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43286_Ward's World+MGH Enzyme

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1 Enzyme Article by: Daniel Wellner, Department of Biochemistry, Cornell University Medical College, Ithaca, New York. Access to this content is available to Ward's World readers for free from McGraw Hill's AccessScience, an award-winning, digital STEM resource that provides immediate, authoritative answers to students' thirst for scientific knowledge on topics such as climate change, virology, pollution, and more. Ward's World and McGraw Hill have partnered to offer educators a no-obligation, free trial subscription to this product. Request your free trial today and discover how valuable AccessScience can be for you and your students. A catalytic protein produced by living cells. Chemical reac- tions involved in the digestion of foods, the biosynthesis of mac- romolecules, the controlled release and utilization of chemical energy, and other processes characteristic of life are all catalyzed by enzymes (Fig. 1). Hundreds of reactions can proceed simulta- neously within a living cell, and the cell contains a comparable number of individual enzymes, each of which controls the rate of one or more of these reactions. In the absence of enzymes, these reactions would not take place at a significant rate. Indeed, enzymes are such efficient catalysts that they acceler- ate chemical reactions measurably even at concentrations too low to be detected by most protein assays. In addition, enzymes promote the chemical processes of life without being them- selves altered or destroyed. Chemical structure and activity Enzymes are proteins with molecular weights that typically range from about 10,000 to more than 1,000,000 daltons. Like other proteins, enzymes consist of chains of amino acids linked together by peptide bonds. An enzyme molecule may contain one or more of these polypeptide chains (Fig. 1). The sequence of amino acids within the polypeptide chains is characteristic for each enzyme and is believed to determine the unique three-dimensional conformation in which the chains are folded. Relatively weak interactions between different parts of the peptide chains and the surrounding medium stabilize this conformation, but they can be disrupted readily by high temperatures, acid or alkaline conditions, or changes in the polarity of the medium. For this reason, enzymes typically work best within an optimal range of temperature and pH. Changes that lead to an unfolding of the peptide chains (denaturation) cause a concomitant loss of enzymatic activity, solubility, and other properties characteristic of the native enzyme. Although enzymes can generally catalyze an indefinite amount of chemi- cal change, most isolated enzymes are relatively unstable, and they often lose activity gradually and need to be replaced. Enzyme denaturation is sometimes reversible under conditions that allow protein refolding back to the native conformation. Many enzymes contain an additional, nonprotein component, termed a coenzyme or prosthetic group. It may be an organic molecule, often a vitamin derivative, or a metal ion. The co- enzyme in most instances participates directly in the catalytic reaction. For example, it may serve as an intermediate carrier of a group being transferred from one substrate to another. Some coenzymes tightly bind to their enzyme's protein component (the apoenzyme), whereas others dissociate readily. When the apoenzyme and the coenzyme are separated, neither possess- es the catalytic properties of the original conjugated protein (the holoenzyme). The same coenzyme may be associated + ward ' s science Key Concepts • Enzymes are catalytic proteins that cells produce to perform hundreds of chemical reactions essential to life. Without enzymes, these reactions would proceed too slowly for cells to survive. • Enzymes are classified on the basis of the types of substrate molecules on which they act and on the types of reactions they enable. • Mechanisms that control enzyme activity include proenzyme activation; feedback inhibition and enzyme repression; and induction. Fig. 1: Structural representation of the enzyme alpha-amylase, which hydrolyzes polysaccha- rides, including starch and glycogen, to glucose and maltose. (Credit: Leonid Andronov/Shutterstock)

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