Reaction velocity specific

The characteristics of enzymes are their catalytic efficiency and their specificity. Enzymes increase the reaction velocities by factors of at least one million compared to the uncatalyzed reaction. Enzymes are highly specific, and consequendy a vast number exist. An enzyme usually catalyzes only one reaction involving only certain substrates. For instance, most enzymes acting on carbohydrates are so specific that even the slightest change in the stereochemical configuration is sufficient to make the enzyme incompatible and unable to effect hydrolysis. 

The discussion above was concerned with the effects of solution conditions on enzyme activity, hence reaction velocity. Equally important for the purpose of assay design is the influence of specific solution conditions on the detection method being used. This latter topic is beyond the scope of the present text. Nevertheless, this is an important issue for screening scientists whose job is often to balance the needs of biochemical rigor and assay practicality in development of an HTS assay. An... 

The mechanism proposed for the aldehyde dehydrogenases includes an enzyme-bound hemiacetal intermediate, possibly via a thioester bond with a cysteine (100). The specificity of the enzyme for aldehydes is quite broad. Apparent Km values for many aliphatic and aromatic aldehydes are in the micromolar range, with the highest reaction velocities observed for aldehydes with electron-with-drawing substituents on the a carbon for aliphatic aldehydes and in the para position for aromatics (99). 

It appears that all these possibilities can be excluded. If reactions (a) or (gf) were rate-limiting the reaction velocity would be independent of the concentration of the substrate, while reaction (e) (identical with (Z)) would predict no catalysis by acids or bases. If reactions (b), (d) or (h) determined the rate the reaction would show specific catalysis by hydrogen or hydroxide ions, in place of the general acid-base catalysis actually observed. Reactions (c), (f) and (m) are unacceptable as rate-limiting processes, since they involve simple proton transfers to and from oxygen. Reactions (j) and (k) might well be slow, but their rates would depend upon the nucleophilic reactivity of the catalyst towards carbon rather than on its basic strength towards a proton as shown in Section IV,D it is the latter quantity which correlates closely with the observed rates. 

Effect of pH on the ionization of the active site The concentra tion of H+ affects reaction velocity in several ways. First, the cat alytic process usually requires that the enzyme and substrate have specific chemical groups in either an ionized or unionized state in order to interact. For example, catalytic activity may require that an amino group of the enzyme be in the protonated form (-NH3+). At alkaline pH, this group is deprotonated, and the rate of the reaction, therefore, declines. 

The number of molecules which react in unit time is far smaller than the number entering into collision this shows that those which suffer transformation are in some way in an exceptional state. The attainment of this exceptional state is very much favoured by increase of temperature molecules of high energy content are thus indicated, since the assumption of specific tautomeric changes into an active form is impossible in the case of quite simple molecules. The law of variation with temperature of the number of molecules the energy of which exceeds an assigned value is, moreover, precisely the same as that of the change in reaction velocity. 

The propagation velocity of detonation in the absence of losses and after selection of a specific state of the reaction products turns out to be dependent on the thermodynamic properties of the original mixture the reaction heat, the change in the number of molecules during the reaction, the specific heat and dissociation t the temperatures which develop. 

According to transition-state theory it is possible to consider reaction velocities in terms of a hypothetical equilibrium between reactants and transition state. It follows that the influence of the isotopic composition of the medium on reaction velocity can be considered to be the same as its influence on the concentration of transition states. The kinetic formulation of the problem can thus be replaced by one couched in equilibrium terms, and the equilibrium theory of the preceding section can be applied with a minimum of modification (Kresge, 1964). The rate constant, or catalytic coefficient, (k) for a catalysed reaction can be written as the product of three factors, viz. the equilibrium constant (K ) for the process forming the transition state from the reactants, the transmission coefficient, and the specific rate of transition state decomposition (kT/h). We recognize that the third factor is independent of the isotopic nature of the reaction and assume that there is no isotope effect on the transmission coefficient. It follows that... 

According to the hypothesis, chemical conversions in a solid occur on the surface of a new crack (or in the layer adjacent to it), that is, the reaction rate in such a reacting system is a certain function of the specific surface area S (active surface area per unit volume). As noted above, the positive feedback in this model manifests itself in the fact that the rate of formation of cracks (i.e., the active surface growth rate in the sample volume) is proportional to the reaction velocity. Therefore, the equation describing the formation of a new surface can be written in a form analogous to that of a branched-chain process ... 

Reaction Velocity and Specific Effect of Reducing and Oxidizing Agents. 

Therefore, affinity changes at the rate of exchanged matter and chemical reaction velocity. Depending on the rate of exchanged matter, the first term in Eq. (9.172) may counterbalance the reaction velocity, and the affinity may become a constant. This represents as system where one of the forces is fixed, and may lead to a specific behavior in the evolution of the whole system. 

The location of the first peak represents the time required to reach a specific viscosity, and, providing the reaction mechanism does not change as a function of temperature, represents the time to reach a fixed chemical conversion for a specific frequency. These peaks can then be used as a measure of the rate of reaction at each temperature where the reaction velocity constant can be treated as inversely proportional to the time to the peak maximum,... 

Specific small molecules or ions can inhibit even nonallosteric enzymes. In irreversible inhibition, the inhibitor is covalently linked to the enzyme or bound so tightly that its dissociation from the enzyme is very slow. Covalent inhibitors provide a means of mapping the enzyme s active site. In contrast, reversible inhibition is characterized by a rapid equilibrium between enzyme and inhibitor. A competitive inhibitor prevents the substrate from binding to the active site. It reduces the reaction velocity by diminishing the proportion of enzyme molecules that are bound to substrate. In noncompetitive inhibition, the inhibitor decreases the turnover number. Competitive inhibition can be distinguished from noncompetitive inhibition by determining whether the inhibition can be overcome by raising the substrate concentration. 

An important consequence of the above situation, specifically arising in electrocatalysis, is that because the reaction velocity can be exponentially modulated by applied potential, the turnover rate at catalyst sites can be varied over a wide range. For example, at 1 mA cm of electrode surface it is 6, whereas for 1 A cm it is 6000, taking about 10 reaction sites cm". ... 

Under IV, Lewis points out that, although the halides of many metals and of boron are important catalysts, there are very few quantitative measurements of reaction velocity. Qualitatively, Rosenheim (51) has shown that D and CNS" are specific catalysts for the reaction SO2 + CjHsO- 5 R0S02 RSO3-... 

In contrast to the general references given above, this chapter is concerned specifically with catalysis of isocyanate reactions. Reactions of isocyanates provide an example of classical catalysis in that a catalyst-reactant complex is first formed which is then able to react with a second reactant molecule with an over-all high reaction velocity and specificity. Factors affecting rate and amount of complex formation, provision of paths of low activation energy, as well as steric and electronic effects, are all important. 

Earlier work demonstrated the effects of metal ions on heat inactivation of VACV. The stability of VACV suspensions was enhanced by the addition of 2M-Na+ [116] vims was protected up to 4 h at 50°C and 24 h at 37°C. Similarly, the reaction velocity of inactivation in suspensions of VACV decreased as the concentration of Na2HP04 increased [67], Further investigating revealed in the presence of metal ions, specifically a mixture of 100 mM-Na+ and 1 niM-Mg21, VACV was significantly more stable at 55°C and 60°C when compared with suspensions without ions. 

The existence of a preequilibrium between catalyst and substrate (leading to a genuine specific catalysis) can, however, be shown unequivocally if ki in the above scheme, i.e., if the substrate has sufficiently marked basic (or acidic) properties, and the concentrations of hydrogen ions (or hydroxyl ions) are sufficiently high. Under these conditions the reaction velocity in solutions of strong acids or bases will increase less rapidly than [H+] or [OH-], corresponding to the conversion of an appreciable proportion of the substrate into its cation or anion [cf. case iib and Eq. (24) above]. The test cannot often be applied, since the... 

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