Desaturation and hydroxylation. Residues 148 and 324 of Arabidopsis FAD2, in addition to substrate chain length, exert a major influence in partitioning of catalytic specificity.

Exchanging the identity of amino acids at four key locations within the Arabidopsis thaliana oleate desaturase (FAD2) and the Lesquerella fendleri hydroxylase/desaturase (LFAH) was shown to influence partitioning between desaturation and hydroxylation (Broun, P., Shanklin, J., Whittle, E., and Somerville, C. (1998) Science 282, 1315-1317). We report that four analogous substitutions in the FAD2 sequence by their equivalents from the castor oleate hydroxylase result in hydroxy fatty acid accumulation in A. thaliana to the same levels as for the wild-type castor hydroxylase. We also describe the relative contribution of these substitutions, both individually and in combination, by analyzing the products resulting from their expression in A. thaliana and/or Saccharomyces cerevisiae. Yeast expression showed that M324V, a change reachable by a single point mutation, altered the product distribution approximately 49-fold, and that residue 148 is also a predominant determinant of reaction outcome. Comparison of residues at position 148 of FAD2, LFAH, and the Ricinus oleate hydroxylase prompted us to rationally engineer LFAH-N149I, a variant with approximately 1.9-fold increase in hydroxylation specificity compared with that of wild-type LFAH. Control experiments showed that the wild-type Arabidopsis thaliana FAD2 desaturase has inherent, low level, hydroxylation activity. Further, fatty acid desaturases from different kingdoms and with different regiospecificities exhibit similar intrinsic hydroxylase activity, underscoring fundamental mechanistic similarities between desaturation and hydroxylation. For LFAH mutants the hydroxylation:desaturation ratio is 5-9-fold higher for 18-carbon versus 16-carbon substrates, supporting our hypothesis that substrate positioning in the active site plays a key role in the partitioning of catalytic specificity.