Enzymes saturation behaviour

Takeuchi et al. reported the preparation of an imprinted polymer for the conversion of the herbicide atrazine (100) in atraton (101), a less toxic compound, by conversion of an atrazine chloride into methoxy [59, 60]. After polymerisation and removal of the template, analysis of the imprinted polymer showed saturation kinetics, suggesting an enzyme-like behaviour of the polymer. 

In this case, Cucurbituril (53) reveals a number of enzymelike features The reaction exhibits saturation behaviour, it becomes independent of substrate concentration with sufficient amounts of 54 and 55, high concentrations of 54 retard the cycloaddition (substrate inhibition), and release of product 56 from its complex with Cucurbituril (53) is the rate determining step. NMR spectroscopic data suggest that both starting materials of the cycloaddition are hydrogen bonded to the carbonyl groups of 53 with their ammonium moiety and that the reactive substituents extend into the interior of Cucurbituril (53). In this cavity the pericyclic reaction takes place to form the 1,2,3-triazole 56. Kinetic data indicate that the formation of the ternary complex of Cucurbituril (53) with the two starting materials 54 and 55 is not strainless. Since the reaction is still accelerated very much it is assumed that the transition state of the reaction corresponds to the size of the cavity more closely than the substrates. This is a further indication that this case is a useful enzyme model. 

Similar saturation behaviour (Figure 16.11) is shown by enzymes (see Chapter 23 on the accompanying website). The enzyme interacts with the reactant, called the substrate, at a specific location in the enzyme called the active site. The three-dimensional shape of the active site fits the shape of the substrate. 

The specific behaviour of unsaturated fatty acids under oxidation is determined by the position and the number of double bonds in the fatty acid molecule. The stepwise oxidation of an unsaturated acid to the position of a double bond in it proceeds in a manner similar to that of saturated acid oxidation. If the double bond retains the same configuration (trans-configuration) and position (A2,3) as those of the enoyl-CoA, which is produced during the oxidation of saturated fatty acids, the subsequent oxidation proceeds via conventional route. Otherwise, the oxidation reaction proceeds with the involvement of an accessory enzyme, A3,4-CiS-A2,3jrans-enoyl-CoA isomerase this facilitates the translocation of the double bond to an appropriate position and alters the double-bond configuration from cis to trans. 

MS is lower than that of M the system is in the regime of substrate saturation addition of more S does not lead to a rate increase. The behaviour of the reaction rate in case B is typical of enzymes and in biochemistry this is referred to as Michaelis-Menten kinetics. The success of the application of the Michaelis-Menten kinetics in biochemistry is based on the fact that indeed only two reactions are involved the complexation of the substrate in the pocket of the enzyme and the actual conversion of the substrate. Usually the exchange of the substrate in the binding pocket is very fast and thus we can ignore the term k2[H2] in the denominator. Complications arise if the product binds to the binding site of the enzyme, product inhibition, and more complex kinetics result. 

In 1913 Michaelis and Menten derived an equation describing the non-linear behaviour of the reaction rate of a substrate-specific enzyme. At low concentrations the velocity of the reaction is nearly linear, while saturation occurs at higher concentrations. For... 

Irrespective of the interpretative approach, it is now widely recognised that many enzymes do show marked deviations from Michaelis-Menten behaviour, and the deviation is often interpretable in terms of regulatory function in vivo. Thus, for example, a number of enzymes, including threonine deaminase [30] and aspartate transcarbamylase [31] as textbook cases, show a sigmoid, rather than hyperbolic dependence of rate upon substrate concentration. This, like the oxygen saturation curve of haemoglobin, permits a response to changes in substrate concentration... 

Molybdenum and tungsten complexes as models for oxygen atom transfer enzymes have been deployed in the full catalytic cycle from Scheme 4.3 predominantly in the early days of this field of research. A selection of the respective determined Michaelis-Menten parameters were expertly reviewed by Holm et al. Since in some cases both forms of model complexes (M and M mimicking the fully reduced or fully oxidized active sites, respectively) are isolable and available in a sufficient amount, the isolated half-reactions are much more often investigated than the whole catalytic cycle. This means that either the reduced form of the enzyme model is oxidized by an oxygen donor substrate like TMAO or the oxidized form is reduced by an oxygen acceptor substrate like triphenylphosphine (PhgP). The observed kinetic behaviour is in some cases described to be of a saturation type. An observation which... 

During the early 1970 s Harold Stewart, Keith Stanley and 1 tried to detect the intermediates of P-oxidation which, as mentioned earher, would be numerous if the process were to involve enzymes which are functionally dissociated. However, intact mitochondria oxidizing palmitoylcamitine were found to contain appreciable quantities only of saturated acyl-CoA derivatives and even these did not show the kinetic behaviour of true intermediates. Unsaturated and 3-hydroxyacyl intermediates were formed only by disrupted mitochondria, or if respiratory inhibitors (such as rotenone) were present. We concluded that the relevant enz3Tnes must be functionally linked in a P-oxidation black box , as would be expected in such an iterative process subsequent work has confirmed those results and, indeed, some multifimctional enzymes of fatty acid oxidation have been isolated from mitochondria. I am glad that Bruce Middleton is one person who has done this. 

The enzyme is saturated and an increase in [S] is not reflected in the effects on the reaction rate. This behaviour resembles that of surface catalysis on solids described in Chapter 11. Figure 14.2 represents the initial rate of enzyme catalysis as a function of substrate concentration. 

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