Enzymes Mechanism of Action free essay sample

Enzymes are biologic polymers that catalyze the chemical reactions that make life as we know it possible. The presence and maintenance of a complete and balanced set of enzymes is essential for the breakdown of nutrients to supply energy and chemical building blocks; the assembly of those building blocks into proteins, DNA, membranes, cells, and tissues; and the harnessing of energy to power cell motility, neural function, and muscle contraction. With the exception of catalytic RNA molecules, or ribozymes, enzymes are proteins. The ability to assay the activity of specific enzymes in blood, other tissue fluids, or cell extracts aids in the diagnosis and prognosis of disease. Deficiencies in the quantity or catalytic activity of key enzymes can result from genetic defects, nutritional deficits, or toxins. Defective enzymes can result from genetic mutations or infection by viral or bacterial pathogens (eg, Vibrio cholerae). Medical scientists address imbalances in enzyme activity by using pharmacologic agents to inhibit specific enzymes and are investigating gene therapy as a means to remedy deficits in enzyme level or function. In addition to serving as the catalysts for all metabolic processes, their impressive catalytic activity, substrate specificity, and stereospecificity enable enzymes to fulfill key roles in other processes related to human health and well-being. The absolute stereospecificity of enzymes is of particular value for use as soluble or immobilized catalysts for specific reactions in the synthesis of a drug or antibiotic. Proteolytic enzymes augment the capacity of detergents to remove dirt and stains. Enzymes play an important role in producing or enhancing the nutrient value of food products for both humans and animals. The protease rennin, for example, is utilized in the production of cheeses while lactase is employed to remove lactose from milk for the benefit of persons who suffer from lactose intolerance as a consequence of a deficiency in this hydrolytic enzyme (Chapter 43). ENZYMES ARE EFFECTIVE HIGHLY SPECIFIC CATALYSTS The enzymes that catalyze the conversion of one or more compounds (substrates) into one or more different compounds (products) enhance the rates of the corresponding non-catalyzed reaction by factors of at least 106. Like all catalysts, enzymes are neither consumed nor permanently altered as a consequence of their participation in a reaction. In addition to being highly efficient, enzymes are also extremely selective catalysts. Unlike most catalysts used in synthetic chemistry, enzymes are specific both for the type of reaction catalyzed and for a single substrate or a small set of closely related substrates. Enzymes are also stereospecific catalysts and typically catalyze reactions of only one stereoisomer of a given compound—for example, D- but not L-sugars, L- but not D-amino acids. Since they bind substrates through at least three points of attachment, enzymes can even convert nonchiral substrates to chiral products. Figure 7–1 illustrates why the enzyme-catalyzed reduction of the nonchiral substrate pyruvate produces L-lactate—rather than a racemic mixture of D- and L-lactate. The exquisite specificity of enzyme catalysts imbues living cells with the ability to simultaneously conduct and independently control a broad spectrum of chemical processes. The commonly used names for most enzymes describe the type of reaction catalyzed, followed by the suffix -ase. For example, dehydrogenases remove hydrogen atoms, proteases hydrolyze proteins, and isomerases catalyze rearrangements in configuration. Modifiers may precede the name to indicate the substrate (xanthine oxidase), the source of the enzyme (pancreatic ribonuclease), its regulation ( hormone-sensitive lipase), or a feature of its mechanism of action ( cysteine protease). Where needed, alphanumeric designators are added to identify multiple forms of an enzyme (eg, RNA polymerase III; protein kinase C ). To address ambiguities, the International Union of Biochemists (IUB) developed an unambiguous system of enzyme nomenclature in which each enzyme has a unique name and code number that identify the type of reaction catalyzed and the substrates involved. Enzymes are grouped into six classes: 1. Oxidoreductases (catalyze oxidations and reductions) 2. Transferases (catalyze transfer of moieties such as glycosyl, methyl, or phosphoryl groups) 3. Hydrolases (catalyze hydrolytic cleavage of C—C, C—O, C—N, and other bonds) 4. Lyases (catalyze cleavage of C—C, C—O, C—N, and other bonds by atom elimination, leaving double bonds) 5. Isomerases (catalyze geometric or structural changes within a molecule) 6. Ligases (catalyze the joining together of two molecules coupled to the hydrolysis of ATP) Despite the clarity of the IUB system, the names are lengthy and relatively cumbersome, so we generally continue to refer to enzymes by their traditional, albeit sometimes ambiguous names. The IUB name for hexokinase illustrates both the clarity of the IUB system and its complexities. The IUB name of hexokinase is ATP:D-hexose 6- phosphotransferase E. C. 2. 7. 1. 1. This name identifies hexokinase as a member of class 2 (transferases), subclass 7 (transfer of a phosphoryl group), sub-subclass 1 (alcohol is the phosphoryl acceptor), and hexose-6 indicates that the alcohol phosphorylated is on carbon six of a hexose. However, we continue to call it hexokinase. PROSTHETIC GROUPS, COFACTORS, COENZYMES PLAY IMPORTANT ROLES IN CATALYSIS Many enzymes contain small nonprotein molecules and metal ions that participate directly in substrate binding or catalysis. Termed prosthetic groups, cofactors, and coenzymes, these extend the repertoire of catalytic