NH3
H
C
COO
R
Chapter 7
Amino Acid Metabolism
The biochemistry and molecular
biology department of CMU
Section 1
Nutritional Function of
Proteins
§ 1.1 The significance 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
§ 1.2 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
trypsin: Arg, Lys (C)
endopeptidases
chymotrypsin: Tyr, Trp,
Phe, Met, Leu (C)
elastase: Ala, Gly, Ser (C)
exopeptidases
carboxypeptidase
aminopeptidase
enterokinase
trypsinogen
chymotrypsinogen
proelastase
procarboxypeptidase
trypsin
chymotrypsin
elastase
carboxypeptidase
amino peptidase endopeptidase
carboxy peptidase
O
O
O
H2N-CH-C-NH-CH--- NH-CH-C-NH-C--- NH-CH-C-NH-CH-COOH
R1
Rn
R
Rn-1
R2
polypeptide
dipeptidase
O
amino acid + H2N-CH-C-NH-CH-COOH
R
R
amino acid
dipeptide
§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
CO2
R CH COOH
NH2
amino acid
bacteria
R CH2 NH2
amine
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
Dietary ab
so
proteins
rp
tio
n
NH3
Ketone bodies
ion
t
a
in
m
a
de
¦Á-Keto acid
Oxidation
Tissue degradation Amino acid
proteins synthesis metabolic pool
Amino acids
synthesized
Urea
dec
arb
oxy
lati
conversion
on
Non- protein nitrogen
compounds
CO2
Glucose
Amine
§ 3.2 Degradation of protein
in cells
1. Lysosomal pathway
• Extracellular proteins, membraneassociated 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
NH3
H C COO
R1
O
+ C COO
R2
amino acid-1 keto acid-1
Amino
transferases
O
C COO
R1
keto acid-2
NH3
+ H C COO
R2
amino acid-2
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 αketoglutarate.
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
α-keto
acid
pyridoxal
phosphate
pyridoxamine
phosphate
Schiff base
Isomer of Schiff base
Two important transaminases:
ALT: Alanine aminotransferase (in liver)
AST: Aspartate aminotransferase (in heart)
pyruvate
glutamate
ALT
alanine
oxaloacetate
AST
a-ketoglutarate
aspartate
(2) Oxidative deamination
COOH
NADH+H+ COOH
C NH
CHNH2
NAD+
(CH2)2
(CH2)2
L-Glu
Dehydrogenase COOH
COOH
L-Glu
H2O
NH3
COOH
C O
(CH2)2
COOH
¦Á-ketoglutarate
(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.
COOH
R-CH-COOH
NH2
¦Á-amino acid
transaminase
CH2
2
NADH + H+ + NH3
C O
COOH
¦Á-ketoglutarate
L-glutamate dehydrogenase
COOH
R-C-COOH
O
¦Á-keto acid
CH2
2
CHNH2
COOH
Glu
NAD+ + H2O
Alanine + α-ketoglutarate
Glutamate + NAD+ + H2O
Net Reaction:
Alanine + NAD+ + H2O
Pyruvate + glutamate
α-ketoglutarate + NADH
+ NH4+
pyruvate + NADH + NH4+
(3) Purine nucleotide cycle (in muscle)
amino
acid
transaminase
¦Á- keto
acid
O
adenylosuccinate
COOH
N
synthetase
HN
HOOCCH2CHCOOH
(CH2)2
NH3
N
N
NH2
CO
R-5'-P
Asp
HOOCCH2CHCOOH
COOH
IMP
AMP
H2O
NH
¦Á- ketoAST
deaminase
glutarate
N
N
CH2COOH
NH2
COOH
N
N
COCOOH
N
(CH2)2
N
R-5'-P
oxaloacetate
adenylosuccinate
CHNH2
N
N
COOH
'
R-5
-P
CHCOOH
L-Glu
CH2COOH
adenyloAMP
succinase
CHCOOH
CHOHCOOH
fumarate
malate
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
Intermediates of TAC
Fumarate
Oxaloacetate
PEP
Glucose
Pyruvate
Acetyl CoA
Acetoacetyl CoA
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
Glucogenic AAs
Glucogenic and
ketogenic AAs
Ketogenic AAs
amino acids
others
Ile, Phe, Tyr, Trp,
Thr
Leu, Lys
Section 4
Metabolism of Ammonia
§ 4.1 Source and outlet of
ammonia (NH3)
1. Sources:
⑴ Endogenous sources:
① Deamination of AAs--main source
② Catabolism of other nitrogen
containing compounds
RCH2NH2
amine oxidase
RCOH + NH3
③ Kidney secretion (Gln)
COOH
CONH2
(CH2)2
Glutaminase
(CH2)2
CHNH2 + H2O
CHNH2 + NH3
COOH
COOH
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
muscle
liver
blood
amino acid
NH3
Glu
¦Á-keto
acid
G
G
pyruvate
pyruvate
Ala
G
Ala
Ala
Glu
NAD+ + H2O
¦Á-keto
acid
NADH + H+
+ NH3
urea
2. Transportation of ammonia by Gln
ATP
Gln synthetase
COOH
(CH2)2
CHNH2
ADP + Pi
(CH2)2
+ NH3
CHNH2
COOH
Glu
CONH2
COOH
Glutaminase
H2O
Gln
§ 4. 3 Formation of urea
1. Site: liver (mitochondria and cytosol)
2. Process --------- ornithine cycle
urea
ornithine
NH3 + CO2
arginase
H2O
H2O
Arg
citrulline
H2O
NH3
① Formation of carbamoyl phosphate
(in mitochondria)
2ATP
NH3 + CO2 + H2O
2ADP+Pi
CPS I
O
H2N-C-O~PO3H2
carbamoyl phosphate
Carbamoyl phosphate synthetase Ⅰ
(CPSⅠ) is an allosteric enzyme and
is absolutely dependent up on Nacetylglutamic acid (AGA) for its
activity.
② Formation of citrulline
(in mitochondria)
NH2
NH2
£¨ CH 2£©
3
CHNH2
O
+ H2N-C-O~PO3H2
COOH
ornithine
carbamoyl
phosphate
Pi
OCT
C O
NH
£¨ CH 2£©
3
CHNH2
COOH
citrulline
OCT: ornithine carbamoyl transferase
③ Formation of arginine (in cytosol)
two sub-steps
NH2
C O
NH
£¨ CH 2£©
3 +
CHNH2
COOH
citrulline
NH2
COOH
H2-N-C-H
CH2
COOH
Asp
ATP
AMP+PPi
ASS
COOH
N-C-H
CH2
NH
£¨ CH £©COOH
C
23
CHNH2
COOH
arginino succinate
ASS: argininosuccinate synthetase
NH2
COOH
NH2
N-C-H
C
CH2
COOH
£¨ CH £©
3
NH
2
CHNH2
COOH
arginino succinate
ASL
COOH
CH
C NH
NH
+ HC
£¨ CH 2£©
COOH
3
fumarate
CHNH2
COOH
Arg
ASL: argininosuccinate lyase
④ Formation of urea (in cytosol)
NH2
C
NH
NH
£¨ CH2£©
3
CHNH2
COOH
Arg
NH2
H2O
arginase
£¨ CH2£©
3
NH2
+ C O
CHNH2
COOH
ornithine
NH2
urea
Total formula：
2NH3 + CO2 + 3ATP + 2H2O
urea + 2ADP + AMP + 2Pi + PPi
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
RCHCOOH
NH2
amino
acid
NH3+H2O2
CO2
decarboxylase
(Vit B6)
H2O+O2
RCH2NH2
amine
1/2O2
RCHO
amine oxidase
RCOOH
organic
acid
1. Glu→γ-aminobutyric acid
(GABA)
COOH
CH2
CH2
CHNH2
CO2
L-glu
decarboxylase
COOH
CH2
CH2
CH2NH2
COOH
L-Glu
GABA
2. Cys→taurine
CH2SH
3[O]
CH2SO3H
CHNH2
CHNH2
COOH
COOH
L-Cys
sulfoalanine
CO2
CH2SO3H
sulfoalanine
decarboxylase
CHNH2
taurine
3. His→histamine
CH2CHCOOH
HN
N
L-His
NH2
CO2
HN
L-His
decarboxylase
CH2CH2NH2
N
histamine
4. Trp→5-hydroxytryptamine (5-HT)
(serotonin)
Tryptophan HO
CH2 CH COOH hydroxylase
N
H
CH2 CH COOH
NH 2
NH 2
N
H
5'-hydroxytryptophan
Trp
decarboxylase
HO
CO2
CH2 CH2 NH 2
N
H
5'-hydroxytryptamine
5. Polyamines
SAM
adenosine
S CH3
COOH
CH NH2
(CH2)3
CO2
NH2
NH2
CO2
adenosine
S CH3
adenosine NH
2
(CH2)3
S CH3
(CH2)3 NH
2
NH
NH2
adenosine(CH )
2 3
S CH3
NH
(CH2)4
(CH2)3
(CH2)4
NH2
NH2
(CH2)4
putrescine
NH2
(CH2)3
spermidine
NH2
spermine
Ornithine
HN
§ 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
methyl
CH2
methylene
CH
methenyl
CHO
formyl
CH
NH
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.
H2N
3
2
1
N
N
4
OH
H
8N
5N
H
7
9
6 CH2
10
HN
H
CO NH C CH2 CH2 COOH
COOH
5
5
10
N
N
N
H
CH3
N5-CH3FH4
5
N
H
5
10
10
N
N
N
CH2
5
CH
10
N , N -CH2-FH4
10
N5, N10 =CH-FH4
5
N
N
CHO
CH=NH
N10-CHOFH4
10
N
H
N5-CH=NHFH4
3. Formation of one carbon unit
(1) Ser→N5,N10-CH2-FH4
CH2
H2O
CH2NH2
5
10
CHNH2 + FH4
+
N , N -CH2-FH4
Ser
COOH
COOH
hydroxymethyl
Ser
Gly
transferase
(2) Gly→N5,N10-CH2-FH4
NADH+H +
NAD +
CH2NH2
COOH + FH4
Gly lyase
Gly
N5, N10-CH2-FH4 + CO2 + NH3
(3) His →N5-CH=NHFH4
NH 3
CH 2CHNH 2COOH
HN
N
CH=CHCOOH
HN
N
His
2H 2O
COOH
CHNH 2
£¨ CH2£©
2
COOH
Glu
N5-CH=NHFH4
FH4
subaminomethyl
transferase
HOOC-CH
HN
CH=CHCOOH
N
subaminomethyl Glu
(4) Trp→N10-CHOFH4
CH2CHNH2COOH
O
O2
CCHNH2COOH
N
H
Trp
NHCHO
N-formyl kynurenine
H2O
ADP+Pi
N10 -CHOFH4
FH2+ATP
N10-CHOFH4
synthetase
HCOOH
O
CCHNH2COOH
NH2
kynurenine
4. One carbon unit exchange
H2O
NH3
N5,N10
N5 CH=NHFH4
CH FH4
NADPH+H+ H2O
NH3
NAPD+
N5,N10
CH2 FH4
NADH+H+
NAD+
N5 CH3 FH4
N10 CHOFH4
5. Significance of one carbon unit
Substance for synthesis of nucleic acid.
N10－CHOFH4
N5,N10－CH2－FH4
§ 5.3 Metabolism of sulfurcontaining AAs
Methionine, cysteine and cystine.
1. Metabolism of Met
Transmethylation and Met cycle
PPi+Pi
S CH3
£¨CH2£©
2
Met
CH NH2
COOH
ATP
A S CH3
£¨CH2£©
2
CH NH2
adenosyl
transferase
COOH
RH
methyl transferase
FH4
N5 -CH3FH4
Met synthase
£¨ VB12£©
SH
£¨CH2£©
2
CH NH2
COOH
homocysteine
A
SAM
RCH3
H2O
A
SH
£¨CH2£©
2
CH NH2
COOH
S-adenosyl
homocysteine
COO
H3N
NH2
CH
N
CH2 N
CH2
S
N
CH2 O
N
CH3
OH
OH
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
Arg
NH2
HN
C
N
Gly
CH3 SAM
CH2
COOH
2. Metabolism of cysteine and
cystine
SH
CH 2
CH NH 2
COOH
cysteine
+
SH
CH 2
2H
CH NH 2
COOH
cysteine
2H
S
CH 2
S
CH 2
CH NH 2 CH NH 2
COOH COOH
cystine
Formation of PAPS
SH
pyruvate NH3
CH2
CH
H2S
NH2
COOH
Cys
[O]
ATP
SO42-
PPi
ATP
adenosine-5'phosphosulfate
(AMPS)
ADP
3'-phosphoadenosine5'-phosphosulfate
(PAPS)
• PAPS is the active sulfate group for
addition to biomolecules.
NH2
N
O
O3S O P O
N
CH2 O
N
N
OH
H2O3PO
OH
3'-phosphoadenosine- 5'-phosphosulfate
(PAPS)
§ 5. 4 Metabolism of aromatic
amino acids
• Phe, Tyr, Trp
NADP+
1. Phe
tetrahydroCH2CHNH2COOH biopterin
+
NADPH+H+
dihydrobiopterin
CH2CHNH2COOH
+
O2
Phe hydroxylase
Phe
OH
Tyr
H2O
OH
N
H
N
5
3
H2N
OH
1
N
CH-CH-CH3
OH OH
HN
8 7
N
H
6
Tetrahydrobiopterin
N
N
5
3
1
N
CH-CH-CH3
OH OH
7
8
N
H
6
Dihydrobiopterin
transaminase
CH2 CH COOH
NH2
¦Á-ketoPhe
glutarate
Glu
CH2 C COOH
O
phenyl pyruvate
• 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.
CH2CHNH 2COOH
CH2CHNH 2COOH
CO2
Tyr
HO
OH
OH
Tyr transaminase
O
CH2CCOOH
OH
hydroxyphenylpyruvate
dopa
CH2CH2NH 2
HO
OH
CH2CHNH 2COOH
dopa
quinone
O
O
dopamine
OH
CH2CH2NH 2
norepinephrine
HO
OH
SAM
OH
NH
CH2COOH
OH homogentisate
O
O
OH
CH2CH2NHCH 3
indole-5,6quinone
HO
fumarate + acetoacetate
melanin
epinephrine
OH
3. Trp
• 5-HT
• One carbon unit
• Nicotinic acid
• Pyruvate and Acetoacetyl CoA
§ 5.5 Metabolism of branchedchain AAs
• Leu, Ile, Val
• They are all essential AAs.
Leu
Val
-NH2
Ile
transamination
formation of ¦Á- keto acid
CO2
decarboxylation
acyl-CoA
oxidation
enoyl-CoA
succinayl CoA
acetyl CoA and
acetoacetyl CoA
succinayl CoA
and acetyl CoA
Summary of metabolism
The sources and fates of acetyl CoA
glucogen
TG
glucose fatty acid glycerol
protein
AAs
acetyl CoA
fatty
acid
cholesterol ketone TAC
bodies
The sources and fates of
oxaloacetic acid
pyruvate
malate
Asp
oxaloacetic
acid
PEP
citrate
citrate