Catalytic behavior ofPseudomonas cepacia lipase in w/o microemulsions

Comparison of enzymatic activities of lipases from Burkholderia plantarii and Burkholderia cepacia

Lipases (EC 3.1.1.3) are enzymes used in the oils and fats industries to modify the physicochemical properties of triacylglycerol (TAG). Lipase-catalyzed interesterification at high temperatures is an effective method for modifying the physicochemical properties of TAG. The lipase from Burkholderia plantarii (BpL) exhibits excellent catalytic activity for non-regiospecific interesterification at high temperatures, with depressed lipase hydrolytic activity. The detailed catalytic mechanism for reactions involving catalytic residues has not been elucidated because of the lack of a conventional method for estimating interesterification activity. We used our original water-in-oil emulsion system to estimate the interesterification activity of lipases. BpL showed 10% hydrolytic and 140% interesterification activities compared to the lipase from Burkholderia cepacia, which has a high sequence homology with BpL. By comparing the sequence and crystal structure data of the lipases, we clarified that two amino acids near the active center are one of the factors controlling the hydrolytic and interesterification activities of the enzyme.

Organic Reactivity in Aot-Stabilized Microemulsions

Microemulsions are highly versatile reaction media, which currently find many applications. In this review, we shall describe recent trends in the use of microemulsions as organic reaction media, and present models for their functioning, in particular the pseudophase model. This model allows a quantitative explanation of organic reactivity in these microheterogeneous media.

Nonionic surfactants: a key to enhance the enzyme activity at cationic reverse micellar interface

The primary objective of the present study is to understand how the different nonionic surfactants modify the anisotropic interface of cationic water-in-oil (W/O) microemulsions and thus influences the catalytic efficiency of surface-active enzymes. Activity of Chromobacterium viscosum lipase (CV-lipase) was estimated in several mixed reverse micelles prepared from CTAB and four different nonionic surfactants, Brij-30, Brij-92, Tween-20, and Tween-80/water/isooctane/n-hexanol at different z ([cosurfactant]/[surfactants]) values, pH 6 (20 mM phosphate), 25 degrees C across a varying range of W0 ([water]/[surfactants]) using p-nitrophenyl-n-octanoate as the substrate. Lipase activity in mixed reverse micelles improved maximum up to approximately 200% with increasing content of non-ionic surfactants compared to that in CTAB probably due to the reduced positive charge density as well as plummeted n-hexanol (competitive inhibitor of lipase) content at the interfacial region of cationic W/O microemulsions. The highest activity of lipase was observed in CTAB (10 mM) + Brij-30 (40 mM)/isooctane/n-hexanol)/water system, k2 = 913 +/- 5 cm3 g-1 s-1. Interestingly, this observed activity is even higher than that obtained in sodium bis (2-ethyl-1-hexyl) sulfosuccinate (AOT)/n-heptane reverse micelles, the most popular W/O microemulsion in micellar enzymology. To ascertain the influence of non-ionic surfactants in improving the activity of surface-active enzymes is not limited to lipase only, we have also investigated the catalytic activity of Horseradish peroxidase (HRP) in different mixed W/O microemulsions. Here also following the similar trend as observed for lipase, HRP activity enhanced up to 2.5 fold with increasing concentration of nonionic surfactants. Finally, the enzyme activity was correlated with the change in the microenvironment of mixed reverse micelles by steady-state fluorescence study using 8-anilino-1-napthalenesulphonic acid (ANS) as probe.

Reverse micelles and protein biotechnology

Reverse micelles are nanometer-sized (1-10 nm) water droplets dispersed in organic media obtained by the action of surfactants. Surfactant molecules organize with the polar part to the inner side able to solubilize water and the apolar part in contact with the organic solvent. Proteins can be solubilized in the water pool of reverse micelles. Studies on the structure-function relationships of proteins in reverse micelles are very important since the microenvironment in which the protein is solubilized has physico-chemical properties distinct from a bulk aqueous solution. Some of the unique characteristics of reverse micelles make them very useful for biotechnological applications. Charge and hydrophilic/hydrophobic characteristics of the protein and the selection of surfactant can be used to achieve selective solubilization of proteins. This has been used to extend the classical liquid-liquid extraction with solvents to protein bioseparation. For biocatalysis the presence of a bulk organic solvent allow synthetic reactions to be performed via the control of water content and the solubilization of hydrophobic substrates. This is accomplished with a higher interfacial area (about 100 m2/mL) than the conventional biphasic systems, minimizing mass transfer problems.

A Procedure for the Joint Evaluation of Substrate Partitioning and Kinetic Parameters for Reactions Catalyzed by Enzymes in Reverse Micellar Solutions

A simple method useful for the joint evaluation of substrate partitioning and kinetic parameters for reactions catalyzed by enzymes entrapped in reverse micelles is proposed. The method is applied to the hydrolysis of 2-naphthyl acetate (2-NA) catalyzed by lipase in sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT)/buffer/heptane reverse micellar solutions. In the presence of micelles, the relationship between the initial reaction rate and the analytical concentration of 2-NA was dependent on AOT concentration at a constant W ([water]/[AOT]) value. The dependence of the initial reaction rate profiles with [AOT] was analyzed according with the method proposed to obtain the partition constant of 2-NA between the micelles and the external solvent, Kp. A value of Kp = 2.7 L mol(-1) was obtained irrespective of the water content of the micelles (W from 5 to 20). The catalytic rate constant kcat in the micellar solutions was independent of [AOT] but slightly decreased with an increase in W from 2 x 10(-6) mol g(-1) s(-1) at W = 5 to 1.2 x 10(-6) mol g(-1) s(-1) at W = 20. The apparent Michaelis constant determined in terms of the analytical concentration of 2-NA increased with [AOT] at a given W and moderately decreased with W at a fixed [AOT]. The increase with [AOT] is accounted for by considering the partitioning of the substrate. After correction for the partitioning of 2-NA values of (Km)corr were obtained as 3.9 x 10(-3) mol L(-1) (W = 5), 4.6 x 10(-3) mol L(-1) (W = 10), 2.3 x 10(-3) mol L(-1) (W = 15), and 1.7 x 10(-3) mol L(-1) (W = 20). The rate parameters in the aqueous phase in the absence of micelles, were obtained as (kcat)aq = 7.9 x 10(-6) mol g(-1) s(-1) and (Km)aq = 2.5 x 10(-3) mol L(-1). In order to compare the efficiency of the enzyme in the micellar solution with that in aqueous phase, the values of (Km)corr were in turn corrected to take into account differences in the substrate activity, obtaining so a set of (Km)*corr values. The efficiency of the enzyme in the micellar solution, defined as the ratio, kcat/(Km)*corr, was found to be higher than in the aqueous phase, even at high water contents (W = 20). This higher efficiency is due to a significant decrease in (Km)*corr values.

Bioorganic reactions in microemulsions

Water-in-oil microemulsions, or reverse micelles, are being evaluated as a reaction medium for a variety of enzymatic reactions. These systems have many potential biotechnological applications. Important examples are the use of various lipase microemulsion systems for hydrolytic or synthetic reactions. This review illustrates the biotechnological applications of microemulsions as media for bioorganic reactions. The principal focus is on lipase catalyzed processes.