An enzyme is a protein which acts as a highly efficient catalyst. Four important points about enzymes are:
An enzyme normally has one particular site at which reactions take place, called the active site. The tertiary structure of an enzyme is so complicated that molecules must have a very particular shape in order are to fit into the active site. These molecules which do fit the active site are called the substrate - the molecule which is reacting. It is another example of molecular recognition. A simple analogy is 'lock and key' in which the enzyme is the 'lock' and the substrate is the 'key'. This explains the specificity of enzymes.
The bonds which bind the substrate to the active site are weak so the binding can be reversed when the products need to leave the active site after the reaction. The bonds are usually hydrogen bonds or ionic interactions.
After the reaction, the product leaves the enzyme which is then free to start again with another molecule of the substrate. This can be illustrated as:
Enzyme + Substrate Enzyme-Substrate complex Enzyme-Product complex Enzyme + Products
E + S ES EP E + P
Enzymes are permanently denatured by heat. Addition of acid or alkali changes the catalytic activity because hydrogen and hydroxide ions add or remove protons from various parts of a peptide chain and thereby upset the interactions between the active site and substrate.
Enzymes work by lowering the activation enthalpy of a reaction.
Enzymes are usually only present in minute traces. If the substrate concentration is high, all the enzyme molecules will have substrate molecules attached. The reaction rate does not depend on the substrate concentration, it is zero order with respect to substrate. The rate equation is:
If the substrate concentration is low, not all active sites will have substrate attached. Increasing concentration of substrate will increase rate. Reaction is first order with respect to substrate. The rate equation is:
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