Many coenzymes are derived from water-soluble vitamins. Coenzymes transport hydrogen atoms and small substrate molecules from one enzyme to another. The rate of enzymatic reactions increases when either the substrate concentration or the enzyme concentration is increased. If the enzyme concentration remains constant, the rate of the reaction increases as the substrate concentration is raised, up to a maximum rate. When the rate of the reaction does not increase upon further addition of substrate, the enzyme is said to be saturated.
Metabolic pathways involve a number of enzyme-catalyzed reactions.
A number of enzymes usually cooperate to convert an initial substrate to a final product by way of several intermediates. If an enzyme is defective due to an abnormal gene, the intermediates formed after the step catalyzed by the defective enzymes will decrease, and the intermediates formed prior to the defective step will accumulate. Diseases that result from defective enzymes are called inborn errors of metabolism. Accumulation of intermediates often results in damage to the organ that contains the defective enzyme. Many metabolic pathways are branched so that one intermediate can serve as the substrate for two different enzymes.