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9A Degradation of Amino Acids—The Fate of the Amino Group

9A Degradation of Amino Acids—The Fate of the Amino Group

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AMINO ACID METABOLISM



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FIGURE 24.9 An Overview of the Catabolism of Amino Acids

Amino acids

Amino acid

catabolism

NH4+

Carbon skeleton



Urea

cycle



Urea



Pyruvate

Acetyl CoA



Citric acid

cycle



The breakdown of amino acids forms NH4+, which enters the urea cycle to form urea, and a

carbon skeleton that is metabolized to either pyruvate, acetyl CoA, or an intermediate in the

citric acid cycle.



• Transamination is the transfer of an amino group from an amino acid to an 𝛂-keto acid,

usually 𝛂-ketoglutarate.

The C–H and C–NH3+ groups are replaced by a C –

– O.

+



NH3

R



C



O

CO2−



+



H

amino acid



+



NH3



O



R' C CO2−

α-keto acid



R C CO2−

α-keto acid



transaminase



+



R'



C



CO2−



H

amino acid



The C –

– O is replaced by C–H and C–NH3+ groups.



In transamination, the amino group of the amino acid and the ketone carbonyl oxygen of

the 𝛂-keto acid are interchanged to form a new amino acid and a new 𝛂-keto acid. As an

example, transfer of an amino group from alanine to α-ketoglutarate forms pyruvate and glutamate, the completely ionized form of the amino acid glutamic acid.

+



NH3

CH3



C



O

CO2−



H

alanine



+



C CO2−

α-ketoglutarate



−O CCH CH

2

2

2



+



NH3



O

transaminase



CH3 C CO2−

pyruvate



+



The amino group is removed

from the amino acid.



−O



2CCH2CH2



C



CO2−



H

glutamate

(ionic form of glutamic acid)



Transamination removes the amino group to form a carbon skeleton that contains only

carbon, hydrogen, and oxygen atoms. These products are then degraded along other catabolic

pathways as described in Section 24.9B.



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CARBOHYDRATE, LIPID, AND PROTEIN METABOLISM



SAMPLE PROBLEM 24.4



What products are formed in the following transamination reaction?

+



NH3

(CH3)2CH



C



O





−O CCH CH

2

2

2



+



CO2



C



CO2−



α-ketoglutarate



H

valine



transaminase



To draw the products of transamination, convert the C H and C NH3+ groups of the amino

acid to a C O, forming an α-keto acid.



ANALYSIS

SOLUTION



new C–NH3+ bond



+



NH3

(CH3)2CH



C



O

CO2−



C CO2−

α-ketoglutarate



−O CCH CH

2

2

2



+



H

valine



+



NH3



O

transaminase



(CH3)2CH C CO2−

α-keto acid



+



−O CCH CH

2

2

2



C



CO2−



H

glutamate



The amino group is removed

from the amino acid.



Draw the products formed when each amino acid undergoes transamination with α-ketoglutarate.



PROBLEM 24.30



+



+



NH3



a.



HOCH2



CO2−



C



+



NH3



b.



CH3SCH2CH2



H

serine



C



NH3

CO2−



c.



HSCH2



H

methionine



C



CO2−



H

cysteine



The glutamate formed by transamination is then degraded by oxidative deamination using NAD+.

• In oxidative deamination, the C H and C NH3+ bonds on the 𝛂 carbon of glutamate are

converted to C O and an ammonium ion, NH4+.

These bonds are broken.



NAD+



+



NH3

−O CCH CH

2

2

2



Cα CO2−



+



H2O



NADH + H+



glutamate

dehydrogenase



H

glutamate



O



enters the urea cycle



−O CCH CH

2

2

2



C CO2−

α-ketoglutarate



+



NH4+



This product can be recycled

for another transamination.



In oxidative deamination, glutamate is re-converted to α-ketoglutarate, which can undergo

transamination with another molecule of an amino acid and the cycle repeats. In this way,

α-ketoglutarate removes an amino group from an amino acid and then loses that amino group

as NH4+ in a second step.

The ammonium ion then enters the urea cycle, where it is converted by a multistep pathway to

urea, (NH2)2C O, in the liver. Urea is then transported to the kidneys and excreted in urine.

• The overall result of transamination and oxidative deamination is to remove an amino

group from an amino acid and form an ammonium ion, NH4+.

+



NH3

R



C



O

CO2



H

amino acid



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[1] transamination

[2] oxidative deamination



R C CO2−

α-keto acid



+



NH4+



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AMINO ACID METABOLISM



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SAMPLE PROBLEM 24.5



What final products are formed when leucine is subjected to transamination followed by

oxidative deamination?

+



NH3

(CH3)2CHCH2



C



CO2−



H

leucine



ANALYSIS



To draw the organic product, replace the C H and C NH3+ on the α carbon of the amino acid

by C O. NH4+ is formed from the amino group.



SOLUTION



+



α carbon



NH3



(CH3)2CHCH2



C



O

[1] transamination

[2] oxidative deamination



CO2−



(CH3)2CHCH2



C



CO2−



+



NH4+



H



PROBLEM 24.31



What products are formed when each amino acid is subjected to transamination followed by

oxidative deamination: (a) threonine; (b) glycine; (c) isoleucine? Use the structures in Table 21.2.



24.9B



DEGRADATION OF AMINO ACIDS—THE FATE OF THE

CARBON SKELETON



Once the nitrogen has been removed from an amino acid, the carbon skeletons of individual

amino acids are catabolized in a variety of ways. There are three common fates of the carbon

skeletons of amino acids, shown in Figure 24.10:

• conversion to pyruvate, CH3COCO2–

• conversion to acetyl CoA, CH3COSCoA

• conversion to an intermediate in the citric acid cycle







FIGURE 24.10 Amino Acid Catabolism

alanine, cysteine,

glycine, serine,

threonine, tryptophan



isoleucine, leucine,

threonine, tryptophan



pyruvate

acetyl CoA



asparagine,

aspartic acid



oxaloacetate



acetoacetyl CoA

(CH3COCH2COSCoA)



citrate



malate



isocitrate

arginine, glutamic acid,

glutamine, histidine,

proline



Citric acid

cycle

phenylalanine,

tyrosine



leucine, lysine,

phenylalanine,

tryptophan, tyrosine



α-ketoglutarate



fumarate



succinate



succinyl CoA



isoleucine, methionine,

threonine, valine



Glucogenic amino acids are highlighted in blue, while ketogenic amino acids are highlighted

in tan. Amino acids that appear more than once in the scheme can be degraded by multiple

routes.



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CARBOHYDRATE, LIPID, AND PROTEIN METABOLISM



Some amino acids such as alanine (Section 24.9A) are catabolized to pyruvate. Since pyruvate is an

intermediate in both glycolysis and gluconeogenesis, pyruvate can be broken down for energy or used

to synthesize glucose. In considering catabolism, amino acids are often divided into two groups.

• Glucogenic amino acids are catabolized to pyruvate or an intermediate in the citric acid

cycle. Since these catabolic products can be used for gluconeogenesis, glucogenic

amino acids can be used to synthesize glucose.

• Ketogenic amino acids are converted to acetyl CoA, or the related thioester

acetoacetyl CoA, CH3COCH2COSCoA. These catabolic products cannot be used to

synthesize glucose, but they can be converted to ketone bodies and yield energy by this

path.



We will not examine the specific pathways that convert the carbon skeletons of individual amino

acids into other products. Figure 24.10 illustrates where each amino acid feeds into the metabolic

pathways we have already discussed.



PROBLEM 24.32



What metabolic intermediate is produced from the carbon atoms of each amino acid?

a. cysteine



b. aspartic acid



c. valine



d. threonine



CHAPTER HIGHLIGHTS

KEY TERMS

Acyl CoA (24.7)

Cori cycle (24.6)

Decarboxylation (24.2)

Fermentation (24.4)

Glucogenic amino acid (24.9)

Gluconeogenesis (24.6)



Glycolysis (24.3)

Isomerase (24.2)

Ketogenesis (24.8)

Ketogenic amino acid (24.9)

Ketone bodies (24.8)

Ketosis (24.8)



Kinase (24.2)

β-Oxidation (24.7)

Oxidative deamination (24.9)

Transamination (24.9)

Urea cycle (24.9)



KEY CONCEPTS

❶ What are the main elements that provide clues to the

outcome of a biochemical reaction? (24.2)

• To understand the course of a biochemical reaction,

examine the functional groups that are added or removed,

the reagents (coenzymes or other materials), and the

enzyme. The name of an enzyme is often a clue as to the

type of reaction.

❷ What are the main aspects of glycolysis? (24.3)

• Glycolysis is a linear, 10-step pathway that converts glucose

to two three-carbon pyruvate molecules. In the energyinvestment phase, steps [1]–[5], the energy from two ATP

molecules is used for phosphorylation and two three-carbon

products are formed. In the energy-generating phase, steps

[6]–[10], the following species are generated: two pyruvate

molecules (CH3COCO2–), 2 NADHs, and 4 ATPs.

• The net result of glycolysis, considering both phases,

is 2 CH3COCO2–, 2 NADHs, and 2 ATPs.



smi26573_ch24.indd 770



❸ What are the major pathways for pyruvate metabolism?

(24.4)

• When oxygen is plentiful, pyruvate is converted to acetyl

CoA, which can enter the citric acid cycle.

• When the oxygen level is low, the anaerobic metabolism of

pyruvate forms lactate and NAD+.

• In yeast and other microorganisms, pyruvate is converted to

ethanol and CO2 by fermentation.

❹ How much ATP is formed by the complete catabolism of

glucose? (24.5)

• To calculate the amount of ATP formed in the catabolism

of glucose, we must take into account the ATP yield from

glycolysis, the oxidation of two molecules of pyruvate to

two molecules of acetyl CoA, the citric acid cycle, and

oxidative phosphorylation.

• As shown in Figure 24.6, the complete catabolism of

glucose forms six CO2 molecules and 32 molecules of ATP.



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