Chapter 7

Amino Acid Metabolism

C

R

H

NH3

COO

Section 1

Nutritional Function of

Proteins

1. Keep the cells and tissues growing, renewing and mending

2. Take part in some kinds of important physiological activities

3. Oxidation and supply energy

§ 1.1 The significance of proteins

§ 1.2 The requirements and nutritious value of proteins

1. Nitrogen balance

Measuring the amount of intake and losses of total nitrogen can help us to know the general situation of protein metabolism.

There are three kinds of conditions:

1) Normal nitrogen balance

intake N = losses N

2) Positive nitrogen balance

intake N > losses N

3) Negative nitrogen balance

intake N < losses N

2. Physical requirements of proteins

• Lowest requirement:

30~50g/day

• Recommend requirement:

80g/day (65kg man)

3. Nutrition value of proteins

(1) Essential amino acids : Amino acids that cannot be synthesized by the body and must be obtained from the diet.

Eight kinds of essential AAs:

Val, Ile, Leu, Phe, Met, Trp, Thr, Lys

(2) Non- essential amino acids

other 12 kinds of AAs

(3) Semi-essential amino acids

Tyr←Phe

Cys←Met

Note: His and Arg are essential AAs for infants and children.

(4) Complementary effect of dietary proteins

• Two or more plant proteins are consumed together which complement each other in essential amino acid content.

Section 2 Digestion,

Absorption and Putrefaction

§2.1 Digestion

site: stomach, small intestine

enzymes: pepsin

Proteolytic enzymes of pancreatic juice

Proteolytic enzymes of pancreatic juice

endopeptidases

exopeptidases

trypsin: Arg, Lys (C)

chymotrypsin: Tyr, Trp, Phe, Met, Leu (C)

elastase: Ala, Gly, Ser (C)

carboxypeptidase

aminopeptidase

trypsin

chymotrypsinogen

elastase

procarboxypeptidase

trypsinogen

enterokinase

chymotrypsin

proelastase

carboxypeptidase

H2N-CH-C-NH-CH---

R2R1 RnR Rn-1

O O O

amino peptidase endopeptidase carboxy peptidase

amino acid + H2N-CH-C-NH-CH-COOH

R R

O

dipeptidase

amino acid

polypeptide

dipeptide

NH-CH-C-NH-C--- NH-CH-C-NH-CH-COOH

§2.2 Absorption

§2.3 Putrefaction of proteins

Concept: Some undigested proteins and no absorbed products are anaerobic decomposed by the bacteria in intestine.

The products are toxic to body except few vitamin and fatty acid.

1. Production of amines

R

NH2

CO2

R

amino acid

bacteria

amine

CH COOH CH2 NH2

2. Production of ammonia (NH3)

• Two sources:

(1) Metabolism on unabsorbed amino acids

(2) Urea hydrolyzed by urease

3. Some other toxic materials

• Tyr → phenol

• Trp → indole

• Cys → hydrogen sulfide (H2S)

Section 3 General

Metabolism of Amino Acid

§ 3.1 The sources and fates of AAs

Amino acid metabolic pool: amino acids in intracellular and extracellular fluids.

1. Sources of amino acids

• Dietary protein from intestine

• Breakdown of tissue protein

• Synthesis in the body

2. Fates of amino acids

• Deamination

• Decarboxylation

• Synthesis of non-protein nitrogen compounds such as purine and pyrimidine

• Synthesis of proteins

Amino acidmetabolic pool

deamination

decarboxylation

NH3

¦Á-Keto acid

Ketone bodies

Oxidation

Glucose

Urea

AmineCO2

conversion

Non- protein nitrogen compounds

absorption

degradation

synthesis

Dietary proteins

Tissue proteins

Amino acids synthesized

§ 3.2 Degradation of protein in cells

1. Lysosomal pathway

• Extracellular proteins, membrane-associated proteins and long-lived proteins

• ATP-independent process

• Cathepsins

2. Cytosol pathway

• Abnormal proteins, damaged proteins and short-lived proteins

• ATP and ubiquitin

• Proteasome

§ 3.3 The catabolism of AAs

1. Deamination of AAs

Four types:

transamination

oxidative deamination

union deamination

non-oxidative deamination

(1) Transamination

C

R1

H

NH3

COO + C

R2

O

COO C

R2

H

NH3

COO+C

R1

O

COO

Aminotransferases

amino acid-1 amino acid-2keto acid-2keto acid-1

Transamination is the process by which an amino group, usually from glutamate, is transferred to an α-keto acid, with formation of the corresponding amino acid plus α-ketoglutarate.

Key points:

① reversible

② Lys and Pro cannot be transaminated.

③ Aminotransferases utilize a coenzyme - pyridoxal phosphate - which is derived from vitamin B6.

Amino acid pyridoxal phosphate

Schiff base

Isomer of Schiff basepyridoxamine phosphate

α-keto acid

ALT: Alanine aminotransferase (in liver)

AST: Aspartate aminotransferase (in heart)

Two important transaminases:

pyruvate

alanine

glutamate

a-ketoglutarate

oxaloacetate

aspartate

ALT AST

(2) Oxidative deamination

(3) Union deamination

The α- amino group of most amino acids is transferred to α- ketoglutarate to form an α- keto acid and glutamate by transaminase. Glutamate is then oxidatively deaminated to yield ammonia and α- ketoglutarate by glutamate dehydrogenase.

R-CH-COOH

NH2

R-C-COOHO

COOH

CH2

COOH

C O

2

COOH

CH2

COOH

CHNH2

2 NAD+ + H2O

NADH + H+ + NH3

¦Á-ketoglutarate¦Á-amino acid

¦Á-keto acid

L-glutamate dehydrogenasetransaminase

Glu

Alanine + α-ketoglutarate Pyruvate + glutamate

Glutamate + NAD+ + H2O α-ketoglutarate + NADH + NH4

+

Net Reaction:

Alanine + NAD+ + H2O pyruvate + NADH + NH4+

(3) Purine nucleotide cycle (in muscle)

amino acid

COOH

(CH2)2

CO

COOH

COOH

(CH2)2

COOH

keto-glutarate¦Á-

ketoacid

¦Á-

L-Glu

trans-aminase

CHNH2

CH2COOH

COCOOH

oxaloacetate

HN

N N

N

O

R-5'-P

N

N N

N

R-5'-P

HOOCCH2CHCOOH

NH2

HOOCCH2CHCOOH

NH

adenylosuccinate

CH2COOH

CHOHCOOHmalate

CHCOOH

CHCOOHfumarate

N

N N

N

NH2

R-5'-P

H2O

NH3

AMP

IMP AMP deaminaseAST

Asp

adenylosuccinate synthetase

adenylo-succinase

2. Metabolism of a-keto acid

(1) Formation of non- essential AAs

(2) Formation of glucose or lipids

(3) Provide energy

catabolites of amino acid

a-Ketoglutarate

Succinyl CoA

Fumarate

Oxaloacetate

Pyruvate

Acetyl CoA

Acetoacetyl CoA

Intermediates of TAC

PEP Glucose

Fatty acid

Ketone bodies

Amino acids of converted into ketone bodies or fatty acids are termed ketogenic amino acids.

Amino acids of converted into glucose are termed glucogenic amino acids.

Amino acids of converted into both glucose and ketone bodies are termed glucogenic and ketogenic amino acids.

Classification

types amino acids

Glucogenic AAs others

Glucogenic and ketogenic AAs

Ile, Phe, Tyr, Trp, Thr

Ketogenic AAs Leu, Lys

Section 4

Metabolism of Ammonia

1. Sources:

⑴ Endogenous sources:

① Deamination of AAs--main source

② Catabolism of other nitrogen containing compounds

③ Kidney secretion (Gln)

RCH2NH2 RCOH + NH3amine oxidase

§ 4.1 Source and outlet of ammonia (NH3)

CONH2

(CH2)2

CHNH2

COOH

+ H2OGlutaminase

COOH

(CH2)2

CHNH2

COOH

+ NH3

Gln Glu

⑵ Exogenous sources ：

① Putrefaction in the intestine.

② Degradation of urea in the intestine

2. Outlets:

(1) Formation of urea

(2) Formation of Gln

(3) Excrete in urine

(4) Synthesis of AA

§ 4. 2 Transportation of NH3

1. Alanine-glucose cycle

2. Transportation of ammonia by Gln

1. Alanine-glucose cycle

protein

amino acid

NH3

pyruvate

¦Á-keto acid

G

muscle

pyruvate

G

NAD+ + H2O

NADH + H+

blood liver

urea

Glu

Ala Ala Ala

Glu

G

¦Á-keto acid

+ NH3

2. Transportation of ammonia by Gln

CONH2

(CH2)2

CHNH2

COOH

Gln synthetaseCOOH

(CH2)2

CHNH2

COOH

+ NH3

ATP ADP + Pi

Glu GlnGlutaminase

H2O

§ 4. 3 Formation of urea

1. Site: liver (mitochondria and cytosol)

2. Process --------- ornithine cycle

ornithine NH3 + CO2

H2O

NH3H2O

H2O

urea

arginase

Arg citrulline

① Formation of carbamoyl phosphate

(in mitochondria)

Carbamoyl phosphate synthetase (CPS ) is an allosteric enzyme anⅠ Ⅰ

d is absolutely dependent up on N-acetylglutamic acid (AGA) for its activity.

H2N-C-O~PO3H2

O2ATP 2ADP+Pi

NH3 + CO2 + H2O

carbamoyl phosphate

CPS I

② Formation of citrulline

(in mitochondria)

OCT: ornithine carbamoyl transferase

H2N-C-O~PO3H2

O

carbamoyl phosphate

+

NH2

£¨CH £©2 3

CHNH2

COOHornithine

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C OPi

citrulline

OCT

③ Formation of arginine (in cytosol)

two sub-steps

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C O

citrulline

+

COOH

H2-N-C-H

CH2

COOH

ATP AMP+PPi

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C

COOH

N-C-H

CH2

COOH

arginino succinate

Asp

ASS

ASS: argininosuccinate synthetase

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C

COOH

N-C-H

CH2

COOH

arginino succinate

ASL

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C

COOH

CH

HC

COOH

NH

+

fumarate

Arg

ASL: argininosuccinate lyase

④ Formation of urea (in cytosol)

NH2

£¨CH £©2 3

CHNH2

COOHornithine

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C NHH2O

arginase+

NH2

NH2

C O

urea

Arg

Total formula ：

3. Summary of urea synthesis

• One nitrogen of urea molecule comes from ammonia, another nitrogen comes from Asp.

• Synthesis of a urea will consume 4 ~P.

• Rate limiting enzyme: ASS

Section 5 Metabolism of Specific Amino Acid

• Decarboxylation of amino acids

• Metabolism of one carbon unit

• Metabolism of sulfur-containing AAs

• Metabolism of aromatic AAs

• Metabolism of branched-chain AAs

§ 5.1 Decarboxylation of amino acids

1. Glu→γ-aminobutyric acid (GABA)

CO2COOH

CH2

L-Glu

L-glu decarboxylase

GABA

CH2

CH2NH2

COOH

CH2

CH2

CHNH2

COOH

2. Cys→taurine

CH2SH

CHNH2

COOH

L-Cys

CH2SO3H

CHNH2

COOH

sulfoalanine

CO2

CH2SO3H

CHNH2

taurine

sulfoalanine decarboxylase

3[O]

3. His→histamine

NHN

CH2CHCOOH

NH2

L-His decarboxylase

L-His

NHN

CH2CH2NH2

histamine

CO2

4. Trp→5-hydroxytryptamine (5-HT)

(serotonin)

NH

CH2 CH COOH

NH2 NH

CH2 CH COOH

NH2

HO

Trp 5'-hydroxytryptophan

decarboxylaseCO2

NH

CH2 CH2 NH2HO

5'-hydroxytryptamine

Tryptophanhydroxylase

5. Polyamines

COOH

CH

(CH2)3

NH2

NH2

Ornithine

CO2NH2

(CH2)4

NH2

putrescine

S

(CH2)3

NH2

adenosine

CH3

S

adenosine

CH3

NH

(CH2)4

NH2

NH2

(CH2)3

S

(CH2)3

NH2

adenosine

CH3

S

adenosine

CH3 NH

(CH2)4

HN

NH2

(CH2)3

(CH2)3

NH2spermidine

spermine

SAM

CO2

§ 5.2 Metabolism of one carbon unit

1. One carbon unit

One carbon units (or groups) are one carbon-containing groups produced in catabolism of some amino acids. They are

CH3 CH2 CH CHO CH NH

methyl methylene methenyl formyl formimino

2. Tetrahydrofolic acid (FH4)

One carbon units are carried by FH4. The N5 and N10 of FH4 participate in the transfer of one carbon units.

NH

HNN

N

H2N

CH2 HN

12

34 5

6

78

9 10

OH

CO NHHC

COOH

CH2 CH2 COOH

N N5 10

CH2

N5-CH3FH4 N5, -CH2-FH4N10

N NH

5 10

CH3

N N5 10

CH

N5, =CH-FH4N10

NH

N5 10

CHO

-CHOFH4N10

N NH

5 10

CH=NH

-CH=NHFH4N5

3. Formation of one carbon unit

(1) Ser→N5,N10-CH2-FH4

CH2

CHNH2

COOH+ FH4

Ser hydroxymethyl transferase

H2O

N5, N10-CH2-FH4 +CH2NH2

COOH

Ser Gly

(2) Gly→N5,N10-CH2-FH4

+ FH4Gly lyase

N5, N10-CH2-FH4 + CO2

CH2NH2

COOH + NH3

NAD+NADH+H+

Gly

(3) His →N5-CH=NHFH4

NHN

CH2CHNH2COOHNH3

NHN

CH=CHCOOH

2H2O

NHNCH=CHCOOHHOOC-CH

FH4

N5-CH=NHFH4

subaminomethyl transferase

CHNH2

COOH

£¨CH £©22

COOHsubaminomethyl Glu

His

Glu

(4) Trp→N10-CHOFH4

N

CH2CHNH2COOH

H

O2

NHCHO

CCHNH2COOH

O

N-formyl kynurenine

H2O

NH2

CCHNH2COOH

O

HCOOH

kynurenine

N10 -CHOFH4 synthetase

FH2+ATPADP+Pi

N10 -CHOFH4

Trp

4. One carbon unit exchange

H2O

CH2 FH4

N5 CH=NHFH4 N10 CHOFH4CH FH4

NH3

NH3

H2O

NADPH+H+

NAPD+

NADH+H+

NAD+

N5 CH3 FH4

N5,N10

N5,N10

5. Significance of one carbon unit

Substance for synthesis of nucleic acid.

N10 － CHOFH4

N5,N10 － CH2 － FH4

§ 5.3 Metabolism of sulfur-containing AAs

Methionine, cysteine and cystine.

1. Metabolism of Met

Transmethylation and Met cycle

N

N N

N

O

OHOH

CH2S

CH2

CH3

CH2

CH

COO

H3NNH2

S-Adenosylmethionine (SAM)

Significance

(1) SAM is the direct donor of methyl in body. Methylation can synthesize many important materials such as: choline, creatine, etc.

(2) N5-CH3FH4 is the indirect donor of methyl in the body.

(3) The free folic acid or VitB12

decrease will cause the decrease of DNA, which will lead to anemia.

Formation of creatine

N

CH2

COOH

CH3

CNH2

HN

SAM

Arg

Gly

2. Metabolism of cysteine and cystine

NH2CH

CH2

SH

COOH

cysteine

NH2CH

CH2

SH

COOH

cysteine

+NH2CH

CH2

S

COOH

NH2CH

CH2

S

COOH

2H

2H

cystine

Formation of PAPS

SO42-

ATPPPi

adenosine-5'-phosphosulfate (AMPS)

ATPADP

3'-phospho- adenosine-5'-phosphosulfate (PAPS)

SH

CH2

CH

COOH

NH2

Cys

H2S

[O]

NH3pyruvate

• PAPS is the active sulfate group for a

ddition to biomolecules.

N

N N

N

O

OHH2O3PO

CH2O

NH2

3'-phosphoadenosine- 5'-phosphosulfate (PAPS)

PO

O

OH

O3S

§ 5. 4 Metabolism of aromatic amino acids

• Phe, Tyr, Trp

1. Phe

CH2CHNH2COOH

+ O2

CH2CHNH2COOH

+

OH

H2O

tetrahydro- biopterin

dihydro- biopterin

Phe hydroxylase

NADPH+H+NADP+

PheTyr

N

N

N

N

CH-CH-CH3

OH

H2N

H

H

OH OH

5

78

6

1

3

N

N

N

N

CH-CH-CH3

OH

HNH

OH OH

5

78

6

1

3

Tetrahydrobiopterin Dihydrobiopterin

CH2 CH COOHNH2

CH2 C COOHOGlu ¦Á-keto-

glutarate phenyl pyruvatePhe

transaminase

• Phe hydroxylase ↓→phenyl pyruvate in the body ↑ → phenylketonuria(PKU) → toxicity of central nervous system →developmental block of intelligence of children

• Treatment: control the input of Phe

2. Tyr

Catecholamines: Dopamine, norepinephrine, epinephrine

Melanin

Tyrosinase decrease will lead to albinism.

CH2CHNH2COOH

OH

CH2CHNH2COOH

OH

HO

CO2CH2CH2NH2

OH

HO

CH2CH2NH2

OH

HO

OH

CH2CH2NHCH3

OH

HO

OH

CH2CHNH2COOH

O

O

OO

NH

CH2CCOOH

OH

O

OH

OH

CH2COOH

dopa dopamine

dopa quinone norepine-

phrine

indole-5,6- quinone

fumarate +acetoacetate

Tyr

epinephrine

SAM

Tyr transaminase

melanin

hydroxy-phenyl-pyruvate

homogentisate

3. Trp

• 5-HT

• One carbon unit

• Nicotinic acid

• Pyruvate and Acetoacetyl CoA

§ 5.5 Metabolism of branched-chain AAs

• Leu, Ile, Val

• They are all essential AAs.

transamination

keto acid¦Á-

decarboxylation

-NH2

CO2

oxidation

enoyl-CoA

succinayl CoA succinayl CoA and acetyl CoA

Val Leu Ile

acyl-CoA

formation of

acetyl CoA and acetoacetyl CoA

Summary of metabolism

The sources and fates of acetyl CoA

TAC

acetyl CoA

glucose fatty acid glycerol

fatty acid

ketone bodies

glucogen TG protein

AAs

cholesterol

The sources and fates of oxaloacetic acid

PEP

pyruvate malate

citratecitrate

oxaloacetic acid

Asp