chapter 7 amino acid metabolism the biochemistry and molecular biology department of cmu
TRANSCRIPT
Chapter 7
Amino Acid Metabolism
The biochemistry and molecular biology department of CMU
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