Chapter 6 ENZYMES
There are two fundamental conditions for life. First, the organism must be able to self-replicate (a topic to be considered in Part III); second, it must be able to catalyze chemical reactions efficiently and selectively. The central importance of catalysis may seem surprising, but it is easy to demonstrate. As described in Chapter 1, living systems make use of energy from the environment. Many of us, for example, consume substantial amounts of sucrose — common table sugar — as a kind of fuel, usually in the form of sweetened foods and drinks. The conversion of sucrose to and in the presence of oxygen is a highly exergonic process, releasing free energy that we can use to think, move, taste, and see. However, a bag of sugar can remain on the shelf for years without any obvious conversion to and . Although this chemical process is thermodynamically favorable, it is very slow. Yet when sucrose is consumed by a human (or almost any other organism), it releases its chemical energy in seconds. The difference is catalysis. Without catalysis, chemical reactions such as sucrose oxidation could not occur on a useful time scale, and thus could not sustain life. Essentially all reactions that occur in cells are, and must be, catalyzed by enzymes, the most remarkable and highly specialized of the proteins.
Our discussion of enzymes is organized around five principles:
Enzymes are powerful biological catalysts. Rate accelerations by enzymes are often far greater than those by synthetic or inorganic catalysts. Like all catalysts, enzymes increase reaction rates, lowering reaction activation barriers. Enzymes do not affect the equilibria of reactions.
Enzymes exhibit a very high degree of specificity. Each enzyme catalyzes only one chemical reaction, or sometimes a few closely related reactions. Reaction activation barriers are thus lowered selectively.
Enzymatic reactions occur in specialized pockets called active sites. These pockets are similar to ligand binding sites, except that a reaction occurs there — the conversion of a substrate, a molecule that is acted on by an enzyme, to a product.
Two concepts explain the catalytic power of enzymes. First, enzymes bind most tightly to the transition state of the catalyzed reaction, using binding energy to lower the activation barrier. Second, enzyme active sites are organized by evolution to facilitate multiple mechanisms of chemical catalysis simultaneously.
Many enzymes are regulated. Regulatory mechanisms include reversible covalent modification, binding of allosteric modulators, proteolytic activation, noncovalent binding to regulatory proteins, and elaborate regulatory cascades. Enzymes are often subject to multiple methods of regulation, which allows for exquisite control of every chemical process that occurs in a cell.
Enzymes are powerful biological catalysts. Rate accelerations by enzymes are often far greater than those by synthetic or inorganic catalysts. Like all catalysts, enzymes increase reaction rates, lowering reaction activation barriers. Enzymes do not affect the equilibria of reactions.
Enzymes exhibit a very high degree of specificity. Each enzyme catalyzes only one chemical reaction, or sometimes a few closely related reactions. Reaction activation barriers are thus lowered selectively.
Enzymatic reactions occur in specialized pockets called active sites. These pockets are similar to ligand binding sites, except that a reaction occurs there — the conversion of a substrate, a molecule that is acted on by an enzyme, to a product.
Two concepts explain the catalytic power of enzymes. First, enzymes bind most tightly to the transition state of the catalyzed reaction, using binding energy to lower the activation barrier. Second, enzyme active sites are organized by evolution to facilitate multiple mechanisms of chemical catalysis simultaneously.
Many enzymes are regulated. Regulatory mechanisms include reversible covalent modification, binding of allosteric modulators, proteolytic activation, noncovalent binding to regulatory proteins, and elaborate regulatory cascades. Enzymes are often subject to multiple methods of regulation, which allows for exquisite control of every chemical process that occurs in a cell.The study of enzymes has immense practical importance. Some diseases, especially inheritable genetic disorders, are the result of a deficiency or even a total absence of one or more enzymes. Other disease conditions may be caused by excessive activity of an enzyme. Measurements of the activities of enzymes in blood plasma, erythrocytes, or tissue samples are important in diagnosing certain illnesses. Many drugs act through interactions with enzymes. Enzymes are also important practical tools in chemical engineering, food technology, and agriculture. Virtually every process studied in a biochemical laboratory involves one or often many enzymes. We now turn to a broader description of these remarkable catalysts.