Enzymes are usually globular proteins that promote specific chemical reactions within cells by lowering the activation energy required to start these reactions. Enzymes can speed metabolic reactions by a factor of a million or more.
Enzymes are required in very small quantities, because as they work, they are not consumed and can, therefore, function repeatedly. Also, each enzyme has specificity, acting only on a particular kind of substance, which is called its substrate. For example, the substrate of an enzyme called catalase (found in the peroxisomes of liver and kidney cells) is hydrogen peroxide, a toxic by-product of certain metabolic reactions. This enzyme's only function is to decompose hydrogen peroxide into water and oxygen, helping prevent accumulation of hydrogen peroxide that might damage cells.
Each enzyme must be able to "recognize" its specific substrate. This ability to identify a substrate. This ability to identify a substrate depends upon the shape of an enzyme molecule. That is, each enzyme's polypeptide chain twists and coils into a unique three-dimensional form, or conformation, that fits the special shape of its substrate molecule.
During an enzyme-catalyzed reaction, regions of the enzyme molecule called active sites temporarily combine with portions of the substrate, forming an enzyme-substrate complex. This interaction strains chemical bonds in the substrate in a way that makes a particular chemical reaction more likely to occur. When it does, the enzyme is released in its original form, able to bind another substrate molecule.
Enzyme catalysis can be summarized as follows:
Substrate + Enzyme is then Enzyme-substrate complex and is then Product (changed substrate) + Enzyme (unchanged)
The speed of an enzyme-catalyzed reaction depends partly on the number of enzyme and substrate molecules in the cell. The reaction occurs more rapidly if the concentration of the enzyme or the concentration of the substrate increases. Also, the efficiency of different kinds of enzymes varies greatly. Thus, some enzymes can process only a few substrate molecules per second, whereas others can handle thousands or nearly a million substrate molecules per second.
Cellular metabolism includes hundreds of different chemical reactions, each controlled by a specific kind of enzyme. Often sequences of enzyme-controlled reactions, called metabolic pathways, lead to synthesis or breakdown of particular biochemicals. Thus, hundreds of different kinds of enzymes are present in every cell.
Enzymes names are often derived from the names of their substrates, with the suffix -ase added. For example, a lipid-splitting enzyme is called a lipase, a protein-splitting enzyme is a protease, and a starch-(amylum) splitting enzyme is an amylase. Similarly, sucrase is an enzyme that splits the sugar sucrose, maltase splits the sugar maltose, and lactase splits the sugar lactose.
Factors that alter enzymes: Almost all enzymes are proteins, and like other proteins, they can be denatured by exposure to excessive heat, radiation, electricity, certain chemicals, or fluids with extreme pH values. For example, many enzymes become inactive at 45 degrees C, and nearly all of them are denatured at 55 degrees C. Some poisons are chemicals that denature enzymes. Cyanide, for instance, can interfere with respiratory enzymes and damage cells by halting their energy-obtaining processes.