The metabolic pathways catalysed by enzymes yield different end products. Enzymes act as biological catalysts and change the rate of reactions without altering themselves. While most enzymes are proteins, some nucleic acids called ribozymes also behave like enzymes.
Like proteins, enzymes have a primary, secondary and tertiary structure. While the primary structure is an amino-acid sequence, the tertiary structure folds and criss-crosses on itself, creating many crevices or pockets. One of these pockets is called the active site. A substrate fits into this active site of the enzyme and enables it to catalyse reactions at a higher rate.
Enzymes are different from inorganic catalysts; they get damaged above forty degrees centigrade temperature and at high pressures. However, thermolysin found in sulphur springs, retain their catalytic power even at eighty to ninety degrees centigrade.
Chemical compounds undergo physical and chemical changes. When a compound changes shape without breaking its existing bonds, it is a physical change. On the other hand, when the bonds in a compound are broken and new bonds are formed, it is a chemical change. The rate of a reaction is defined as the amount of a product formed per unit time. It can be expressed as delta p upon delta t. Various factors such as temperature and pressure influence the rate of reaction. This rate generally doubles or decreases by half for every ten degree change in temperature in either direction.
A catalyst greatly influences the rate of a reaction. Hence, the rate of enzyme-catalysed reactions is significantly higher than the un-catalysed reactions. For example, carbonic anhydrase accelerates the reaction between carbon dioxide and water to form carbonic acid, ten million times.
There are also multistep reactions, where each step is catalysed by a single or multiple enzyme(s). For example, pyruvate yields different end products using different enzyme-catalysed reactions. Pyruvate dehydrogenase converts pyruvate to acetyl Co A and carbon dioxide during aerobic respiration, while lactate dehydrogenase converts pyruvate to lactic acid during anaerobic respiration in the muscles. On the other hand pyruvic decarboxylase converts pyruvate to alcohol during anaerobic respiration in yeast.