Enzymes and Metabolism

Enzymes are proteins that function as biological catalysts.

They permit biochemical reactions to occur rapidly at normal body temperatures. Enzymes were initially given somewhat arbitrary names, still with us, such as pepsin

To appreciate the effect of an enzyme, think of what happens when paper burns. Paper is composed mainly of glucose (in the form of cellulose). The burning of glucose can be represented by the equation

Paper does not spontaneously burst into flame, because few of its molecules have enough kinetic energy to react. Lighting the paper with a match, however, raises the kinetic energy enough to initiate combustion (rapid oxidation). The energy needed to get the reaction started, supplied by the match, is called activation energy (fig. 2.26a).

In the body, we carry out the same reaction and oxidize glucose to water and carbon dioxide to extract its energy. We could not tolerate the heat of combustion in our bodies, however, so we must oxidize glucose in a more controlled way at a biologically feasible and safe temperature. Enzymes make this happen by lowering the activation energy—that is, by reducing the barrier to glucose oxidation (fig. 2.26b)—and by releasing the energy in small steps rather than a single burst of heat.

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 2 The Chemistry of Life 83

Reaction occurring without a catalyst

Reaction occurring with a catalyst

Energy level-of products

Time-

Energy level-of products

Time-

Figure 2.26 Effect of an Enzyme on Activation Energy. (a) Without catalysts, some chemical reactions proceed slowly because of the high activation energy needed to get molecules to react. (b) A catalyst facilitates molecular interaction, thus lowering the activation energy and making the reaction proceed more rapidly.

Does an enzyme release more energy from its substrate than an uncatalyzed reaction would release?

Enzyme Structure and Action

Substrates bind to pockets called active sites in the enzyme surface and create a temporary enzyme-substrate complex. The enzyme may break covalent bonds and convert the substrate to a reaction product, or it may hold two or more substrates close together, in adjacent active sites, thus enabling the substrates to react with each other (fig. 2.27). The enzyme then releases the reaction products and is free to begin the process again. Since enzymes are not consumed by the reactions they catalyze, one enzyme molecule can convert millions of substrate molecules, and at astonishing speeds. A single molecule of carbonic anhy-drase, for example, breaks carbonic acid (H2CO3) down to H2O and CO2 at a rate of 36 million molecules per minute.

A substrate fits an enzyme somewhat like a key fits a lock. A given enzyme is very selective—that is, it exhibits enzyme-substrate specificity. An enzyme that oxidizes glucose, for example, will not act on the similar sugar galactose, which does not fit its active site.

Factors that change the shape of an enzyme—notably temperature and pH—tend to alter or destroy the ability of the enzyme to bind its substrate. They disrupt the hydrogen bonds and other weak forces that hold the enzyme in its proper conformation, essentially changing the shape of the "lock" (active site) so that the "key" (substrate) no longer fits. Enzymes vary in optimum pH according to where in the body they normally function. Thus salivary amylase, which digests starch in the mouth, functions best at pH 7 and is inactivated when it is exposed to stomach acid; pepsin, which works in the acidic environment of the stomach, functions best around pH 2; and trypsin, a digestive enzyme that works in the alkaline environment of the small intestine, has an optimum pH of 9.5. Our internal body temperature is nearly the same everywhere, however, and all human enzymes have a temperature optimum (that is, they produce their fastest reaction rates) near 37°C.

_Think About It_

Why is homeostasis important for enzyme function?

Cofactors

Many enzymes cannot function without nonprotein partners called cofactors—for example, iron, copper, zinc, magnesium, or calcium ions. By binding to an enzyme, a cofactor may stimulate it to fold into a shape that activates its active site. Coenzymes are organic cofactors usually

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

84 Part One Organization of the Body

(substrates)

Enzyme (products)

Substrate A

Substrate A

Substrate B

Enzyme

(a) Enzyme and substrates

Substrate B

Product C Ü
Unity Vitamin Complex

Product D

Enzyme

Product D

(a) Enzyme and substrates

(b) Enzyme-substrate complex

(c) Enzyme (unchanged) and reaction products

Figure 2.27 The Three Steps of an Enzymatic Reaction. (a) One or more substrate molecules bind to the enzyme's active sites. (b) The substrates and enzyme form a temporary enzyme-substrate complex and the substrates react chemically with each other. (c) The enzyme releases the reaction products and is available to catalyze the same reaction again.

derived from niacin, riboflavin, and other water-soluble vitamins. They accept electrons from an enzyme in one metabolic pathway and transfer them to an enzyme in another pathway. For example, cells partially oxidize glucose through a pathway called glycolysis. A coenzyme called NAD+,28 derived from niacin, shuttles electrons from this pathway to another one called aerobic respiration, which uses energy from the electrons to make ATP (fig. 2.28). If NAD+ is unavailable, the glycolysis pathway shuts down.

Metabolic Pathways

A metabolic pathway is a chain of reactions with each step usually catalyzed by a different enzyme. A simple metabolic pathway can be symbolized a p 7 A ^ B ^ C ^ D

where A is the initial reactant, B and C are intermediates, and D is the end product. The Greek letters above the reaction arrows represent enzymes that catalyze each step of the reaction. A is the substrate for enzyme a, B is the substrate for enzyme p, and C for enzyme 7. Such a pathway

28nicotinamide adenine dinucleotide

Glycolysis Aerobic respiration

Glycolysis Aerobic respiration

Mcgraw Hill Glycolysis
Figure 2.28 The Action of a Coenzyme. A coenzyme such as NAD acts as a shuttle that picks up electrons from one metabolic pathway (in this case, glycolysis) and delivers them to another pathway (in this case, aerobic respiration).

can be turned on or off by altering the conformation of any of these enzymes, thereby activating or deactivating them. This can be done by such means as the binding or dissociation of a cofactor, or by an end product of the pathway binding to an enzyme at an earlier step (product D binding to enzyme a and shutting off the reaction chain at that step, for example). In these and other ways, cells are able to turn on metabolic pathways when their end products are needed and shut them down when the end products are not needed.

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • richard
    Why is homeostasis important for enzyme function saladin?
    9 years ago

Post a comment