Trichomes amino acids

The discovery of a novel pathway for biosynthesis of medium and short chain fatty acids in plants (a-keto acid elongation pathway, 1) raises the possibility (however unlikely) that medium-chain fatty acids (mcFAs) of certain oil seeds producing them may be derived by this pathway. Alternatively, these may be formed after release of elongating fatty acid chains from fatty acid synthase mediated biosynthesis (FAS) by specific medium chain thioesterases [2, 3,4]. Thus far the aKAE pathway is only known to occur in trichome glands of plants in the family Solanaceae. In the aKAE pathway, iso-, anteiso- or straight-chain keto acid products of branched-chain amino acid metabolism are elongated by one carbon (via acetate) per cycle. The final step is predicted to be oxidative decarboxylation to yield CoA activated acids. The mechanism that determines the chain length of aKAE products is not understood [1]. 

The A -stearoyl (18 0)-ACP desaturase is found in nearly all plant tissues. In addition, several other acyl-ACP desaturases have been identifi in plants, including the A -palmitoyl (16 0)-ACP desaturase of coriander seed (1), the A -16 0-ACP desaturase of Thunbergia data seed (2), and the A -myristoyl (14 0)-ACP desaturase of geranium trichomes (3). These enzymes share >70% amino acid sequence similarity. Given this high degree of structural relatedness, comparisons of the amino acid sequences of these enzymes may provide clues as to which residues dictate the fatty acid chain-length and double bond positional specificities of acyl-ACP desaturases, especially when used in conjunction with the recently determined crystal structure of the A -18 0-ACP desaturase (4). 

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