Kinetics, of enzyme-catalysed reaction

Experimental studies on the effect of substrate concentration on the activity of an enzyme show consistent results. At low concentrations of substrate the rate of reaction increases as the concentration increases. At higher concentrations the rate begins to level out and eventually becomes almost constant, regardless of any further increase in substrate concentration. The choice of substrate concentration is an important consideration in the design of enzyme assays and an understanding of the kinetics of enzyme-catalysed reactions is needed in order to develop valid methods. 

As with normal chemical reactions, there are many reasons for studying the kinetics of enzyme-catalysed reactions to be able to predict how the rate of reaction will be affected by changes in reaction conditions to aid in the determination of the... 

The Michaelis-Menten model shown below is the simplest mechanism for describing the kinetics of enzyme catalysed reactions ... 

The kinetics of enzyme-catalysed reactions have been very intensively studied, not least because this is one of the few parts of... 

Microemulsions have larger structures than micelles and a large oil drop centre for binding hydrophobic molecules. However, the photoionization behaviour of molecules solubilized in microemulsion aggregates is similar to that in micelles. The kinetics of enzyme-catalysed reactions solubilized in water pools in organic solvents by surfactants have been analysed. ... 

Finally, it is useful to note that the catalytic efficiency reaches a maximum when the formation of the enzyme-substrate complex ES is rate-determining. This corresponds to the situation where k2> > k i, so that every ES which is formed is converted to product, none re-dissociates to E + S. However, the rate cannot exceed the rate of the collisions of E and S, and this upper limit is determined by the rate of diffusion in the solution. Experiments on the kinetics of enzyme-catalysed reactions demonstrate that a number of them achieve this state of catalytic perfection [2, p. 372],... 

Sculley MJ, Morrison JF. 1986. The determination of kinetic constants governing the slow, tight-binding inhibition of enzyme-catalysed reactions. Biochim Biophys Acta 874 44. 

To avoid dealing with curvilinear plots of enzyme-catalysed reactions, Lineweaver and Burk introduced an analysis of enzyme kinetics based on the following rearrangement of the Michaelis-Menten equation ... 

In order to see whether or not these polymers display typical enzyme-analogue properties, we investigated the kinetics of the catalysed reaction in the presence... 

Recently, enormous progress has been made in understanding the systematics of in vivo isotope discrimination and in understanding and even predicting isotopic patterns of natural compounds [33, 56, 89, 111]. As has been demonstrated, these fingerprints of origin and biosynthesis of natural compounds are created by the overlap of influences from equilibrium isotope effects under conditions of metabolic steady state, from kinetic isotope effects in coimection with branching of synthesis chains and from mechanisms of enzyme catalysed reactions. 

Experimentally, the initial rate, v, of enzyme catalysed reactions is found to show saturation kinetics with respect to the concentration of the substrate, S. At low concentrations of substrate the initial rate increases with increasing concentration of S but becomes independent of [S] at high or saturating concentrations of S (Fig. 2). This observation was interpreted by Michaelis and Menten in terms of the rapid and reversible formation of a non-covalent complex (ES), from the substrate (S) and enzyme (E), which then decomposes into products (P) (Eqn. 1). 

Obedience to Michaelis-Menten kinetics yields interesting conclusions about the specificity of enzyme-catalysed reactions. In vitro non-specific substrates are sometimes described as poor because they show a low value of or a high value of K. However, in vivo specificity results from a competition of substrates for the active site of the enzyme. If two substrates, S and S, compete for the same enzyme different conclusions could be reached about their relative specificity if rates of reaction or the K s of the individual substrates are compared instead of their relative values of K. If the enzyme catalyses the reaction of both S and S (Eqn. 15) the relevant equations may be obtained by the usual procedures (Eqns. 16-18). 

Obedience to Michaelis-Menten kinetics yields interesting conclusions about the specificity of enzyme-catalysed reactions. In vitro non-specific substrates are sometimes described as poor because they show a low value of high value... 

Enzyme kinetics the mathematical treatment of enzyme-catalysed reactions. A great deal of information about reaction mechanisms can be obtained from kinetic experiments and evaluation of the data... 

Program for estimation of kinetic parameters of enzyme-catalysed reaction S P Michaelis-Mentonrate equation Reaction rate is (-ra) = (kl Cs)/(Km + Cs)... 

Most enzymes catalyse reactions and follow Michaelis-Menten kinetics. The rate can be described on the basis of the concentration of the substrate and the enzymes. For a single enzyme and single substrate, the rate equation is ... 

Transition state theory has been useful in providing a rationale for the so-called kinetic isotope effect. The kinetic isotope effect is used by enzy-mologists to probe various aspects of mechanism. Importantly, measured kinetic isotope effects have also been used to monitor if non-classical behaviour is a feature of enzyme-catalysed hydrogen transfer reactions. The kinetic isotope effect arises because of the differential reactivity of, for example, a C-H (protium), a C-D (deuterium) and a C-T (tritium) bond. 

Hydrogen motion, H+, H or H, is often involved in the rate-limiting step of many enzyme catalysed reactions. Here, QM tunnelling can be important and is reflected in the values of the measured kinetic isotope effects (KIEs) [75], Enzyme motion... 

M. A. Savageau, Development of fractal kinetic theory for enzyme catalysed reactions and implications for the design of biochemical pathways. BioSystems 47(1 2), 9 36 (1998). 

Kinetics is the study of the factors which influence reaction rates. Enzyme-catalysed reactions are subject to the same principles of rate regulation as any other type of chemical reaction. For example, the pH, temperature, pressure (if gases are involved) and concentration of reactants all impact on the velocity reactions. Unlike inorganic catalysts, like platinum for example, there is a requirement for the substrate (reactant) to engage a particular region of the enzyme known as the active site. This binding is reversible and is simply represented thus ... 

Enzyme kinetics Michaelis constant, symbol iCm maximum velocity of an enzyme catalysed reaction, Vm DC inhibitor constant, symbol X Michaelis-Menten equation and graph in the absence and the presence of inhibitors. Lineweaver-Burke and Eadie-Hofstee plots. 

We think about metabolic pathways as linear or cyclical sequences of reactions as described in Chapter 1. Individual reactions within a pathway are often dependant upon at least one other reaction. For example, we know from our studies of enzyme kinetics in Chapter 2 that the rate of an enzyme catalysed reaction is determined in part by the concentration of substrate. Remember, the substrate for one reaction is usually the product of a previous reaction, so the activity of an enzyme is affected by the activity of the preceding enzyme in the sequence. 

In terms of kinetic parameters, reversible inhibitors act to alter the rate of certain enzyme-catalysed reactions by changing the Km or Vmax values (designated ] warent Qr K m and /illIl ll silrcnL or respectively to distinguish these changed values from the real values. (See Chapter 2 for a more detailed account.)... 

The order of reaction is not always integral and sometimes the rate cannot be expressed in a simple form. For example, some enzyme-catalysed reactions obey Michaelis—Menten kinetics and the rate is given by... 

The kinetics of an enzyme catalysed reactions in a w/o-microemulsions is dependent on several parameters. For example, the substrates and enzymes distribute within the different parts of a one-phase microemulsion with different concentrations. The enzymes are located in the water and hydrophobic substrates are mainly dissolved in the oil. Additionally, the choice of oil and surfactant, the water concentration, and the structure of the interfacial layer can influence the activity and stability of biocatalysts. The influences of the main parameters on the kinetics will be discussed in this chapter. 

The natural cycles of the bioelements carbon, oxygen, hydrogen, nitrogen and sulphur) are subjected to various discrimination effects, such as thermodynamic isotope effects during water evaporation and condensation or isotope equilibration between water and CO2. On the other hand, the processes of photosynthesis and secondary plant metabolism are characterised by kinetic isotope effects, caused by defined enzyme-catalysed reactions [46]. 

All reactions are to some degree reversible, and many enzyme-catalysed reactions can take place in either direction inside a cell. It is therefore interesting to compare the forward and back reactions, especially when the reaction approaches equilibrium, as in an enzyme reactor. Haldane derived a relationship between the kinetic and equilibrium constants. The derivation of the relationship is shown in Appendix 5.4. 

Reversible inhibition occurs rapidly in a system which is near its equilibrium point and its extent is dependent on the concentration of enzyme, inhibitor and substrate. It remains constant over the period when the initial reaction velocity studies are performed. In contrast, irreversible inhibition may increase with time. In simple single-substrate enzyme-catalysed reactions there are three main types of inhibition patterns involving reactions following the Michaelis-Menten equation competitive, uncompetitive and non-competitive inhibition. Competitive inhibition occurs when the inhibitor directly competes with the substrate in forming the enzyme complex. Uncompetitive inhibition involves the interaction of the inhibitor with only the enzyme-substrate complex, while non-competitive inhibition occurs when the inhibitor binds to either the enzyme or the enzyme-substrate complex without affecting the binding of the substrate. The kinetic modifications of the Michaelis-Menten equation associated with the various types of inhibition are shown below. The derivation of these equations is shown in Appendix S.S. 

The steady-state kinetics of a simple single-substrate, single-binding site, single-intermediate-enzyme catalysed reaction in the presence of competitive inhibitor are shown in Scheme A5.5.1. 

A reactant in an enzyme catalysed reaction is known as substrate. According to the mechanism of enzyme catalysis, the enzyme combines with the substrate to form a complex, as suggested by Henri (1903). He also suggested that this complex remains in equilibrium with the enzyme and the substrate. Later on in 1925, Briggs and Haldane showed that a steady state treatment could be easily applied to the kinetics of enzymes. Some photochemical reactions and some enzymic reactions are reactions of the zero order. 

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