Hydrophilic enzymes

After membrane ruffling and formation of the so-called pseudopodia, the material is engulfed by the cell and is further transported to vesicles (phagosomes/macropinosomes) that have the ability to become acidified. These vesicles fuse rapidly with late endosomes and/or lysosomes, exposing their contents to the hydrophilic enzymes. 

The sensing microzone of the flow-through sensor depicted in Fig. 5.9.B1 integrates gas-diffusion and detection with two analytical reactions [28], viz. (a) the urease-catalysed formation of ammonium ion by hydrolysis of urea (the analyte), which takes places on a hydrophilic enzyme membrane in contact with the sample-donor stream, which contains a gel where the enzyme is covalently bound and (b) an acid-b reaction that takes place at the microzone on the other side of the diffusion membrane and involves Bromothymol Blue as indicator. This is a sandwich-type sensor including a hydrophilic and a hydrophobic membrane across which the sample stream is circulated —whence it is formally similar to some enzyme electrodes. Since the enzymatic conversion of the analyte must be as efficient as possible, deteetion (based on fibre optics) is performed after the donor and acceptor streams have passed through the sensor. Unlike the previous sensor (Fig. 5.9.A), this does not rely on the wall-jet approach in addition, each stream has its own outlet and the system includes two sensing microzones... 

The water-shell-model, strictly speaking, will only apply to very hydrophilic enzymes which do not contain hydrophobic parts. Many enzymes, like lipases, are surface active and interact with the internal interface of a microemulsion. In fact, lipases need a hydrophobic surface in order to give the substrate access to the active site of the enzyme. Nevertheless, Zaks and Klibanov found out that it is often not necessary to have a monolayer of water on the enzyme surface in order to perform a catalytic reaction in an organic solvent [98]. 

Here km and kw are the second-order rate constants in the micellar pseudo-phase and the aqueous phase, respectively, Phrp and Prfc are the partition coefficients for HRP and RFc, respectively, between the micellar and aqueous phases (PA= [AJm/fA],, A = HRP or RFc), C is the total surfactant concentration without cmc (C = [surfactant] t-cmc), and V is the molar volume of micelles. Equation (39) simplifies assuming Phrp <3C 1 and PRFc 1. In fact, the hydrophilic enzyme molecule is expected to be in the aqueous phase, while hydrophobic, water-insoluble ferrocenes have a higher affinity to the micellar pseudo-phase. Taking also into account that relatively low surfactant concentrations are used, i.e., CV <5iC 1, Eq. (39) transforms into Eq. (40). 

In the last Section 6.4 new supramolecular approaches to construct synthetic biohybrid catalysts are described. So-called giant amphiphiles composed of a (hydrophilic) enzyme headgroup and a synthetic apolar tail have been prepared. These biohybrid amphiphilic compounds self-assemble in water to yield enzyme fibers and enzyme reaction vessels, which have been studied with respect to their catalytic properties. As part of this project, catalytic studies on single enzyme molecules have also been carried out, providing information on how enzymes really work. These latter studies have the potential to allow us to investigate in precise detail how slight modifications ofthe enzyme, e.g., by attaching a polymer tail, or a specific mutation, actually infiuence the catalytic activity. 

Reverse micellar extraction (RME) has been gaining popularity as an attractive hquid-hquid extraction process [108-111]. This is mainly due to the fact that enzymes can be solubilized in organic solvents with the aid of reverse micellar aggregates [112, 113]. Their inner core contains an aqueous micro-phase, which is able to solubilize polar substances, e.g., hydrophilic enzymes [114]. In many cases not only the enzymes retained their activity in organic environment in some cases they seem to perform even better if they are entrapped into reverse micellar aggregates [111]. One of the remarkable findings that gave this field a major boost is that the solubilization of different proteins into micellar solutions is a selective process [112]. 

In the processing of foods, liposomes accelerate cheese ripening and increase the yield in bioconversion through uniform distribution of hydrophilic enzymes in hydrophobic medium. 

In W/O microemulsions the enzyme molecules are invariably located (entrapped) in the water droplet region, but the exact position may vary depending on the hydrophilic-lipophilic balance of the enzyme (and to some extent on the nature of the solvent). A truly hydrophilic enzyme will be located in the water core of the droplet, surrounded by a water layer. a-Chymotrypsin is an example of such a protein. A surface-active enzyme, such as lipase, has a strong driving force for the oil/water interface, where it competes with... 

It was later demonstrated that JVq is not the only factor governing enzymatic activity in W/O microemulsions. Both a hydrophilic enzyme, a-chymotrypsin, and a lipophilic enzyme, hydroxysteroid dehydrogenase, varied in activity with surfactant concentrations at constant Wq [36]. Evidently, at least two parameters, molar water-to-surfactant ratio (Wq) and surfactant concentration, are decisive for enzymatic activity in these systems. 

There have been many attempts to explain the bell-shaped curve of enzyme activity versus Wo. It is likely that several factors contribute and that the relative importance of different parameters varies with the type of enzyme studied [40,41]. However, it seems probable that diffusion effects play a major role, and a diffusion model applicable to a hydrophilic enzyme located in the core of the water droplet and hydrophilic substrates also situated in the droplets was worked out by Walde and coworkers [42,43]. Before the enzyme-catalyzed reaction can take place, two different diffusion processes must occur. In the first of these, an interdroplet diffusion step, drops containing the substrate and drops containing the enzyme must collide. In the second process, an intradroplet diffusion step, the substrate reaches the enzyme s active site. Whereas the rate of the first process increases with droplet radius, the reverse is true for the second process. These two counteracting dependencies of reaction rate on droplet size (and thus on Wo at constant surfactant concentration) may lead to a bell-shaped activity versus Wo curve. 

Michaelis-Menten kinetics have been derived for the case of hydrophilic enzyme and hydrophilic substrate both entrapped in water droplets [55]. The normal kinetic scheme for enzymatic reactions in aqueous solution, shown in Fig. 8 (top) is the basis for the treatment, and with consideration taken of droplet collision and disintegration the kinetic scheme becomes more involved (Fig. 8, bottom). Collision of droplets containing enzyme and substrate is followed by decomposition of the transient dimer formed, leading to a droplet containing both enzyme and substrate. The enzyme-substrate complex, E S, will form within the droplet, and the complex will subsequently decompose into enzyme and product. Finally, the droplet containing both E and P will decompose into droplets containing E and droplets containing P. Reversibility of all processes except the last step is taken into consideration. The treatment leads to the prediction that hyperbolic (Michaelis-Menten)... 

In every synthetic reaction where a net amount of water is formed (such as an ester synthesis from an alcohol and a carboxylic acid [38 0]) physicochemical problems arise. Due to the fact that the lipophilic solvent (log F > 1.5) is unable to accommodate the water which is gradually produced during the course of the reaction, it is collected at the hydrophilic enzyme surface. As a consequence, the water gradually forms a discrete aqueous phase which encompasses the enzyme, finally separating substrate and enzyme from each other by a polar interface, which is difficult to penetrate for lipophilic substrate/product molecules. Thus, the rate... 

In recent years, W/O microemulsions have found numerous applications as microreactors for specific reactions (for comprehensive reviews, see Refs. 94 and 95). Thus, it has been shown that hydrophilic enzymes can be solubilized without loss of enzymatic activity and used to catalyze various chemical and photochemical reactions [96,97]. Other interesting applications involve the polymerization of solubilizates in microemulsions [98] and the preparation of micro-porous polymeric materials by polymerization of single-phase microemulsions [99]. Furthermore, microemulsions have been used as microreactors for the synthesis of nanosized particles for various applications [93,95] such as metal clusters (Pt, Pd, Rh, Au) for catalysis [100,101], semiconductor clusters [102-104] (ZnS, CdS, etc.), silver halides [105], calcium carbonates, and calcium fiuoride [106]. Recently it was shown [107,108] that it is possible to use W/O microemulsions for the control of polymorphism of water-soluble organic compounds. In most of these appUcations, one or more reactants are solubilized within a microemulsion and then a reaction is initiated. Depending on its molecular structure. 

V. Mozhaev, K. Poltevsky, V. Slepnev, G. Badun, A. Levashov, Homogeneous solutions of hydrophilic enzymes in nonpolar organic solvents. New systems for fundamental studies and bio-catalytic transformations, FEBS Lett. 292 (1991) 159-161. 

Protein stability may be regarded as the opposite of denaturation. The stability of enzymes (and proteins) can be increased in many ways, e.g., by microenvironmental changes, immobilization, and protein engineering (78). Enzymes are more stable in the presence of polyols (ethylene glycol, glycerol, erythritol, and sorbitol), polymers (PEG, dextrans), and carbohydrates (sucrose, lactose, and trehalose). Hydrophilic enzymes are stabilized by the presence of salts (LiCl, NaCl, and KCl), whereas hydrophobic enzymes are hardly affected by salts. Proteins are also stabilized by compounds that bind specifically to the folded conformation. Most of the metalloenzymes and the enzymes that have an anion-binding site fall into this category. 

When solubilized in the water pool of reverse micelles, hydrophilic enzymes experience a new microenvironment, which is different from that of the bulk aqueous solution. As a result, the activity of the enzyme may increase or decrease... 

Figure 4.4 Schematic diagram of the structure of the a/p-barrel domain of the enzyme methylmalonyl-coenzyme A mutase. Alpha helices are red, and p strands are blue. The inside of the barrel is lined by small hydrophilic side chains (serine and threonine) from the p strands, which creates a hole in the middle where one of the substrate molecules, coenzyme A (green), binds along the axis of the barrel from one end to the other. (Adapted from a computer-generated diagram provided by P. Evans.)...

This ester was developed to impart greater hydrophilicity in C-terminal peptides that contain large hydrophobic amino acids, since the velocity of deprotection with enzymes often was reduced to nearly useless levels. Efficient cleavage is achieved with the lipase from R. niveus (pH 7, 37°, 16 h, H2O, acetone, 78-91% yield)... 

Because skin exhibits many of the properties of a lipid membrane, dermal penetration can often be enhanced by increasing a molecule s lipophilicity. Preparation of an ester of an alcohol is often used for this purpose since this stratagem simultaneously time covers a hydrophilic group and provides a hydrophobic moiety the ready cleavage of this function by the ubiquitous esterase enzymes assures availability of the parent drug molecule. Thus acylation of the primary alcohol in flucinolone (65) with propionyl chloride affords procinonide (66) the same transform... 

Drugs that are too highly hydrophilic are often absorbed rather poorly from the gastrointestinal tract. It is sometimes possible to circumvent this difficulty by preparing esters of such compounds so as to change their water lipid partition characteristics in order to enhance absorption. Once absorbed, the esters are cleaved by the numerous esterase enzymes in the bloodstream, releasing free drug. 

Porous glass (PG) modified with covalently adsorbed poly(p-nitrophenyl acrylate), as described in Sect. 4.1, turned out to be a highly suitable carrier for immobilization of various biospecific ligands and enzymes. When the residual active ester groups of the carrier were blocked by ethanolamine, the immobilized ligands when bound to the solid support via hydrophilic and flexible poly(2-hydroxyethyl acrylamide). The effective biospecific binding provided by the ligands... 

In this case study, an enzymatic hydrolysis reaction, the racemic ibuprofen ester, i.e. (R)-and (S)-ibuprofen esters in equimolar mixture, undergoes a kinetic resolution in a biphasic enzymatic membrane reactor (EMR). In kinetic resolution, the two enantiomers react at different rates lipase originated from Candida rugosa shows a greater stereopreference towards the (S)-enantiomer. The membrane module consisted of multiple bundles of polymeric hydrophilic hollow fibre. The membrane separated the two immiscible phases, i.e. organic in the shell side and aqueous in the lumen. Racemic substrate in the organic phase reacted with immobilised enzyme on the membrane where the hydrolysis reaction took place, and the product (S)-ibuprofen acid was extracted into the aqueous phase. 

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

---