Enzymes facilitate key processes in the biochemistry of organisms which, otherwise, would take place too slowly for the maintenance of life. Enzymes speed up chemical reactions of substrates by lowering the activation energy. They have complex tertiary structures which are held in shape by weak chemical bonds between the polypeptide chains.
Each enzyme has a specific surface configuration with one or more clefts known as active sites to which only certain substrates can bind. Each enzyme is highly specific to the reaction it catalyzes as the substrate must fit precisely into the active site. If the active site loses its unique shape, it can no longer provide a point of attachment for its substrate and the enzyme is said to be denatured. This can happen if the enzyme is subjected to temperatures or pH levels outside of the narrow range in which it normally operates. Many enzymes require the assistance of certain accessory substances, known as cofactors and coenzymes, to function properly.
Shape and activity of enzymesBy changing the shapes of enzymes it is possible to inactivate them, and thus stop certain reactions from occurring at a noticeable rate. For example, the important protein-digesting enzyme chymotrypsin occurs in an inactive form called chymotrypsinogen. Only when a few amino acids that make up this protein are removed does it adopt the catalytic shape of chymotrypsin. This change is triggered by the presence of food in the digestive tract. If the chymotrypsin were active all the time it would rapidly digest the intestinal wall while waiting for food to arrive.
In many biochemical processes a molecule is passed from enzyme to enzyme before it becomes an end product. At each stage, an intermediate compound is formed. Sometimes the final product, or one of the intermediates, can combine with an enzyme farther back along the chain and switch it off. This feedback is like automation in a factory that ceases production when enough of a particular material has been made. Other small molecules may combine with an enzyme molecule to increase its activity.
Lock-and-key and induced modelsThe "lock and key" model of enzymes, first described more than a century ago by Emil Fischer, comes surprisingly close to the actual mechanism of enzyme-substrate interaction. In the more recent and refined model, known as induced fit, an enzyme assumes a complementary shape to that of its substrate only after the substrate binds to the enzyme; hence, this is a more dynamic scenario compared to the lock-and-key hypothesis.
Summary of key enzyme properties
Related category• BIOCHEMISTRY
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