Chapter 7

After studying this chapter, you should be able to:

• Illustrate the structural relationships between B vitamins and coenzymes.
• Outline the four principal mechanisms by which enzymes achieve catalysis.
• Describe how an “induced fit” facilitates substrate recognition and catalysis.
• Outline the underlying principles of enzyme-linked immunoassays.
• Explain how coupling an enzyme to an NAD(P)+-dependent dehydrogenase can simplify assay of its activity.
• Identify enzymes and proteins whose plasma levels are used for the diagnosis and prognosis of a myocardial infarction.
• Describe the application of restriction endonucleases and of restriction fragment length polymorphisms in the detection of genetic diseases.
• Explain the utility of site-directed mutagenesis for the identification of residues involved in catalysis, in the recognition of substrates or allosteric effectors, or in the mechanism of enzyme action.
• Describe how the addition of fused affinity “tags” via recombinant DNA technology can facilitate purification of a protein expressed from its cloned gene.
• Indicate the function of specific proteases in the purification of affinity-tagged enzymes.
• Discuss the events that led to the discovery that RNAs can act as enzymes.

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. The vast majority of enzymes are proteins. Notable exceptions include ribosomal RNAs and a handful of self-cleaving or self-splicing RNA molecules known collectively as ribozymes. 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 a particular value for use as soluble or immobilized catalysts for specific reactions in the synthesis of a drug or antibiotic. Proteases and amylases 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 lactose-intolerant persons deficient in ...

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