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Chapter 5 Enzymes Enzymes are catalytic proteins that increase the rate of biological reactions within the cells of the body. Enzymes digest our foods, contract our muscles and synthesize hemoglobin. They catalyze reactions that supply all necessary biomolecules and energy for our survival. A catalyst increases the reaction rate by changing the way a reaction takes place without itself undergoing any net change. An uncatalyzed reaction in a cell may take place eventually, but not at a rate fast enough for survival. For example, the hydrolysis of proteins in our diet would eventually occur without a catalyst, but not fast enough to meet the body’s requirements for amino acids. The chemical reactions in our cells must occur at incredibly fast rates under the mild conditions of pH 7.4 and a body temperature of 37 °C. As catalysts, enzymes lower the activation energy for a chemical reaction (Figure 1). Less energy is required to convert reactant molecules to products, which increases the rate of a biochemical reaction compared to the rate of the uncatalyzed reaction. For example, carbonic anhydrase, an enzyme in the blood converts large amounts of carbon dioxide and water to carbonic acid. In the presence of the enzyme, this reaction occurs 10 million times, as rapidly as it does in the absence of the enzyme. One molecule of carbonic anhydrase can hydrate about 1 million molecules of CO. a second. K,., uneatalyzed Big: Catalyzed HCO; Reaction progress!) > Figure 1. The enzyme carbonic anhydrase lowers the activation energy needed for the reaction of CO, and H,0. Types of Enzymes 1. simple proteins — consist only of a polypeptide chain; it is the tertiary protein structure of the simple enzyme that makes it biologically active. 2. conjugated proteins - protein portion is inactive without a cofactor; the cofactor is the nonprotein portion of an enzyme, such as a metal ion that is necessary for enzyme activity; if the cofactor is an organic compound, usually a vitamin, it is called a coenzyme. = Active Enzyme Metal ion or Organic (holoenzyme) molecule Inactive protein {apoenzyme) © apoenzyme - protein portion of the enzyme, inactive (Figure 2) e holoenzyme - apoenzyme + cofactor, active enzyme e cofactors - organic compounds derived from B vitamins (coenzymes) (Table 1) - inorganic metal ions (Table 2) 81 
