In synthesis reaction (a) the shapes of substrate molecules fit the shape of the enzyme's active site. (b) When the substrate molecules temporarily combine with the enzyme, a chemical reaction occurs. The result is (c) product molecule and an unaltered enzyme. Many enzymatic reactions are reversible.

Enzyme B


Figure 4.5

A metabolic pathway consists of a series of enzyme-controlled reactions leading to formation of a product.

a lipid-splitting enzyme is called a lipase, a proteinsplitting enzyme is a proiease, and a starch-(amylum) splitting enzyme is an amylase. Similarly, sucrase is an enzyme that splits the sugar sucrose, maliase splits the sugar maltose, and laciase splits the sugar lactose.

Cofactors and Coenzymes

Often an enzyme is inactive until it combines with a nonprotein component that either helps the active site attain its appropriate shape or helps bind the enzyme to its substrate. Such a substance, called a cofactor, may be an ion of an element, such as copper, iron, or zinc, or it may be a small organic molecule, called a coenzyme (ko-en'zlm). Coenzymes are often composed of vitamin molecules or incorporate altered forms of vitamin molecules into their structures.

Vitamins are essential organic substances that human cells cannot synthesize (or may not synthesize in sufficient quantities) and therefore must come from the diet. Since vitamins provide coenzymes that can, like enzymes, function again and again, cells require very small quantities of vitamins. Chapter 18 (pp. 750-757) discusses vitamins further.

Factors That Alter Enzymes

Almost all enzymes are proteins, and like other proteins, they can be denatured by exposure to excessive heat, ra diation, electricity, certain chemicals, or fluids with extreme pH values. For example, many enzymes become inactive at 45° C, and nearly all of them are denatured at 55° C. Some poisons are chemicals that denature enzymes. Cyanide, for instance, can interfere with respiratory enzymes and damage cells by halting their energy-obtaining processes.

Certain microorganisms, colorfully called "ex-tremophiles," live in conditions of extremely high or low heat, salinity, or pH. Their enzymes have evolved to function under these conditions and are useful in industrial processes that are too harsh to use other enzymes.

What is an enzyme?

How can an enzyme control the rate of a metabolic reaction?

How does an enzyme "recognize" its substrate? What is the role of a cofactor? What factors can denature enzymes?


Energy (en'er-je) is the capacity to change something; it is the ability to do work. Therefore, we recognize energy by what it can do. Common forms of energy include heat, light, sound, electrical energy, mechanical energy, and chemical energy.

Although energy cannot be created or destroyed, it can be changed from one form to another. An ordinary incandescent light bulb changes electrical energy to heat and light, and an automobile engine changes the chemical energy in gasoline to heat and mechanical energy.

Changes occur in the human body as a characteristic of life—whenever this happens, energy is being transferred. Thus, all metabolic reactions involve energy in some form.

Release of Chemical Energy

Most metabolic processes depend on chemical energy. This form of energy is held in the chemical bonds that link atoms into molecules and is released when these bonds break. Burning a marshmallow over a campfire releases the chemical energy held within the bonds of substances in the marshmallow as heat and light. Similarly, when a marshmallow is eaten, digested, and absorbed, cells "burn" glucose molecules from that marshmallow in a process called oxidation (ok"si-da'shun). The energy released by oxidation of glucose is used to promote cellular metabolism. There are obviously some important differences between the oxidation of substances inside cells and the burning of substances outside them.

Burning in nonliving systems (such as, starting a fire in a fireplace) usually requires a relatively large amount of energy to begin, and most of the energy released escapes as heat or light. In cells, enzymes initiate oxidation by decreasing the activation energy. Also, by transferring energy to special energy-carrying molecules, cells are able to capture almost half of the energy released in the form of chemical energy. The rest escapes as heat, which helps maintain body temperature.

Cellular respiration is the process that releases energy from molecules such as glucose and makes it available for cellular use. The chemical reactions of cellular respiration must occur in a particular sequence, each one controlled by a different enzyme. Some of these enzymes are in the cell's cytosol, whereas others are in the mitochondria. Such precision of activity suggests that the enzymes are physically positioned in the exact sequence as that of the reactions they control. Indeed, the enzymes responsible for some of the reactions of cellular respiration are located in tiny, stalked particles on the membranes (cristae) within the mitochondria (see chapter 3, p. 77).

D What is energy?

^9 How does cellular oxidation differ from burning? ^9 Define cellular respiration.

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Keep Your Weight In Check During The Holidays

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