Enzymes activation energies

J. Kim, D. S. Clark, and J. S. Dordick, 2000, Intrinsic effects of solvent polarity on enzymic activation energies, Biotechnd. Bioeng. 67, 112-116. 

Source of enzyme Activation energy (Kcal.) Temperature range of measurements, degrees... 

Enzymes act by lowering the overall activation energy of a reaction sequence by involving a series of intermediates, or a mechanism, different from the spontaneous uncatalysed reaction. 

Craig D B, Arriaga E A, Wong J C Y, Lu H and Dovichi N J 1996 Studies on single alkaline phosphatase molecules reaction rate and activation energy of a reaction catalyzed by a single molecule and the effect of thermal denaturation—the death of an enzyme J. Am. Chem. See. 118 5245-53... 

Enzymes decrease the activation energy of chemical reactions... 

Figure 11.2 Enzymes accelerate chemical reactions by decreasing the activation energy. The activation energy is higher for a noncatalyzed reaction (a) than for the same reaction catalyzed by an enzyme (b). Both reactions proceed through one or several transition states, S. Only one transition state is shown in (a), whereas the two bumps in (b) represent two different transition states.

Enzymes increase the rate of chemical reactions by decreasing the activation energy of the reactions. This is achieved primarily by the enzyme preferentially binding to the transition state of the substrate. Catalytic groups of the enzyme are required to achieve a specific reaction path for the conversion of substrate to product. 

FIGURE 16.1 Enzymes catalyze reactions by lowering the activation energy. Here the free energy of activation for (a) the uncatalyzed reaction, AGu, is larger than that for (b) the enzyme-catalyzed reaction, AG,". 

In thermodynamic terms, a spontaneous reaction AG < 0) may proceed only slowly without enzymes because of a large activation energy (EJ. Adding enzymes to the system does not change the free energy of either the substrates or products (and thus does not alter the AG of the reaction) but it does lower the activation energy and increase the rate of the reaction. 

Figure 5.8 An energy diagram for a typical, enzyme-catalyzed biological reaction (blue curve) versus an uncatalyzed laboratory reaction (red curve). The biological reaction involves many steps, each of which has a relatively small activation energy and small energy change. The end result is the same, however.

An enzyme—usually a large protein—is a substance that acts as a catalyst for a biological reaction. Like all catalysts, an enzyme doesn t affect the equilibrium constant of a reaction and can t bring about a chemical change that is otherwise unfavorable. An enzyme acts only to lower the activation energy for a reaction,... 

Acifluorfen, synthesis of, 683 Acrolein, structure of, 697 Acrylic acid, pKa of, 756 structure of. 753 Activating group (aromatic substitution), 561 acidity and, 760 explanation of, 564-565 Activation energy, 158 magnitude of, 159 reaction rate and, 158-159 Active site (enzyme), 162-163 citrate synthase and, 1046 hexokinase and, 163... 

Enzymes, like all other catalysts, lower the activation energy for reaction. They can be enormously effective it is not uncommon for the rate constant to increase by a factor of... 

At temperatures above Tm, chemical and enzymatic degradation of microbial exopolysaccharides is enhanced. The apparent enhanced stability of microbial exopolysaccharides in their ordered confirmation is thought to be due to the glycosidic bonds in the backbone of the polymer which raises the activation energy. This restricted movement would also restrict access of enzymes and chemicals to the backbone. 

A transition state is an unstable, high-energy configuration assumed by reactants in a chemical reaction on the way to making products. Enzymes can lower the activation energy required for a reaction by binding and stabilizing the transition state of the substrate. 

An interesting point that emerges from Fig. 5.6 is the relation between Ag and (AAgsol)w. p. As seen from the figure, the lowering of the activation energy for the reaction is almost linearly proportional to the stabilization of the ionic resonance form (AAg )w. p. An enzyme which is designed to accelerate a proton transfer between A and B will simply stabilize the (B 1—H A-) state more than water. 

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