The Basics of Enzymatic Catalysis

Mimetic catalysis designs a real model (a mimic) which simulates objects and processes of enzymatic catalysis by their basic (but deficient) characteristics (selectivity, mildness of condition, active site action mechanism, etc.). Since only definite properties of the enzyme are simulated, it does not profess to a complete enzyme description, though optimal parameters by some properties may be approached. The mimetic model of enzyme helps in synthesizing suitable catalysts using inaccurate and sometimes ambiguous information. 

The present example of heterogeneous catalysis shows the unity of acidic-basic and redox mechanisms, which is typical of enzymatic catalysis. 

The basic concept is the intuition that, whether homogeneous or heterogeneous, catalysis is primarily a process controlled by a molecular phenomenon since it implies the catalyzed transformation of molecules into other molecules. It follows that on the surface of metals or metal oxides, sulfides, carbides, nitrides usually involved as heterogeneous catalysts, the relevant surface species and the mechanism of their mutual reactions must be of molecular character, as occurs in homogeneous or enzymatic catalysis. 

Since the beginning of biochemical investigation enzymes have held a special fascination for chemists and biologists. How can these easily destroyed substances catalyze reactions with such speed and without formation of significant quantities of side products Some enzymes increase the velocity of a single chemical reaction of a specific compound by a factor of as much as 1010. How can a protein do this In this chapter we ll consider both ways of measuring enzymatic activity and basic mechanisms of catalysis. 

The mechanism of an enzymatic reaction is ultimately defined when all the intermediates, complexes, and conformational states of the enzyme are characterized and the rate constants for their interconversion are determined. The task of the kineticist in this elucidation is to detect the number and sequence of these intermediates and processes, define their approximate nature (that is, whether covalent intermediates are formed or conformational changes occur), measure the rate constants, and, from studying pH dependence, search for the participation of acidic and basic groups. The chemist seeks to identify the chemical nature of the intermediates, by what chemical paths they form and decay, and the types of catalysis that are involved. These results can then be combined with those from x-ray diffraction and NMR studies and calculations by theoretical chemists to give a complete description of the mechanism. 

Hi) Enzymatic Esterifications. A major alternative to the classical basic catalysis is the use of enzymes for esterification, in particular with proteases and lipases.110 112 To make these enzymes, which normally hydrolyze amide or ester linkages, work in the reverse direction of esterification, the reactions have to be performed in organic media, with only the small amount of water necessary to preserve their active conformation. In such reactions, the difficulty is to find those conditions of solvent and temperature compatible with both the solubility of the substrates and the stability and the activity of the enzyme.113,114 In the case of sucrose (Scheme 9), most proteases lead selectively to monoesters at position p nl-ii5,ii6 Ypggg reactions are often performed in DMF, but examples in Me2SO, which is much less toxic, have also been reported, despite the ability... 

Bronsted acid/base catalysis is the most common enzymatic mechanism, since nearly all enzymatic reactions involve a proton transfer. This means that nearly all enzymes have acidic and/or basic groups in their active site. In add catalysis, the substrate is protonated by one of the amino add residues at the active site (typically aspartic acid, glutamic acid, histidine, cysteine, lysine, or tyrosine). This residue itself must therefore be protonated at the readion pH (typically between pH 5 and 9), with a pKa just above this value. Conversely, in base catalysis, the pJCa of the deprotonating residue must be just below the physiological pH. Some enzymes can even carry out bifunctional catalysis, by protonating and deprotonating two different sites on the same substrate molecule simultaneously. 

The area between enzymatic and chemical catalyses, associated with simulation of biochemical processes by their basic parameters, is accepted as mimetic catalysis. The key aspect of the mimetic catalyst is diversity of enzyme and biomimetic function processes, which principally distinguishes the mimetic model from traditional full simulation. Based on the analysis of conformities and diversities of enzymatic and chemical catalysis, the general aspects of mimetic catalysis are discussed. An idealized model of the biomimetic catalyst and the exclusive role of the membrane in its structural organization are considered. The most important achievements in the branch of catalysis are shown, in particular, new approaches to synthesis and study of biomimetic catalase, peroxidase and monooxidases reactions. 

The basic question of mimetic catalysis is the determination of physicochemical properties of enzymes to be simulated in the synthesized biomimic in accordance with the reaction modeled. Firstly, let us try to answer one of the key questions that arise in biomimic construction how important is it to reach the enzymatic specificity in the synthesis of their analogs To put it another way, should the dynamic (tertiary) structure of the enzyme responsible for the selectivity control mechanism be simulated by active site protection from competitive admixtures This feature of the enzymes distinguishes them from usual catalysts. 

In the framework of general BRC theory in the example of PPFe3+0H/Al203, the unified picture of two-proton transfer to acidic-basic groups of the carrier (A1203) with electron transfer to the active site (PPFe3+OH) is observed. Finally, the substrate is redox converted. It is typical that in enzymatic catalysis conditions without acidic-basic groups redox processes are suppressed. 

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