Identification of catalytically important residues of the carotenoid 1,2-hydratases from Rubrivivax gelatinosus and Thiocapsa roseopersicina.

Carotenoid 1,2-hydratases (CrtC) catalyze the selective addition of water to an isolated carbon-carbon double bond. Although their involvement in the carotenoid biosynthetic pathway is well understood, little is known about the mechanism by which these hydratases transform carotenoids such as lycopene into the corresponding hydroxyl compounds. Key residues were identified at positions His239, Trp241, Tyr266, and Asp268 in CrtC from Rubrivivax gelatinosus (and corresponding positions in Thiocapsa roseopersicina). Alanine mutants at these positions were found to be completely inactive, suggesting their direct involvement in the catalytic reaction. Our resulting mechanistic hypothesis is in analogy with the recently studied class of terpenoid cyclase enzymes containing a highly acidic aspartic residue in their active site. We propose that a similar aspartic acid residue, which is conserved through all putative CrtCs, is involved in initial protonation of the double bond in lycopene.

Enzymatic hydration activity assessed by selective spectrophotometric detection of alcohols: a novel screening assay using oleate hydratase as a model enzyme.

Hydroxy fatty acids (HFAs) are high-added-value compounds, which are incorporated in polymers, lubricants, emulsifiers and stabilizers and have potential medicinal use. In nature, HFAs are regio-specifically synthesized by several enzymes, including P450 monooxygenases, lipoxygenases, hydratases, 12-hydroxylases, and diol synthases. The growing demand for HFAs warrants the development of simple and efficient analytical methods that enable high-throughput detection of the hydroxylated product in the presence of its unsaturated precursor. Herein a novel high-throughput assay for the detection of alcohols is described using oleate hydratase (OHase, EC 4.2.1.53) from Elizabethkingia meningoseptica as the model enzyme. The developed assay is based on the selective spectrophotometric detection of alkyl nitrites formed upon the reaction between the hydroxyl group and nitrous acid. The assay proved to discriminate between unsaturated fatty acids as well as small cyclic and acyclic unsaturated alkenes and their corresponding alcohols. Lower detection limits were 1.5-3 mM with excellent Z'-factors. Enzymatic reactions using OHase with oleic acid resulted in somewhat lower Z-factors for various enzyme preparations. This small scale assay can enable fast discovery of new microorganisms or improved enzymes from mutant libraries and will be useful for biocatalytic strategies involving fatty acid (de)hydrating enzymes.

Biochemical characterization of the carotenoid 1,2-hydratases (CrtC) from Rubrivivax gelatinosus and Thiocapsa roseopersicina.

Two carotenoid 1,2-hydratase (CrtC) genes from the photosynthetic bacteria Rubrivivax gelatinosus and Thiocapsa roseopersicina were cloned and expressed in Escherichia coli in an active form and purified by affinity chromatography. The biochemical properties of the recombinant enzymes and their substrate specificities were studied. The purified CrtCs catalyze cofactor independently the conversion of lycopene to 1-HO- and 1,1'-(HO)(2)-lycopene. The optimal pH and temperature for hydratase activity was 8.0 and 30°C, respectively. The apparent K (m) and V (max) values obtained for the hydration of lycopene were 24 µM and 0.31 nmol h(-1) mg(-1) for RgCrtC and 9.5 µM and 0.15 nmol h(-1) mg(-1) for TrCrtC, respectively. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis revealed two protein bands of 44 and 38 kDa for TrCrtC, which indicate protein processing. Both hydratases are also able to convert the unnatural substrate geranylgeraniol (C20 substrate), which functionally resembles the natural substrate lycopene.