Reverse enzyme catalysis

Elucidating Mechanisms for the Inhibition of Enzyme Catalysis An inhibitor interacts with an enzyme in a manner that decreases the enzyme s catalytic efficiency. Examples of inhibitors include some drugs and poisons. Irreversible inhibitors covalently bind to the enzyme s active site, producing a permanent loss in catalytic efficiency even when the inhibitor s concentration is decreased. Reversible inhibitors form noncovalent complexes with the enzyme, thereby causing a temporary de-... 

These bonds are sufficiendy strong to direct molecular iateractions such as the attraction between an enzyme and its substrate, but sufficiently weak to be reversibly made and broken, as ia enzyme catalysis (11). 

In the classical world (and biochemistry textbooks), transition state theory has been used extensively to model enzyme catalysis. The basic premise of transition state theory is that the reaction converting reactants (e.g. A-H + B) to products (e.g. A + B-H) is treated as a two-step reaction over a static potential energy barrier (Figure 2.1). In Figure 2.1, [A - H B] is the transition state, which can interconvert reversibly with the reactants (A-H-l-B). However, formation of the products (A + B-H) from the transition state is an irreversible step. 

Let us consider the basic enzyme catalysis mechanism described by the Michaelis-Menten equation (Eq. 2). It includes three elementary steps, namely, the reversible formation and breakdown of the ES complex (which does not mean that it is at equilibrium) and the decomposition of the ES complex into the product and the regenerated enzyme ... 

A more realistic but still relatively simple model of enzyme catalysis includes binding of both substrate and product as described by Equation 11.9. This reaction is characterized by five individual rate constants k and k2, and k4 and k5, correspond to the forward and reverse binding steps of the substrate S and product P to the enzyme E, respectively, while k3 expresses the irreversible chemical conversion at the enzyme active site ... 

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviom" and the corresponding activity of enzymes the application of these media can lead to favomable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes. 

Microemulsions with different structures, like micelles, reverse micelles or bicontinuous networks, can be used for several inorganic, organic [72] or catalytic reactions which require a large contact area between oil and water. Besides enzyme catalysis, this can be the formation of nanoparticles [54, 73, 74], hydro-formylation reactions [75] or polymerisations [76-78]. 

The surfactant mass fraction in a microemulsion defines the size of the interfacial area between the water and oil. The reaction rate of organic reactions in microemulsions can be dramatically enhanced by increasing the specific interfacial area [95]. Enzyme catalysis in microemulsions is usually not influenced by the size of the interfacial area because only a small fraction of the reverse micelles are hosting a bio-molecule. Most investigations published so far were made with low enzyme concentrations resulting in a low population of enzymes per reverse micelle. 

For a better understanding of the enzyme catalysis in nature, experimental and theoretical studies characterize the free energy profiles and catalytic efficiencies of enzymes under different conditions, which may define the performance of an enzyme in maintaining a constant flux or a constant pool concentration of the product, working under irreversible or reversible conditions etc. (Albery and Knowles, 1976 Stackhouse et al., 1985 Pettersson, 1992 Somogyi, Welch and Damjanovich, 1984). Only a few enzyme reactions have been analyzed in detail and further experimental investigations are necessary to characterize the enzymes, to draw general conclusions, and to deduce how much their evolution approximated the requirements for optimal catalysis . 

Enzymes and micelles resemble each other with respect to both structure (e.g., globular proteins and spherical aggregates) and catalytic activity. Probably the most common form of enzyme catalysis follows the mechanism known in biochemistry as Michaelis-Menton kinetics. In this the rate of the reaction increases with increasing substrate concentration, eventually leveling off. According to this mechanism, enzyme E and substrate A first react reversibly to form a complex EA, which then dissociates to form product P and regenerate the enzyme ... 

In an enzyme reaction, initially free enzyme E and free substrate S in their respective ground states initially combine reversibly to an enzyme-substrate (ES) complex. The ES complex passes through a transition state, AGj, on its way to the enzyme-product (EP) complex and then on to the ground state of free enzyme E and free product P. From the formulation of the reaction sequence, a rate law, properly containing only observables in terms of concentrations, can be derived. In enzyme catalysis, the first rate law was written in 1913 by Michaelis and Menten therefore, the corresponding kinetics is named the Michaelis-Menten mechanism. The rate law according to Michaelis-Menten features saturation kinetics with respect to substrate (zero order at high, first order at low substrate concentration) and is first order with respect to enzyme. 

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