Christopher K. Savile

Converting an Esterase into an Epoxide Hydrolase

Entering the fold: A common structural motif in hydrolytic enzymes is the alpha,beta-hydrolase fold. The interconversion of one enzyme into another by introduction of mechanistically important residues is not enough; only substitution of a loop allows epoxide hydrolase activity in the esterase scaffold to be formed (see picture; structure comparison of epoxide hydrolases (green) with the esterase (orange)). The result is an enantioselective chimeric enzyme.

Remote interactions explain the unusual regioselectivity of lipase from Pseudomonas cepacia toward the secondary hydroxyl of 2'-deoxynucleosides.

Lipase from Pseudomonas cepacia (PCL) surprisingly favors acylation of the secondary hydroxyl at the 3′‐position over the primary hydroxyl at the 5′‐position in 2′‐deoxynucleosides by up to >98:1. Catalytically productive tetrahedral intermediate analogues for both orientations were found by molecular modeling. However, acylation of the 3′‐hydroxyl places the thymine base in the alternate hydrophobic pocket of PCLs substrate‐binding site where it can hydrogen bond to the side‐chain hydroxyls of Tyr23 and Tyr29 and the main chain carbonyl of Leu17. Conversely, acylation of the 5′‐hydroxyl leaves the thymine base in the solvent where there is no favorable binding to the enzyme. We propose that these remote stabilizing interactions between the thymine base and PCLs substrate‐binding site stabilize the 3′‐acylation transition state and thus account for the unusual regioselectivity.

Substrate specificity, regioselectivity and cryptoregiochemistry of plant and animal ω-3 fatty acid desaturases

In order to define the substrate requirements, regiochemistry and cryptoregiochemistry of the omega-3 fatty acid desaturases involved in polyunsaturated fatty acid formation, the genes Fad3 and fat-1 from Brassica napus and the nematode Caenorhabditis elegans respectively were expressed in bakers yeast (Saccharomyces cerevisiae). Various fatty acids, including deuterium-labelled thia-fatty acids, were supplied to growing cultures of transformed yeast. The results from GC-MS analysis of the desaturated products indicate that both the plant and animal desaturases act on unsaturated substrates of 16-20 carbons with a preference for omega-6-unsaturated fatty acids. The regioselectivities of both enzymes were confirmed to be that of omega-3 desaturases. The primary deuterium kinetic isotope effects at C-15 and C-16 of a C(18) fatty acid analogue were measured via competitive incubation experiments. Whereas k(H)/k(D) at the omega-3 position was shown to be large, essentially no kinetic isotope effect at the omega-2 position was observed for the plant or the nematode enzymes. These results indicate that omega-3 desaturation is initiated by an energetically difficult C-H bond cleavage at the carbon closer to the carboxyl terminus. These results will be discussed in the context of a general model relating the structure and function of membrane-bound fatty acid desaturases featuring different regioselectivities.

Mechanistic characterization of ω-3 desaturation in the green alga Chlorella vulgaris

alpha-Linolenic acid (ALA, 9(Z),12(Z),15(Z)-octadecatrienoic acid) derivatives are important plant lipids which play a critical key role in cold tolerance. The final steps of ALA biosynthesis feature a series of regio- and stereoselective dehydrogenation reactions which are catalyzed by a set of enzymes known as fatty acid desaturases. In conjunction with ongoing research into the structural biology of these remarkable catalysts, we have examined the mechanism of double bond introduction at C15,16 as it occurs in a model photosynthetic organism, Chlorella vulgaris. The individual deuterium kinetic isotope effects associated with the C-H bond cleavages at C-15 and C-16 of a thialinoleoyl analogue were measured via competition experiments using appropriately deuterium-labelled 7-thia substrates. A large kinetic isotope effect (KIE) (k(H)/k(D)=10.2+/-2.8) was observed for the C-H bond-breaking step at C-15 while the C-H bond cleavage at C-16 was found to be relatively insensitive to deuterium substitution (k(H)/k(D)=0.8+/-0.2). These results point to C-15 as the site of initial oxidation in omega-3 desaturation and imply that the Chlorella and corresponding plant systems share a common active site architecture.