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Enzymes as Catalysts

In living system chemical reactions proceed at normal physiological temperatures which are quite low (in most cases 37oC). The same reactions, in vitro, will proceed at considerably higher temperature and pressure. The remarkable property of the cells to carry on reactions at low temperatures, low pressure, and in dilute solution, is due to the catalytic function of enzymes. Enzyme catalysis follows the same general rules as observed for non-catalyzed reaction which are however very slow. Both kinds of reactions carry out forward and backward reactions to reach a state of equilibrium. Nevertheless, the difference processes at considerably low temperature with maximum economy.

Activation Energy

All thermodynamically favorable reactions do not occur with the same speed, some are slow while others occur at rapid rates. Some molecules are unstable even at room temperature, hence they spontaneously undergo a chemical reaction. Stable molecules, on the other hand, require a controlled condition to initiate reaction at a favorable speed and require enzymes to do so.
For example, urea when dissolved in water will not react rapidly and its dissociation into ammonia and  CO2 will take place slowly with release of considerably energy.

activation energy

In the cell, however, the same reaction will proceed at higher speed in the presence of the enzyme urease lower the energy barrier to favour the reactants being converted into products.

The concept of catalysis was explained by Berzelius in 1853 to explain the speeding up of chemical reaction even if the reactants are in low concentration. We can take up an example of a reaction in which glucose and ATP are reactants and glucose-6-phosphate is the product. The reactants come together to form the glucose-ATP activation complex with the help of activation energy(EA). Since at room temperature the reactants are in a state of random motion, they possess sufficient energy to form an activation complex at a slow rate. However, if the reactants are allowed to collide with one another with sufficient energy to overcome the energy barrier, they will react successfully at a faster rate to form the products. One way of  lowering the energy barrier is to increase the temperature, but at room temperature enzymes can bring about the speeding up of reaction by lowering the energy barrier, permitting a large number of molecules to react at a given temperature.

Reaction coordinate

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