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After studying this chapter, you should be able to:

  • Describe the structural relationships between specific B vitamins and certain coenzymes.

  • Outline the four principal catalytic mechanisms and how they can be combined by enzymes to facilitate chemical reactions.

  • Explain the concept of an “induced fit” and how it facilitates catalysis.

  • Outline the underlying principles of enzyme-linked immunoassays.

  • Describe how coupling an enzyme to the activity of a dehydrogenase can simplify many enzyme assays.

  • Identify proteins whose plasma levels are used for diagnosis and prognosis.

  • Describe the application of restriction endonucleases and of restriction fragment length polymorphisms in the detection of genetic diseases.

  • Illustrate the utility of site-directed mutagenesis for the identification of aminoacyl residues that are involved in the recognition of substrates or allosteric effectors, or in the mechanism of catalysis.

  • Describe how “affinity tags” 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, and briefly describe the evolutionary concept of an “RNA world.”


Enzymes, which catalyze the chemical reactions that make life on the earth possible, participate in 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. Almost all enzymes are proteins. Notable exceptions include ribosomal RNAs and a handful of RNA molecules imbued with endonuclease or nucleotide ligase activity known collectively as ribozymes. The ability to detect and to quantify the activity of specific enzymes in blood, other tissue fluids, or cell extracts provides information that complements the physician’s ability to diagnose many diseases. Many pathologic conditions are the direct consequence of changes in the quantity or in the catalytic activity of key enzymes that result from genetic defects, nutritional deficits, tissue damage, toxins, or infection by viral or bacterial pathogens (eg, Vibrio cholerae).

In addition to serving as the catalysts for all metabolic processes, the impressive catalytic activity, substrate specificity, and stereospecificity of enzymes enables them to fulfill unique roles in human health and well-being. The protease rennin, for example, is utilized in the production of cheeses, while lactase is employed to remove lactose from milk to benefit lactose-intolerant individuals. Proteases and amylases augment the capacity of detergents to remove dirt and stains, while other enzymes can participate in the stereospecific synthesis of complex drugs or antibiotics.


The enzymes that catalyze the conversion of one or more compounds (substrates) into one or more different compounds (products) generally enhance the rates of the corresponding noncatalyzed reaction by factors of 106 or more. Enzymes may undergo transient modification ...

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