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Amino Acid Metabolism
Introduction
Role of Amino Acids
• Protein monomeric units• Energy source• Precursors of other
biological molecules
Protein Monomeric Units
+H3N C
R1
H
COO– +H3N C
R2
H
COO–
H2O
+H3N C
R1
H
C NH C COO–
R2O
H
+
Energy Source
Amino Acids Waste Products
ADP + Pi ATP
Precursors(Nitrogen-containing Compounds)
• Heme• Nucleotides• Amines• Nucleotide
Coenzymes• Glutathione
Precursors(-ketoacids)
Amino Acids
Pyruvate
Oxaloacetate
-Ketoglutarate
GlucoseFatty AcidsKetone Bodies
Classification(Mammals)
• Essential amino acids• Non-essential amino
acids
Amino Acid Deamination(First Reaction in Amino Acid
Breakdown)
+H3N C
R
COO–
H
R C
O
COO–
OAAH2N
CNH2
O
-keto acid
Glutamate
Aspartate
Fumarate
UreaaKG
KG
Nitrogen-containing compounds
UreaCycle
Aminotransferases(Transaminases)
Aminotransferases(Transaminases)
Amino Acid + -Ketoglutarate -Keto Acid + Glutamate
Glutamate + Oxaloacetate -Ketoglutarate + Aspartate
Oxidative Deamination
Glutamate + NAD(P)+ + H2O -Ketoglutarate + NH4+ + NAD(P)H + H+
GlutamateDehydrogenase
Amino Acid Oxidase
Amino Acid + FAD + H2O -Keto Acid + NH3 + FADH2
FADH2 + O2 FAD + H2O2
Transamination(Reactions)
Amino Acid + Enzyme - + –Keto Acid Enzyme NH2
–Enzyme NH2
+ -Ketoglutarate + Enzyme Glutamate
Transamination(Enzyme Cofactor)
N CH3
H
H2
C
C
H O
OH
Pyridoxal-5'-P
(PLP)
PO
N CH3
H
H2
C
H2
C
OHHO
OH
Pyridoxine
(Vitamin B6
)
N CH3
H
H2
C
H2
C
OHPO
NH2
Pyridoxamine-5'-P
(PMP)
Transamination
Enzyme Mechanism
(1) Transimination
N CH3
H
P
C
H N
H
O
–
(CH2
)4
Enzyme–B:
R C
H
NH2
COO
–
+
N CH3
H
P
C
H N
H
O
–
R C
H
COO
–
– :Enzyme B
- Amino Acid
-Enzyme PLP
Schiff Base
- Amino Acid PLP Schiff Base
( )aldimine
Lys
(2) Tautomerization
N CH3
H
P
C
H N
H
O
–
R C
H
COO
–
– :Enzyme BLys
N CH3
H
P
C
H N
H
O
–
R C
COO
–
–Enzyme BH
+
Lys
Resonance-stabilized Intermediate
N CH3
H
P
C
H N
H
O
–
R C
COO
–
–Enzyme BH
+
Lys
N CH3
H
P
C
H N
H
O
–
R C
COO
–
–Enzyme BH
+
Lys
Resonance-stabilized Intermediate
N CH3
H
P
C
H N
H
O
–
R C
COO
–
–Enzyme BH
+
Lys
N CH3
H
P
C
H N
H
O
–
R C
COO
–
– :Enzyme BLys
H
Ketimine
(3) Hydrolysis
N CH3
H
P
C
H N
H
O
–
R C
COO–
– :Enzyme BLys
H
N CH3
H
P
C
O–
H H
NH
C
R COO–
O
H
– :Enzyme BLys
OH–
N CH3
H
P
C
O–
H H
NH2
R C
O
COO–
– :Enzyme BLys
Pyridoxamine Phosphate
( ) PMP Enzyme
- Keto Acid
(4) Transfer of Amine
PMP-Enzyme + -ketoglutarate - + PLP Enzyme Glutamate
Resonance Stabilization
N CH3
H
P
C
H N
H
O
–
R C
H
COO
–
Resonance Stabilization
• Transaminaton• Decarboxylation• Removal of R
group
Summary
Amino Acid + -Ketoglutarate - + Keto Acid Glutamate
Degradative Fates of GlutamateRegeneration of -Ketoglutarate
OAA
NH4+
H2NC
NH2
O
CO2
Glutamate
Aspartate
Fumarate
UreaKG
Nitrogen-containing compounds
UreaCycle
-ketoacid
Amino Acid
-KG
Carbon and Energy
Glutamate-Aspartate Aminotransferase
Glutamate + Oxaloacetate - + Ketoglutarate Aspartate
Glutamate Dehydrogenase(Oxidative Deamination)
C COO
–
CH2
H2
C
+
H3
N
H
COO
–
Glutamate
C COO
–
CH2
H2
C
H2
N
COO
–
NAD(P)
+
NAD(P)H + H
+
C COO
–
CH2
H2
C
O
COO
–
H2
O NH4
+
-Ketoglutarate -Iminoglutarate
Formation of Urea
Aspartate + HCO3- + NH4
+ UreaUrea Cycle
Degradative Fates of GlutamateRegeneration of -Ketoglutarate
OAA
NH4+
H2NC
NH2
O
CO2
Glutamate
Aspartate
Fumarate
UreaKG
Nitrogen-containing compounds
UreaCycle
-ketoacid
Amino Acid
-KG
Carbon and Energy
Urea Cycle
Urea Cycle(Introduction)
Catabolism
(Oxidation)
ADP
ATP
NADP+
NADPH
Intermediates
Anabolism
(Biosynthesis)
Proteins
Fats
Carbohydrates
(Nutrients)
Waste
(CO2
/Urea/etc.)
Nitrogen Waste Products
NH3
H2
N
C
NH2
O
N
NN
N
O
O
H
O
Ammonia Urea
(Vertebrates)
Uric Acid
(Birds and Reptiles)
Require less water for excretion
Classification of Organisms(Nitrogen Excretion Patterns)
• Ammonotelic: ammonia excreting
• Ureotelic: urea excreting• Uricotelic: uric acid excreting
Overall Urea Cycle(Liver)
NH3
+ HCO3
–
+
H2
N
C
NH2
O
H3
N C COO
–
H2
C
H
COO
–
3ATP 2ADP + 2Pi
+ AMP + PPi
+
HC COO
–
CH
–
OOC
AspartateUrea Fumarate
+
Glutamate Dehydrogenase(Generation of NH3)
Glutamate + NAD(P)
+
+ H2
O - + Ketoglutarate NH3
+ ( ) + NAD P H H
+
Carbamyl Phosphate Synthetase (CPS)
(Mitochondrion)
H2N C
O
OP
Carbamyl Phosphate
2ATP 2ADP + Pi
NH3 + HCO3–
Carbamyl Phosphate Synthetase (CPS)
• CPSI (Mitochondria)– Uses NH3
– Urea Cycle
• CPSII (Cytosol)– Uses Glutamine– Pyrimidine Biosynthesis
Carbamyl Phosphate Synthetase (CPS)
Mechanism
Activation of HCO3– by ATP
HO C
O
O
–
Bicarbonate
(CO2
)
+
–
O P
O
O
–
O ADP
ADP
HO C
O
O P
Carbonyl-P
Displacement of Phosphate by NH3
HO C
O
O P + NH3
Pi
H2
N C
O
O
–
Carbamate
Carbonyl-P
Phosphorylation of Carbamate by ATP
H2N C
O
O–
ATP ADP
H2N C
O
OP
Carbamyl PhosphateCarbamate
Ornithine Transcarbamylase (OTC)
(Mitochondrion)
H2
N C
O
O P +
+
H3
N C COO
–
H
(CH2
)3
NH3
+
Ornithine
Pi
+
H3
N C COO
–
H
(CH2
)3
NH
C
O NH2
Citrulline
Carbamoyl P
Non-standard amino acids
(not present in proteins)
Citrulline Carrier
(Ornithine Antiport)
Argininosuccinate Synthetase(Cytosol)
+
H3
N C COO
–
H
(CH2
)3
NH
C
O NH2
Citrulline
+ C
+
H3
N COO
–
H
H2
C COO
–
Aspartate
ATP AMP + PPi
+
H3
N C COO
–
H
(CH2
)3
NH
C
H2
N
H
N C
H
COO
–
H2
C COO
–
Argininosuccinate
Mechanism
+
H3
N C COO
–
H
(CH2
)3
NH
C
O NH2
Citrulline
CH2
N COO
–
H
H2
C COO
–
Aspartate
+
H3
N C COO
–
H
(CH2
)3
NH
C
H2
N
H
N C
H
COO
–
H2
C COO
–
Argininosuccinate
AMP P P
PPi
+
H3
N C COO
–
H
(CH2
)3
NH
C
O NH2
AMP
Citrullyl-AMP
AMP
+
Activation of oxygen atom as a leaving group
Argininosuccinase(Cytosol)
+
H3
N C COO
–
H
(CH2
)3
NH
C
H2
N
H
N C
H
COO
–
H2
C COO
–
Argininosuccinate
+
H3
N C COO
–
H
(CH2
)3
NH
C
H2
N NH2
Arginine
Fumarate
C C
COO
–
H
H
–
OOC
Arginase(Cytosol)
+
H3
N C COO
–
H
(CH2
)3
NH
C
H2
N NH2
Arginine
+
H3
N C COO
–
H
(CH2
)3
NH3
+
Ornithine
H2
N
C
NH2
O
UreaH
2O
Ornithine Transport
(Citrulline Antiport)
Summary
NH3
+ HCO3
–
+
H2
N
C
NH2
O
H3
N C COO
–
H2
C
H
COO
–
3ATP 2ADP + 2Pi
+ AMP + PPi
+
HC COO
–
CH
–
OOC
AspartateUrea Fumarate
+
Recovery of ATP
Glutamate Dehydrogenase
Glutamate + NAD
+
+ H2
O - + Ketoglutarate NH3
+ + NADH H
+
Regeneration of Aspartate(Cytosol)
Fumarate Malate OxaloacetateAspartate
NAD
+
NADH + H
+
Fumarase Malate
Dehydrogenase
Glu-Asp
Aminotransferase
Oxidation of 2 NADH Yields 6 ATP
Regulation of the Urea Cycle
Carbamyl Phosphate Synthetase I (CPSI)
(Rate-limiting Reaction)
Activator
H3
C C
O
N
H
C
H
COO
–
CH2
H2
C COO
–
N -Acetylglutamate
Formation of N-acetylglutamate
H3
C C
O
N
H
C
H
COO
–
CH2
H2
C COO
–
N -Acetylglutamate
+
H3
N C COO
–
CH2
H2
C COO
–
G lutamate
H
H3
C C
O
CoA CoA
H2
O
H3
C C
O
O
–
N-Acetylglutamate
Synthase
Specific
Hydrolase
Deficiencies of the Urea Cycle
Metabolic Breakdown of Individual Amino
Acids
toCitric Acid Cycle Intermediates
ortheir Precursors
Products of Amino Acid Breakdown
• Glucogenic– Pyruvate– -Ketoglutarate– Succinyl-CoA– Fumarate– Oxaloacetate
• Ketogenic – Acetyl-CoA– Acetoacetate
Animals cannot carryout net synthesis of precursors of
gluconeogenesis from acetyl-CoA or acetoacetate
Conversion of Pyruvate and Oxaloacetate to PEP
(Gluconeogenesis)
COOH
CH2
CO COOH
H3C C COOH
O
HCO3–
(CO2)
ATP
H2C C COO–
OP
COOH
CH2
CO COOH
Oxaloacetate
ADP + Pi
+
PyruvatePyruvate
Carboxylase
OxaloacetatePhosphoenolpyruvate
(PEP)
GlucosePEP
Carboxykinase"Gluconeogenesis"
GTP GDP + CO2
Degradation to Pyruvate
Alanine, Cysteine, Glycine, Serine and Threonine
Cysteine
(Various Routes)
Alanine
+
H3
N C
H
C H3
C O O
–
H3
C C C O O
–
O
K G G l u
A l a n i n e
P y r u v a t e
A l a n i n e A m i n o t r a n s f e r a s e
Glycine, Threonine and Serine
N C H3
H
P
C
H N
H
O
–
C C O O
–
H
T h r e o n i n e
G l y c i n e
S e r i n e
" C H2
" + H2
O P L P
Serine ——> Pyruvate
