Organic Chemistry

1036 CHAPTER 25 The Organic Mechanisms of the Coenzymes • Metabolism

CO 2

CO 2

NADH

Stage 1

fats

fatty acids

Stage 3

Stage 4

Stage 2

3 ATP + NAD+

carbohydrates

monosaccharides

pyruvate

acetyl-CoA

citric acid

cycle

proteins

amino acids

Figure 25.1N
The four stages of catabolism:

1. digestion (hydrolysis of polymers
   to monomers)


2. conversion of monomers to
   compounds that can enter the citric
   acid cycle


3. the citric acid cycle


4. oxidative phosphorylation

Catabolism can be divided into four stages. The first stage of catabolismis called

digestion. The reactants required for all life processes ultimately come from our diet.

In that sense, we really are what we eat. In this first stage, fats, carbohydrates, and pro-

teins are hydrolyzed into fatty acids, monosaccharides, and amino acids, respectively

(Figure 25.1).

In the second stage of catabolism, these hydrolysis products—fatty acids, mono-

saccharides, and amino acids—are converted to compounds that can enter the citric

acid cycle. To enter the citric acid cycle, a compound must be either one of the com-

pounds in the cycle itself (called a citric acid cycle intermediate), acetyl-CoA, or

pyruvate. Acetyl CoA is the only non–citric acid cycle intermediate that can enter

the cycle (Section 17.20); pyruvate can enter the citric acid cycle only because it can

be converted to acetyl-CoA or to oxaloacetate, a citric acid cycle intermediate

(Sections 25.4 and 25.5). Fatty acids are converted to acetyl-CoA, monosaccharides

are converted to pyruvate via glycolysis (Figure 25.2), and amino acids are convert-

ed to acetyl-CoA, pyruvate, and/or citric acid cycle intermediates, depending on the

amino acid.

The third stage of catabolismis the citric acid cycle (also known as the Krebs

cycle or the tricarboxylic acid (TCA) cycle; Figure 25.3). For every acetyl-CoA that

enters the cycle, two molecules of are formed.

Metabolic energy is measured in terms of adenosine triphosphate (ATP). How the

body uses ATP is described in Sections 27.2 and 27.3. Very little ATP is formed in the

first three stages of catabolism. However, in the fourth stage of catabolism,every

NADH (see Section 25.2) that is formed in the process of carrying out oxidation reac-

tions in the earlier stages is converted into three ATPs in a process known as oxidative

phosphorylation. Thus, the bulk of the energy provided by fats, carbohydrates, and

proteins is obtained in this fourth stage.

CO 2