Any substance that can diminish the velocity of an enzyme is known as an inhibitor. The two main types of inhibition are competitive, non-competitive. In competitive inhibition, the enzyme and inhibitor compete to bind to the same active site. Hence if substrate concentration is increased the chances of substrate binding to the enzyme will increase. Thus if (S) increases ${\text{K}}_{\text{m}}$ (Michaelis constant) also increases. In non-competitive inhibition, the substrate and inhibitor bind at different sites hence do not compete with each other. This cannot be reversed by increasing the substrate concentration and hence ${\text{K}}_{\text{m}}$ remains constant. This velocity of reaction also decreases. An example of a competitive inhibitor is malic acid. This competes with succinate (substrate) to bind at the active site of enzyme succinate dehydrogenase.