Enzymes are biological catalysts made of protein – they act as a boost to speed up chemical reactions which cannot be speeded up by temperature, high pressures, extremes of PH and by maintaining high concentrations of the reacting molecules.
It is the presence of enzymes in cells and organisms that enables these reactions to occur at incredible speeds, in an orderly manner, yielding products that the organism requires, when they are needed. Enzymes speed up the rate at which an equilibrium position is reached.
Enzymes operate in many areas within the body, some are exported from cells, such as the digestive enzymes. These enzymes which are secreted and parcelled and work externally are called extracellular enzymes. Although most enzymes work within the cells, and are therefore called intracellular enzymes. These are found inside the sub-structures within cells known as organelles, in the membranes of organelles, in the fluid medium around the organelles, and in our plasma membrane. The chemical reactions of life are called metabolism. Since each reaction of metabolism can only occur in the presence of a specific enzyme, we know that if an enzyme is not present than the reaction it catalyses cannot occur. Some enzymes are only made at particular conditions or at certain stages.
The enzyme-substrate complex
Enzymes work by binding to a specific substance, known as their substrate molecule, at a specifically formed pocket in the enzyme, called the active site. As the enzyme and substrate form a complex, this immediately breaks down to form the products, plus the unchanged enzyme.
To emphasise this, the binding of enzyme and substrate is referred to the ‘lock and key’ hypothesis of enzyme action. The enzyme is the lock and the substrate is the key that fits the lock. Enzymes are typically large molecules while substrates are typically smaller.