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Table of Contents

(Subject Area: Biochemistry)

Article

Authors

Pages in the

Encyclopedia

Bioenergetics

Richard E. McCarty

and Eric A. Johnson

Pages 99-115

Enzyme Mechanisms

Stephen J. Benkovic

and Ann M. Valentine

Pages 627-639

Food Colors

Pericles Markakis

Pages 105-120

Glycoconjugates and

Carbohydrates

Eugene A. Davidson

Pages 833-849

Ion Transport Across

Biological Membranes

George P. Hess

Pages 99-108

Lipoprotein/Cholesterol

Metabolism

Alan D. Attie

Pages 643-660

Membrane Structure

Anna Seelig and

Joachim Seelig

Pages 355-367

Natural Antioxidants

In Foods

Eric A. Decker

Pages 335-342

Sankar Mitra_ Tapas

K. Hazra and Tadahide

Nucleic Acid Synthesis

Pages 853-876

Protein Folding

Maurice Eftink and

Susan Pedigo

Pages 179-190

Protein Structure

Ivan Rayment

Pages 191-218

Protein Synthesis

Paul Schimmel and

Rebecca W. Alexander

Pages 219-240

Vitamins and

Coenzymes

David E. Metzler

Pages 509-528

Bioenergetics

Richard E. McCarty

Eric A. Johnson

Johns Hopkins University

I. Catabolic Metabolism: The Synthesis of ATP

II. Photosynthesis

III. Origin of Mitochondria and Chloroplasts

IV. Illustrations of the Uses of ATP: Ion Transport,

Biosynthesis, and Motility

V. Concluding Statements

GLOSSARY

BIOENERGETICS , an amalgamation of the term biolog-

ical energetics , is the branch of biology and biochemistry

that is concerned with how organisms extract energy from

their environment and with how energy is used to fuel the

myriad of life’s endergonic processes. Organisms may be

usefully divided into two broad groups with respect to

how they satisfy their need for energy. Autotrophic organ-

isms convert energy from nonorganic sources such as light

or from the oxidation of inorganic molecules to chemical

energy. As heterotrophic organisms, animals must ingest

and break down complex organic molecules to provide the

energy for life.

Interconversions of forms of energy are commonplace

in the biological world. In photosynthesis, the electro-

magnetic energy of light is converted to chemical energy,

largely in the form of carbohydrates, with high overall

efﬁciency. The energy of light is used to drive oxidation–

reduction reactions that could not take place in the dark.

Light energy also powers the generation of a proton

electrochemical potential across the green photosynthetic

Adenosine 5 -triphosphate (ATP) The carrier of free

energy in cells.

Bioenergetics The study of energy relationships in living

systems.

Chloroplasts The sites of photosynthesis in green

plants.

Ion transport The movement of ions across biological

membranes.

Metabolism The total of all reactions that occur in cells.

Catabolic metabolism is generally degradative and ex-

ergonic, whereas anabolic metabolism is synthetic and

requires energy.

Mitochondria Sites of oxidative (catabolic) metabolism

in cells.

Photosynthesis Light-driven synthesis of organic mole-

cules from carbon dioxide and water.

Plasma membrane The barrier between the inside of

cells and the external medium.

100

Bioenergetics

FIGURE 1 Central role of adenosine 5 -triphosphate (ATP) in metabolism. Catabolic (degradative) metabolism

is exergonic and provides the energy needed for the synthesis of ATP from adenosine 5 -diphosphate (ADP)

and inorganic phosphate (P i ). The exergonic hydrolysis of ATP in turn powers the endergonic processes of

organisms.

membrane. Thus, electrical work is an integral part of pho-

tosynthesis. Chemical energy is used in all organisms to

drive the synthesis of large and small molecules, motility

at the microscopic and macroscopic levels, the genera-

tion of electrochemical potentials of ions across cellular

membranes, and even light emission as in ﬁreﬂies.

Given the diversity in the forms of life, it might be ex-

pected that organisms have evolved many mechanisms to

deal with their need for energy. To some extent this ex-

pectation is the case, especially for organisms that live in

extreme environments. However, the similarities among

organisms in their bioenergetic mechanisms are as, or even

more, striking than the differences. For example, the sugar

glucose is catabolized (broken down) by a pathway that

is the same in the enteric bacterium Escherichia coli as

it is in higher organisms. All organisms use adenosine

5 -triphosphate (ATP) as a central intermediate in energy

metabolism. ATP acts in a way as a currency of free en-

ergy. The synthesis of ATP from adenosine 5 -diphosphate

(ADP) and inorganic phosphate (P i ) is a strongly en-

dergonic reaction that is coupled to exergonic reactions

such as the breakdown of glucose. ATP hydrolysis in

turn powers many of life’s processes. The central role of

ATP in bioenergetics is illustrated in Fig. 1 . Partial struc-

tures of sev

metabolism are shown in Fig. 2 .

In this article, the elements of energy metabolism will

be discussed with emphasis on how organisms satisfy their

energetic requirements and on how ATP hydrolysis drives

otherwise unfavorable reactions.

I. CATABOLIC METABOLISM:

THE SYNTHESIS OF ATP

Metabolism may be deﬁned as the total of all the chem-

ical reactions that occur in organisms. Green plants can

synthesize all the thousands of compounds they contain

eral compounds that play important roles in

Bioenergetics

101

G 0 of about 4 kcal/mol, at pH 7.0 and 25 ◦ C. (Note

that the biochemist’s standard state differs from that as

usually deﬁned in that the activity of the hydrogen ion is

taken as 10 − 7 M , or pH 7.0, rather than 1 M , or pH 0.0.

pH 7.0 is much closer to the pH in most cells.) This prob-

lem is neatly solved in cells by using ATP, rather than P i ,

as the phosphoryl donor:

Glucose

AT P

←→

Glucose 6-phosphate

ADP

G 0 for this reaction, which is catalyzed by the en-

zyme hexokinase, is approximately

4 kcal/mol. Thus the

phosphorylation of glucose by ATP is an energetically fa-

vorable reaction and is one example of how the chemical

energy of ATP may be used to drive otherwise unfavorable

reactions.

Glucose 6-phosphate is then isomerized to form fruc-

tose 6-phosphate, which in turn is phosphorylated by ATP

at the 1-position to form fructose 1,6-bisphosphate. It

seems odd that a metabolic pathway invests 2 mol of ATP

in the initial steps of the pathway when ATP is an im-

portant product of the pathway. However, this investment

pays off in later steps.

Fructose 1,6-bisphosphate is cleaved to form two triose

phosphates that are readily interconvertible. Note that the

oxidation–reduction state of the triose phosphates is the

same as that of glucose 6-phosphate and the fructose phos-

phates. All molecules are phosphorylated sugars. In the

next step of glycolysis, glyceraldehyde 3-phosphate is ox-

idized and phosphorylated to form a sugar acid that con-

tains a phosphoryl group at positions 1 and 3. The oxidiz-

ing agent, nicotinamide adenine dinucleotide (NAD + ), is

a weak oxidant ( E 0 , at pH 7.0 of

−

FIGURE 2 Some important reactions in metabolism. Shown are

the phosphorylation of ADP to ATP, NAD + , NADH, FAD, FADH 2

acetate, CoA, and acetyl CoA. For clarity, just the parts of the

larger molecules that undergo reaction are shown. NAD + , nicoti-

namide adenine dinucleotide; NADH, nicotinamide adenine dinu-

cleotide (reduced form); FAD, ﬂavin adenine dinucleotide; FADH 2 ,

ﬂavin adenine dinucleotide (reduced form); CoA, coenzyme A;

AMP, adenosine monophosphate.

from carbon dioxide, water, and inorganic nutrients. The

discussion of the complicated topic of metabolism is

somewhat simpliﬁed by separation of the subject into

two areas—catabolic and anabolic metabolism. Catabolic

metabolism is degradative and is generally exergonic. ATP

is a product of catabolic metabolism. In contrast, an-

abolic metabolism is synthetic and requires ATP. Fortu-

nately, there are relatively few major pathways of energy

metabolism.

340 mV). The oxida-

tion of the aldehyde group of glyceraldehyde 3-phosphate

to a carboxylate is a favorable reaction that drives both

the oxidation and the phosphorylation. This is the only

oxidation–reduction reaction in glycolysis.

The hydrolysis of acyl phosphates, such as that of

position 1 of 1,3-bisphosphoglycerate, is characterized

by strongly negative

−

A. Glycolysis and Fermentation

G 0 values. That for 1,3-bisphos-

phoglycerate is approximately

Carbohydrates are a major source of energy for organisms.

The major pathway by which carbohydrates are degraded

is called glycolysis. Starch, glycogen, and other carbohy-

drates are converted to the sugar glucose by pathways that

will not be considered here. In glycolysis, glucose, a six-

carbon sugar, is oxidized and cleaved by enzymes in the

cytoplasm of cells to form two molecules of pyruvate, a

three-carbon compound (see Figs. 3 and 4). The overall

reaction is exergonic and some of the energy released is

conserved by coupling the synthesis of ATP to glycolysis.

Before it may be metabolized, glucose must ﬁrst be

phosphorylated on the hydroxyl residue at position 6.

Under intracellular conditions, the direct phosphorylation

10 kcal/mol, which is

signiﬁcantly more negative than the

−

G 0 for the hydrol-

ysis of ATP to ADP and P i . Thus, the transfer of the acyl

phosphate from 1,3-bisphosphoglycerate to ADP to form

3-phosphoglycerate and ATP is a spontaneous reaction.

Since two sugar acid bisphosphates are formed per glu-

cose metabolized, the two ATP invested in the beginning

of the pathway have been recovered.

In the next steps of glycolysis, the phosphate on the

3-position of the 3-phosphoglycerate is transferred to the

hydroxyl residue at position 2. Removal of the elements

of water from 2-phosphoglycerate results in the formation

of an enolic phosphate compound, phospho(enol)pyruvate

of glucose by P i is an unfavorable reaction, characterized

by a

The

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