Desaturases: Emerging Models for Understanding Functional Diversification of Diiron-containing Enzymes

Desaturases and related enzymes perform O₂-dependent dehydrogenations initiated at unactivated C-H groups with the use of a diiron active site. Determination of the long-sought oxidized desaturase crystal structure facilitated structural comparison of the active sites of disparate diiron enzymes. Experiments on the castor desaturase are discussed that provide experimental support for a hypothesized ancestral oxidase enzyme in the context of the evolution of the diiron enzyme diverse functionality. We also summarize recent analysis of a castor mutant desaturase that provides valuable insights into the relationship of proposed substrate-binding modes with respect to a range of catalytic outcomes.Desaturase enzymes perform dehydrogenation reactions that result in the introduction of double bonds into fatty acids that are initiated by the energy-demanding abstraction of a hydrogen from a methylene group (1–3). To achieve this, desaturase enzymes recruit and activate molecular oxygen with the use of an active-site diiron cluster (4). The diiron center is common to a variety of proteins, including methane monooxygenase, ribonucleotide reductase, rubrerythrins, and a variety of oxidase enzymes (5). Valuable insights regarding the tuning of diiron centers with respect to diverse chemical reactivity (6) have been made via comparisons of the diiron centers of diiron-containing enzymes (7); however, differences in amino acid sequence, multiple protein-protein interactions, and reaction outcomes complicate the analysis. The study of fatty-acid desaturases and related enzymes presents a unique opportunity for performing enzyme structure-function studies because relatively close homologs perform diverse reactions on similar substrates (8, 9).Desaturase enzymes have evolved independently twice (10); the acyl-ACP² desaturases are soluble enzymes found in the plastids of higher plants, whereas the more widespread class of integral membrane desaturases is found in endomembrane systems in prokaryotes and eukaryotes (9). In addition to forming distinct homology groups, their diiron centers possess distinct primary ligation spheres (11). The availability of crystal structures for acyl-ACP desaturases (12) makes this system amenable to detailed structure-function studies. Crystal structures are available for the 18:0 Δ⁹-desaturase³ (12, 13) from Ricinus communis (castor) and a bifunctional desaturase from Hedera helix (ivy) (14, 15). These desaturases are homodimeric proteins, with each monomer folded into a compact single domain composed of nine helices. The diiron active site of these enzymes is buried within a core four-helix bundle and is positioned alongside a deep, bent, narrow hydrophobic cavity in which the substrate is bound during catalysis. It is a textbook example of a lock-and-key type of binding site in which the bound fatty acid moiety is poised for formation of the cis-fatty acid product.Nobel Laureate Konrad Bloch observed, “The stereospecific removal of hydrogen in the formation of oleate, although predictable on principle grounds would seem to approach the limits of the discriminatory power of enzymes” (16). Bloch''s statement underscores that desaturase enzymes perform highly regio- and stereo-selective reactions on long-chain fatty acids composed of essentially equivalent methylene chains that lack distinguishing landmarks close to the site of desaturation. We will review structural features of the diiron active site of the acyl-ACP desaturases in the context of those of other diiron enzymes, discuss recent insights into the evolution of acyl-ACP desaturases, and summarize recent discoveries relating to the evolution of selectivity and functional diversity within desaturase enzyme families.     

Coherent ontogenic dynamics of geraniol acetyltransferase activity and geranyl acetate concentration in flowers and leaves of aroma grass Cymbopogon martinii var. Motia

We have investigated the correlation between the concentration of geranyl acetate (GA) and acetyl CoA: geraniol acetyltransferase (GAAT) activity in palamarosa (Cymbopogon martini var. Motia) inflorescence and leaves at their different physiological stages. The results on GA concentration and the GAAT activity have been expressed on per gram fresh weight, per spikelet pair or leaf and per unit area of the phylloplane also incase of leaf. The percentage of geranyl acetate and geraniol in the volatile oil has also been considered. GA concentration was found to be highest in unopened floral spikelets and on the decline in fully open spikelets matching the trend of GAAT activity. Similarly, highest concentration of GA and maximum GAAT activity were found in leaves at mid-stage of development (stage II). The regression analysis curve between GA concentration (mg gFw⁻¹) and GAAT activity (IU 10⁻³ gFw⁻¹) gave an estimate of correlation coefficient (at 95% confidence) value of 0.79 for flowers and 0.92 for leaf. The results suggest that volatile ester (like geranyl acetate) synthesis in foliage and flowers of the aroma oil plant is controlled by the existent catalytic levels GAAT rather than the availability of geraniol. The study also indicates that the GAAT to be a good target to over-express for improvement of oil quality in terms of GA linked to fruit-fresh olfactory note of the oil.     

High level expression of a functionally active cholera toxin B: rabies glycoprotein fusion protein in tobacco seeds

A synthetic DNA construct containing cholera toxin B subunit, genetically fused to the surface glycoprotein of rabies virus was expressed in tobacco plants from a seed specific (legumin) promoter. Seed specific expression was monitored by real-time PCR, GM1-ELISA and Western blot analyses. The fusion protein accumulated in tobacco seeds at up to 1.22% of the total seed protein. It was functionally active in binding to the GM1-ganglioside receptors, suggesting its assembly into pentamers in seeds of the transgenic plants. Immunoblot analysis confirmed that the ~80.6 kDa monomeric fusion polypeptide was expressed in tobacco seeds and accumulated as a ~403 kDa pentamer. Evaluation of its immunoprotective ability against rabies and cholera is to be examined.     

Methoxylated isoflavones,cajanin and isoformononetin,have non‐estrogenic bone forming effect via differential mitogen activated protein kinase (MAPK) signaling

Following a lead obtained from stem‐bark extract of Butea monosperma, two structurally related methoxyisoflavones; cajanin and isoformononetin were studied for their effects in osteoblasts. Cajanin had strong mitogenic as well as differentiation‐promoting effects on osteoblasts that involved subsequent activation of MEK‐Erk and Akt pathways. On the other hand, isoformononetin exhibited potent anti‐apoptotic effect in addition to promoting osteoblast differentiation that involved parallel activation of MEK‐Erk and Akt pathways. Unlike genistein or daidzein, none of these two compounds appear to act via estrogen receptors in osteoblast. Once daily oral (by gavage) treatment for 30 consecutive days was given to recently weaned female Sprague–Dawley rats with each of these compounds at 10.0 mg kg^?1 day^?1 dose. Cajanin increased bone mineral density (BMD) at all skeletal sites studied, bone biomechanical strength, mineral apposition rate (MAR) and bone formation rate (BFR), compared with control. BMD levels at various anatomic positions were also increased with isoformononetin compared with control however, its effect was less potent than cajanin. Isoformononetin had no effect on the parameters of bone biomechanical strength although it enhanced MAR and BFR compared with control. Isoformononetin had very mild uterotrophic effect, whereas cajanin was devoid of any such effect. Our data suggest that cajanin is more potent than isoformononetin in accelerating peak bone mass achievement. To the best of our knowledge, this work represents the first attempt to elucidate structure‐activity relationship between the two methoxylated isoflavones regarding their effects in osteoblasts and bone formation. J. Cell. Biochem. 108: 388–399, 2009. © 2009 Wiley‐Liss, Inc.     

Biochemical composition of the marine conditioning film: implications for bacterial adhesion

The conditioning film formed on glass panels was analysed for total carbohydrates (CFCHO), total proteins (CFP) and total uronic acids (CFURA). The influence of these compounds on the adhesion of three marine bacterial cultures, Pseudomonas sp. CE-2, Pseudomonas sp. CE-10 and Bacillus sp. SS-10 was also evaluated. One-way analysis of variance suggested a significant increase in the attachment of all three cultures to conditioned glass panels. Moreover, CE-2 (r = 0.874) and CE-10 (r = 0.879) showed a significant positive correlation with CFCHO. Conversely, SS-10 (r = -0.69) showed a significant negative correlation with CFCHO. Backward multiple linear regression analysis indicated that CFCHO were the most predictive component of the conditioning film in explaining bacterial adhesion to the conditioned glass panels.     

Biosorption of Cd(II) and Pb(II) onto brown seaweed, Lobophora variegata (Lamouroux): kinetic and equilibrium studies

The present work deals with the biosorption performance of raw and chemically modified biomass of the brown seaweed Lobophora variegata for removal of Cd(II) and Pb(II) from aqueous solution. The biosorption capacity was significantly altered by pH of the solution delineating that the higher the pH, the higher the Cd(II) and Pb(II) removal. Kinetic and isotherm experiments were carried out at the optimal pH 5.0. The metal removal rates were conspicuously rapid wherein 90% of the total sorption occurred within 90 min. Biomass treated with CaCl₂ demonstrated the highest potential for the sorption of the metal ions with the maximum uptake capacities i.e. 1.71 and 1.79 mmol g⁻¹ for Cd(II) and Pb(II), respectively. Kinetic data were satisfactorily manifested by a pseudo-second order chemical sorption process. The process mechanism consisting of both surface adsorption and pore diffusion was found to be complex. The sorption data have been analyzed and fitted to sorption isotherm of the Freundlich, Langmuir, and Redlich–Peterson models. The regression coefficient for both Langmuir and Redlich–Peterson isotherms were higher than those secured for Freundlich isotherm implying that the biosorption system is possibly monolayer coverage of the L. variegata surface by the cadmium and lead ions. FT-IR studies revealed that Cd(II) and Pb(II) binding to L. variegata occurred primarily through biomass carboxyl groups accompanied by momentous interactions of the biomass amino and amide groups. In this study, we have observed that L. variegata had maximum biosorption capacity for Cd(II) and Pb(II) reported so far for any marine algae. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effect of gamma irradiation on microbial load,aflatoxins and phytochemicals present in Trigonella foenum-graecum

The effects of gamma irradiation on microbial load, total aflatoxins and phytoconstituents content of Trigonella foenum-graecum have been studied. Gamma irradiation at a dose of 2.5 kGy resulted in 2 log reduction of the total aerobic microbial count. A complete sterilization was, however, observed at 10 kGy. The total aflatoxin level decreased gradually with increase in gamma irradiation dose as compared to its un-irradiated counterparts, whereas the high performance liquid chromatography (HPLC) profile showed no change in the levels of phytochemicals up to the gamma irradiation dose of 10 kGy. HPLC profiles, however, differed in peak areas, and retention times of the components. These results suggest that gamma irradiation at a dose of 5.0 kGy was very effective for microbial decontamination because it did not adversely affect the active components of T. foenum-graecum.