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DESATURATION AND RELATED MODIFICATIONS OF FATTY ACIDS1
KEY WORDS: unsaturated fatty acid, protein engineering, binuclear iron, nonheme, oxygenase
ABSTRACT
Desaturation of a fatty acid first involves the enzymatic removal of a hydrogen from a methylene group in an acyl chain, a highly energy-demanding step that requires an activated oxygen intermediate. Two types of desaturases have been identified, one soluble and the other membrane-bound, that have different consensus motifs. Database searching for these motifs reveals that these enzymes belong to two distinct multifunctional classes, each of which includes desaturases, hydroxylases, and epoxidases that act on fatty acids or other substrates. The soluble class has a consensus motif consisting of carboxylates and histidines that coordinate an active site diiron cluster. The integral membrane class contains a different consensus motif composed of histidines. Biochemical and structural similarities between the integral membrane enzymes suggest that this class also uses a diiron cluster for catalysis. Soluble and membrane enzymes have been successfully re-engineered for substrate specificity and reaction outcome. It is anticipated that rational design of these enzymes will result in new and desired activities that may form the basis for improved oil crops.
INTRODUCTION
Unsaturated fatty acids contain one or more double bonds, each of which lacks two hydrogen atoms relative to its saturated counterpart. Double bonds in fatty acids are predominantly of the cis (or Z) configuration. The number and position of double bonds in fatty acids profoundly affects their physical and therefore their physiological properties (50, 83).
Various mechanisms have evolved for the introduction of double bonds into fatty acids. Many prokaryotes, including Escherichia coli, introduce double bonds into fatty acids anaerobically (12). The advent of an aerobic environment several billion years ago allowed eukaryotes, cyanobacteria, and some bacilli to desaturate the methylene groups of long-chain fatty acids using enzymes called fatty acid desaturases (13,44). Oxidative desaturation is more energy demanding than the anaerobic introduction of double bonds into fatty acids. However, the transition from anaerobic fermentation to aerobic respiration yielded greater than an order of magnitude of energy efficiency, leaving a surplus available for processes such as desaturation. In addition, the ability to regulate membrane fluidity by controlling the number of double bonds in fatty acids within the membrane in response to...