Similarities to Enzymes

Both enzymes and receptors are proteins, and therefore both biological structures share many fundamental similarities. Regardless their roles within a biological system are distinct from one another. 

---

Zeolites are used in various applications such as household detergents, desiccants and as catalysts. In the mid-1960s, Rabo and coworkers at Union Carbide and Plank and coworkers at Mobil demonstrated that faujasitic zeolites were very interesting solid acid catalysts. Since then, a wealth of zeolite-catalyzed reactions of hydrocarbons has been discovered. Eor fundamental catalysis they offer the advantage that the crystal structure is known, and that the catalytically active sites are thus well defined. The fact that zeolites posses well-defined pore systems in which the catalytically active sites are embedded in a defined way gives them some similarity to enzymes. 

Abstract In the first part of this mini review a variety of efficient asymmetric catalysis using heterobime-tallic complexes is discussed. Since these complexes function at the same time as both a Lewis acid and a Bronsted base, similar to enzymes, they make possible many catalytic asymmetric reactions such as nitroal-dol, aldol, Michael, Michael-aldol, hydrophosphonyla-tion, hydrophosphination, protonation, epoxide opening, Diels-Alder and epoxi-dation reaction of a, 3-unsaturated ketones. In the second part catalytic asymmetric reactions such as cya-nosilylations of aldehydes... 

These protruding structures may form catalytic sites or binding sites for small molecules. Remarkably, some polysaccharide modification enzymes of pathogenic bacteria are similar to enzymes of their host cells. For example, during cell development, pectin, a principal component in the primary cell wall of plants, is modified by its own pectin methylesterases that have /1-solenoid structures (Johansson et al, 2002) and in this respect, resemble pectin lyases secreted by bacteria to break down these structures (Lietzke et al., 1996). 

The catalytic principle of micelles as depicted in Fig. 6.2, is based on the ability to solubilize hydrophobic compounds in the miceUar interior so the micelles can act as reaction vessels on a nanometer scale, as so-called nanoreactors [14, 15]. The catalytic complex is also solubihzed in the hydrophobic part of the micellar core or even bound to it Thus, the substrate (S) and the catalyst (C) are enclosed in an appropriate environment In contrast to biphasic catalysis no transport of the organic starting material to the active catalyst species is necessary and therefore no transport limitation of the reaction wiU be observed. As a consequence, the conversion of very hydrophobic substrates in pure water is feasible and aU the advantages mentioned above, which are associated with the use of water as medium, are given. Often there is an even higher reaction rate observed in miceUar catalysis than in conventional monophasic catalytic systems because of the smaller reaction volume of the miceUar reactor and the higher reactant concentration, respectively. This enhanced reactivity of encapsulated substrates is generally described as micellar catalysis [16, 17]. Due to the similarity to enzyme catalysis, micelle and enzyme catalysis have sometimes been correlated in literature [18]. 

Polymer catalysts showing interactions with the substrate, similar to enzymes, were prepared and their catalytic activities on hydrolysis of polysaccharides were investigated. Kinetical analyses showed that hydrogen bonding and electrostatic interactions played important roles for enhancement of the reactions and that the hydrolysis rates of polysaccharides followed the Michaelis-Menten type kinetics, whereas the hydrolysis of low-molecular-weight analogs proceeded according to second-order kinetics. From thermodynamic analyses, the process of the complex formation in the reaction was characterized by remarkable decreases in enthalpy and entropy. The maximum rate enhancement obtained in the present experiment was fivefold on the basis of the reaction in the presence of sulfuric acid. 

Similar to enzymes, the biomimetic catalysts mentioned operate in liquids. Their activity depends on the diluter origin, reaction mixture pH and cell effects. Gas-phase oxidation is free from these effects, which can be considered in the first approximation as oxidation under quasi-ideal conditions [53], The study of resonance Raman spectra [54] of PPFe3+ 0H/A1203 catalase mimic indicated its clear analogy with the fifth coordinate high-spin heme Fe3+ ion, bonded to tyrosine in catalase. 

Note that the monooxygenase model reactions described above are performed in non-aqueous diluters. Therefore, hematin form is absent and only hemin is present, of which, apparently, formation of an intermediate shaped as Hm=0 is typical. Model catalysts of cytochrome P-450 operate in liquids, similar to enzymes themselves. Their activity depends on many factors, including diluter origin, reaction mixture pH and cell effects. As indicated [1], the gas-phase version of the oxidation process is much freer from these effects. 

Other alkylalumoxanes (e.g., ethylalumoxane or isobutylalumoxane) were also used as cocatalysts instead of MAO (67) but show a much lower polymerization activity. The combination of zirconocenes with MAO is evidently optimal. The three-dimensional structure plays a role (there is a rough similarity to enzymes)—changing either the metal or the alkyl groups leads to lower activity, as does changing the amino acids in enzymes. 

Our study on the distribution of electron transferring proteins in animal sources is still in progress. From present knowledge, adrenodoxin can be found in adrenal cortexes from pig, beef, and rat. Further, a similar protein was isolated from pig testis (see II-A-2), and it was also found in the ovary. However, brain, heart, liver, kidney, and pancreas appear to lack adrenodoxin-like protein. If this is correct, the proteins of the ferredoxin family are located solely in the glands which directly act in the biosynthesis of steroid hormones. It is of interest that adrenodoxin-like protein does not participate in the steroid hydroxylation involved in cholesterol and cholic acid biosyntheses. All of these reactions without the participation of adrenodoxin are similar to enzymes responsible for microsomal non-specific hydroxylation, which consist of the following sequence of electron transfer ... 

A receptor is a part of a large molecule (often a protein) or structure which binds another, usually smaller, molecule. This binding site will be specific for a particular chemical or group of chemicals, normally hormones or transmitters, and the binding may be very tight. The receptor is like a lock into which the chemical substance (the ligand) fits as a key. So receptors are similar to enzymes. 

Although the emphasis in this review has been on the manipulation of synthetic polymers it would not be appropriate to omit entirely the complementary work on biopolymers. Two important strategies are evolving here, both of which involve molecular imprinting. The first concerns specifically chemically modified proteins, and the second the generation of catalytic antibodies with functions similar to enzymes, but potentially with much greater scope in terms of the reactions catalysed. 

In general, these molecules base their recognition on a specialized binding site and, similarly to enzymes, they catalyse a large variety of reactions, displaying a similar specificity, stereo specificity, kinetics and competitive inhibition, than their enzyme counterparts [453]. However, the rate of accelerations obtained by transition state stabilization still remains lower than that obtained with enzymes, i.e., 10 -10 fold [454]. In search of improved efficiency and since no stable transition state analogues can reproduce all the characteristics of the transition state analogues, new and more sophisticated alternatives have been developed to elicit catalytic antibodies [455]. These include... 

The activation energy for proton transfer can be viewed as a lattice oxygen Lewis-base and proton Br0nsted-acid synergetic event [3]. One generally finds that activation energies of proton-activated reactions arc rather high between 100 and 200 kJ/mol for proton-activated elementary reaction steps in hydrocarbon conversion catalysis. ITiis is the main reason for the relatively low TOP per proton ( 102 s ) for this type of reaction. Similarly to enzymes [31], the weak van der Waals-type interaction determines the size- and shape-dependent behavior. 

Kinetics of carrier-mediated transport processes is similar to enzyme-substrate reactions and can be described by the Michaelis-Menten equation (Eq. (9.2)), assuming that each transport system has one specific binding site for its substrates. Maximum transport velocity (Vmax) is reached when all binding sites of the respective carrier proteins are occupied by substrate molecules. Substrate turnover can be delineated by the Michaelis constant Km corresponding to the substrate concentration [S], at which half-maximum transport velocity has been reached (Figure 9.5). Km also depends on pH and temperature. In cotransport systems transferring several substrates, the transport protein has a characteristic Km for each molecule transported. 

Microbial Cell-containing Membranes for Molecular Recognition. Suzuki et al. (87) have proposed a miocrobial sensor which consists of membrane-bound microbial cells and an electrochemical device. The assemblies of microbial sensors are similar to enzyme sensors. Two types of microbial sensors have been developed as presented In Figure 9. The first monitors the respiration activity of membrane-bound microbial cells with a Clark-type oxygen electrode. The... 

Micelles are used in many applications. Their largest industrial use is in emulsion polymerization, as detailed in Section 5.9 below. On the other hand, micelles made of ionic surfactants can trap hydrocarbon wastes in polluted water, since these hydrocarbon molecules prefer to be in the hydrocarbon interior of the micelle in an aqueous environment. In addition, ionic wastes dissolved in water adsorb onto the polar heads of these micelles. The resulting waste-filled micelles may be removed by simple ultrafiltration. As an example of another application, micelles can affect the rate of several chemical reactions and are used in micellar catalysis, similar to enzyme catalysis, in biochemistry. The rate of the chemical reaction increases with increasing micelle concentration, eventually leveling off. Nevertheless, micellar catalysts are less specific than enzymes. 

In which way are cooperative enzymes similar to enzymes that do not display cooperativity Enzymes a) Obey the equation V=Vmax [S]/(AM+[S])... 

Enzyme-catalyzed Claisen-type condensations are formally similar to enzyme-catalyzed aldol condensations, with the exception that the nucleophilic substrate is an ester or thioester [Eq. (35)] ... 

Structure determination of nickel(II)-containing ARD has revealed a mononuclear nickel(II) center ligated by four protein ligands His-96, His-98, Glu-102, and His-140. The substrate is believed to coordinate to the two vacant coordination sites as the dianion. The protein fold is similar to enzymes of the cupin superfamily. 

Physical adsorption of molecules onto solid surfaces possesses certain similarities to enzyme-substrate binding. The atoms in a solid present a closely packed surface and in some cases, for example graphite, the interatomic distance is similar to that found in covalently bonded molecules. Instead of the normal dependence of the London attractive potential between a pair of molecules there is a r dependence for the interaction between a molecule and a solid. This longer range interaction is the basic reason why gases are adsorbed at pressures lower than those at which they condense to liquids or solids. The differential heat of adsorption is often of the order of twice the heat of condensation of the adsorbed vapour. 

Transporter proteins, like enzymes, exhibit saturation kinetics when all the binding sites on all of the transporter proteins in the membrane are occupied, the system is saturated and the rate of transport reaches a plateau (the maximum velocity). By analogy to enzymes, the concentration of a transported compound required to reach Vz the maximum velocity is often called the (Fig. 10.10). Facilitative transporters are similar to enzymes with respect to two additional features they are relatively specific for the compounds they bind and they can be inhibited by compounds that block their binding sites or change their conformation. 

Somewhat similar to enzyme kinetics, but definitely not the same, is the area of microbial kinetics. Here one is concerned with reactions between entities that may not be of the same level of organization (i.e., not atom-atom, atom-molecule, etc,). Indeed, microbial kinetics is more concerned with interactions between populations of living organisms, and some of the problems seem more akin to population dynamics than to chemical kinetics. In fact, in much of the discussion below we are concerned with population-changing processes. First, we need to define a few terms. 

The final aim in the application of these polymers is the use as catalysts working in a fashion similar to enzymes. The first steps in this direction are reached. 

---