Learning Objectives for this Section
To live, grow, and reproduce, microorganisms undergo a variety of chemical changes. They alter nutrients so they can enter the cell and they change them once they enter in order to synthesize cell parts and obtain energy.
Metabolism (def) refers to all of the organized chemical reactions in a cell. Reactions in which chemical compounds are broken down are called catabolic reactions (def) while reactions in which chemical compounds are synthesized are termed anabolic reactions (def). All of these reactions are under the control of enzymes.
Enzymes are substances present in the cell in small amounts that function to speed up or catalyze chemical reactions. On the surface of the enzyme is usually a small crevice that functions as an active site (def) or catalytic site to which one or two specific substrates are able to bind. (Anything that an enzyme normally combines with is called a substrate (def).) The binding of the substrate to the enzyme causes the flexible enzyme to change its shape slightly through a process called induced fit to form a tempory intermediate called an enzyme-substrate complex (see Fig. 7).
Enzymes speed up the rate of chemical reactions because they lower the energy of activation (def), the energy that must be supplied in order for molecules to react with one another (see Fig. 8). Enzymes lower the energy of activation by forming an enzyme-substrate complex allowing products of the enzyme reaction to be formed and released (see Fig. 7).
Many enzymes require a nonprotein cofactor (def) to assist them in their reaction. In this case, the protein portion of the enzyme, called an apoenzyme (def), combines with the cofactor to form the whole enzyme or haloenzyme (def) (see Fig. 9). Some cofactors are ions such as Ca++, Mg++, and K+; other cofactors are organic molecules called coenzymes (def) which serve as carriers for chemical groups or electrons. NAD+, NADP+, FAD, and coenzyme A (CoA) are examples of coenzymes.
A. Characteristics of Enzymes
a. Chemically, enzymes are generally globular proteins. (Some RNA molecules called ribozymes (def) can also be enzymes. These are usually found in the nuclear region of cells.)
b. Enzymes are catalysts (def) that breakdown or synthesize more complex chemical compounds. They allow chemical reactions to occur fast enough to support life. Enzymes speed up the rate of chemical reactions because they lower the energy of activation (def), the energy that must be supplied in order for molecules to react with one another. Anything that an enzyme normally combines with is called a substrate (def). Enzymes are very efficient. An enzyme generally can typically catalyze between 1 and 10,000 molecules of substrate per second.
c. Enzymes are only present in small amounts in the cell since they are not altered during their reactions.
d. Enzymes are highly specific for their substrate. Generally there is one specific enzyme for each specific chemical reaction.
B. Enzyme Activity
Enzyme activity is affected by a number of factors including:
a. the concentration of enzyme
Assuming a sufficient concentration of substrate is available, increasing enzyme concentration will increase the enzyme reaction rate.
b. the concentration of substrate
At a constant enzyme concentration and at lower concentrations of substrates, the substrate concentration is the limiting factor. As the substrate concentration increases, the enzme reaction rate increases. However, at very high substrate concentrations, the enzymes become saturated with substrate and a higher concentration of substrate does not increase the reaction rate.
c. the temperature
Each enzyme has an optimum temperature at which it works best. A higher temperature generally results in an increase in enzyme activity. As the temperature increases, molecular motion increases resulting in more molecular collisions. If, however, the temperature rises above a certain point, the heat will denature (def) the enzyme, causing it to lose its three-dimensional functional shape by denaturing its hydrogen bonds. Cold temperature, on the other hand, slows down enzyme activity by decreasing molecular motion.
d. the pH
Each enzyme has an optimal pH that helps maintain its three-dimensional shape. Changes in pH may denature enzymes by altering the enzyme's charge. This alters the ionic bonds of the enzyme that contribute to its functional shape.
e. salt concentration
Each enzyme has an optimal salt concentration. Changes in the salt concentration may also denature enzymes.
© Gary E. Kaiser
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Updated: June 27, 2001