Exotic biomodification of fatty acids

Many biotransformations of mid- to long chain fatty acyl derivatives are intrinsically interesting because of their high selectivity and novel mechanisms. These include one carbon transfer, hydration, isomerization, hydrogenation, ladderane and hydrocarbon formation, thiolation and various oxidative transformations such as epoxidation, hydroxylation and desaturation. In addition, hydroperoxidation of polyunsaturated fatty acids leads to a diverse array of bioactive compounds. The bioorganic aspects of selected reactions will be highlighted in this review; 210 references are cited

Catalytic diversity of fatty acid desaturases

The highly selective oxidation chemistry carried out by fatty acid desaturases is a potentially important source of novel biocatalytic activity. Recent progress in the mechanistic understanding of this set of reactions will help to guide ongoing protein engineering experiments designed to modify desaturases for specific requirements

Fatty acid desaturases: Selecting the dehydrogenation channel

Desaturation (O 2-dependent dehydrogenation) of long chain fatty acyl derivatives is a ubiquitous biotransformation which generates a wide variety of oxidized lipidic natural products with important biological properties. Recent advance

Deciphering the cryptoregiochemistry of oleate Δ12 desaturase: A kinetic isotope effect study

The intermolecular primary deuterium isotope effects on the individual C-H bond cleavage steps involved in linoleic acid biosynthesis were determined using a suitably transformed strain of Saccharomyces cerevisiae containing a functional oleate Δ12 desaturase from Arabidopsis thaliana. Mass spectral analysis of the methyl 7-thialinoleate fraction obtained from competition experiments involving methyl 7-thiastearate, methyl [12,12-2H2]-7-thiastearate and methyl [13,13-2H2]-7-thiastearate showed that cleavage of the C12-H bond is very sensitive to isotopic substitution (k(H)/k(D) = 7.3 ± 0.4) while a negligible isotope effect (k(H)/k(D) = 1.05 ± 0.04) was observed for the C13-H bond breaking step. This result strongly suggests that the site of initial oxidation for Δ12 desaturation is at C-12. The possible relationship between castor oleate 12-hydroxylase and microsomal Δ12 oleate desaturases is discussed in the context of a common mechanistic paradigm. Our methodology may be also be useful in deciphering the cryptoregiochemistry of other desaturase systems

Castor Stearoyl-ACP Desaturase Can Synthesize a Vicinal Diol by Dioxygenase Chemistry

In previous work, we identified a triple mutant of the castor (Ricinus communis) stearoyl-Acyl Carrier Protein desaturase (T117R/G188L/D280K) that, in addition to introducing a double bond into stearate to produce oleate, performed an additional round of oxidation to convert oleate to a trans allylic alcohol acid. To determine the contributions of each mutation, in this work we generated individual castor desaturase mutants carrying residue changes corresponding to those in the triple mutant and investigated their catalytic activities. We observed that T117R, and to a lesser extent D280K, accumulated a novel product, namely erythro-9,10-dihydroxystearate, that we identified via its methyl ester through gas chromatography-mass spectrometry and comparison with authentic standards. The use of 18O2 labeling showed that the oxygens of both hydroxyl moieties originate from molecular oxygen rather than water. Incubation with an equimolar mixture of 18O2 and 16O2 demonstrated that both hydroxyl oxygens originate from a single molecule of O2, proving the product is the result of dioxygenase catalysis. Using prolonged incubation, we discovered that wild-type castor desaturase is also capable of forming erythro-9,10-dihydroxystearate, which presents a likely explanation for its accumulation to ∼0.7% in castor oil, the biosynthetic origin of which had remained enigmatic for decades. In summary, the findings presented here expand the documented constellation of di-iron enzyme catalysis to include a dioxygenase reactivity in which an unactivated alkene is converted to a vicinal diol