of this fatty acid was detected. In addition, Δ⁶16:0-ACP desaturase activity in T. alata endosperm extracts was dependent on the presence of ferredoxin and molecular oxygen and was stimulated by catalase. To further characterize this enzyme, a cDNA encoding a diverged acyl-ACP desaturase was isolated from a T. alata endosperm cDNA library using polymerase chain reaction with degenerate oligonucleotides corresponding to conserved amino acid sequences in Δ⁹stearoyl (18:0)- and Δ⁴16:0-ACP desaturases. The primary structure of the mature peptide encoded by this cDNA shares 66% identity with the mature castor Δ⁹18:0-ACP desaturase and 57% identity with the mature coriander Δ⁴16:0-ACP desaturase. Extracts of Escherichia coli that express the T. alata cDNA catalyzed the Δ⁶ desaturation of 16:0-ACP. These results demonstrate that 16:1Δ⁶ in T. alata endosperm is formed by the activity of a soluble Δ⁶16:0-ACP desaturase that is structurally related to the Δ⁹18:0- and Δ⁴16:0-ACP desaturases. Implications of this work to an understanding of active site structures of acyl-ACP desaturases are discussed.