Download Amino Acid Catabolism: C

Amino Acid Catabolism II:
Fate of Carbon Skeleton
Long-term protein overfeeding accelerates the status
insulin resistance
Extra glucose in the fed state
protein
rich
Quantitative aspects of amino acids catabolism
1. Amino acids only undergo partial oxidation
in the liver
2. Partial oxidation - ATP in the fed state
3. Hepatic gluconeogenesis
4.Urea synthesis and gluconeogenesis from
dietary amino acids on the same pathway.
Glucogenic and ketogenic amino acids
each endproduct can yield
a new oxaloacetate
can yield FA or ketone body
CH3
HC
COO
COO
CH2
CH2
CH2
NH3+
COO
alanine
+
C
CH3
O
COO
C
CH2
O
COO
+
HC
NH3+
COO
a-ketoglutarate
pyruvate glutamate
Aminotransferase (Transaminase)
The 3-C a-keto acid pyruvate is produced from alanine, cysteine, glycine,
serine and partly from tryptophan (indolylalanine). Glucoplastic
Alanine via transaminase directly yields pyruvate.
Major pathways of serine in humans
glutathion
creatine
hem
bile acids
O
Serine
pyruvate
O
transamination
R1
C
H2C
O
O
CH
H2C
C
R2
O
O
P
CH3
O
CH2
O
CH2
+
N CH3
CH3
phosphatidylcholine
HCO3
3 S-adenosyl-methionine
choline
ethanolamine
Serine
H2O
neurotransmitter synthesis
Serine
active C1 transfer
Glycine
glycine cleavage
HCO3+NH+4
transaminase
glyoxalate
oxalate, transaminase deffect: kidney stones
Sulfur containing amino-acids I. Role in protein structure
Methionine – mostly found in the hydrophobic core of proteins, membrane
spanning domains, if surface exposed, susceptible to oxidation,
e.g.: elastase inhibitor
- initiating protein for eukaryotic protein synthesis
Cysteine
- forms inter-intrachain disulfide bonds with other cysteine residues
Cystine
Sulfur-containing amino acid (cysteine, methionine) II. Major pathways
Met and Cys incorporate into
proteins, homocysteine and taurine
do not. Met- essential a.a.
1.
2.
re
methy
lation
path
way
4
Transmethylation pathway
(ubiquitous in all cells)
3.
5.
Transsulfuration pathway
irreversible
(limited expression: liver, kidney
intestine,pancreas)
MAT: Meth adenosyltransferase
SAHH: S-adenosylhomocysteine hydrolase
CBS:
Cystathionine  synthase
CGL:
Cystathionine  lyase
MTHFR: Methylenetetrahydrofolate reductase
MS:
Methionine synthase
BHMT: Betaine:homocysteine methyltransferase
SHMT: Serine hydroxymethyltransferase
6
antioxidant
conjugates with bile
COO
COO
COO
CH2
COO
CH2
CH2
CH2
CH2
CH2
HC
NH3+
COO
aspartate
+
C
O
C
O
+
COO
COO
a-ketoglutarate
oxaloacetate
HC
NH3+
COO
glutamate
Aminotransferase (Transaminase)
The 4-C oxaloacetate is produced from aspartate and asparagine.
Asp - other transamination reactions.
Asp - fumarate in the ornithine cycle.
Fumarate – oxaloacetate -Asp connects TCA and ornithine cycle.
The 4-C TCA cycle intermediate succinyl-CoA is produced from
isoleucine, valine, threonin (branched chain amino acids (BCCA) and
methionine.
BCCA – branched-chain amino acids: leucine, isoleucine, valine
Role in protein structure I.
Leu
Ile
Val
Why BCCA?
- Nonlinear structure - most hydrophobic amino acids – interior of globular proteins
membranous proteins, surfactants - interaction with phospholipids (lung surfactant protein B)
- All essential amino acids (~ 20% BCCA in all dietary proteins)
- Stability of folded proteins, effect the folding pathway to form mature protein, thermostability
- Function of proteins: create a non-aqueous environment, phospholipid binding, oxygen
binding in myoglobin and hemoglobin
BCCA and protein structure II.
- Coiled-coiled a-helices: fibrinogen, myosine, keratin, transcription factors
Leucine-zippers: permit formation of homodimers/heterodimers of transcription factors
The bZip family of transcription factors consist of a
basic region which interacts with the major groove of a
DNA molecule through hydrogen bonding, and a leucine
zipper region which is responsible for dimerization.
Tissue distribution of BC aminotransferase and dehydrogenase
Ile, Val
Gln synthesis
BCAA in the diet are metabolized extrahepatically, major site - muscle
Catabolism of BCAA
- BCAA metabolism escapes hepatic metabolism
- regulatory role in muscle protein
synthesis, insulin secretion, brain amino acid
uptake-leucine.
- no unique biologically active degradation
product
- catabolized in lockstep
- two common steps: BCAT, BCKDH, all 3
regulated at BCKDH, catabolism is not driven
by the need of glucose or ketone bodies.
Val
Ile
Leu
- genetic deffect of BCKDH: maple
sirup urine disease (odor of keto acids)
Ile, Val
Propionyl-CoA
Methylmalonyl-CoA
Carboxylase (Biotin)
Racemase
H
H
HCO 3
C
CH3
C
S-CoA
O
COO
H
ATP ADP
+ Pi
propionyl-CoA
C
CH3
C
S-CoA
O
H
Methylmalonyl-CoA
Mutase (B12)
H
COO
C
C
H
H
C
S-CoA
H
COO
H
C
C
CoA-S
C
H
O
D-methylmalonyl-CoA L-methylmalonyl-CoA
H
O
succinyl-CoA
Propionyl-CoA - carboxylated to methylmalonyl-CoA.
Racemase - L-isomer.
Methylmalonyl-CoA Mutase - molecular rearrangement-linear chain of succinylCoA.
Coenzyme B12 (vitamin B12+ATP, adenosylcobalamine)- cofactor of
Methylmalonyl-CoA Mutase.
COO
COO
COO
CH2
COO
CH2
CH2
CH2
CH2
CH2
HC
NH3+
COO
+
C
O
COO
C
O
COO
+
HC
NH3+
COO
aspartate a-ketoglutarate oxaloacetate glutamate
Aminotransferase (Transaminase)
The 5-C TCA Cycle intermediate a-ketoglutarate is produced
from arginine, glutamate, glutamine, histidine and prolin.
NO transport of amino acid N histamin glutamate
creatine Gla (- carboxy
semialdehyde
glutamate
glutaminic acid)vitamin K
glutamate
GABA (amino butyrate)
HC
Histidine
C
COO
histidine
histidine lyase
N
N-formiminoglutamate is
converted to glutamate by
transfer of the formimino group
to THF - N5-formimino-THF.
CH2
H
C
NH3+
NH
C
H
NH4+
H2O
urokanase
H2O
H
C

OOC
HN
CH2
CH2 COO
N-formiminoglutamate
NH
C
H
THF
N 5-formimino-THF

OOC
H
C
CH2
NH3+
CH2 COO
glutamate
C1 unit transfer in amino acid catabolism
What is folate?
THF+ derivatives
H2N
N
H
H
N
8
H
H
Tetrahydrofolate (THF)
7
H
HN
5
O
N
H
H
pteridine
6
H
H
9
CH2
COO
O
10
HN
C
-aminobenzoate
N
H
C
H
C C COO
H2 H2
glutamate
- Tetrahydrofolate (THF), a reduced form of folate.
- C1 unit transfer “active carbon” attached to N5 or N10.
- C1 units: methyl, methylene, formyl,formimino, methenyl
can transform in each other as donors, acceptors.
- C1 donated for synthesis of nucleotides, in amino acid
metabolism
C1 attached to THF
Interconversion of derivatized THF, role
in amino acid metabolism
S-Adenosyl-Methionine as methyl donor and its metabolic versatility
The methyl group’s transfer at N-5 of THF is insufficient, S-Adenosylmethionine is preferred for methyl transfer
methionine adenosyl transferase
biotine
lipoic acid
S+
cal
methyl
synthesis in plants
,Creatine
Liver: SAM is a precursor for glutathione
aminoisopropyl group
Bulk of SAM is used in methyltransferase reactions I :
- creatine synthesis
- phosphatidylcholine synthesis
HO
OH
HO
CH
CH2
NH3+
norepinephrine
S-adenosylmethionine
S-adenosylhomocysteine
HO
OH
HO
CH
CH2
H
N
CH3
epinephrine
- 0.6-1.6% of all genes code for methyltransferases at present 25% identified
…….
Transfer of one carbon atom units (C1)
histidine
glycine
C1
serine
tryptophan
C1
C1
C1
tetrahyrofolic acid (THF4)
C1
purine ring
C1
Vitamin
C1 B12
thymidilate synthase
dTMP formation
Methyl cycle
S-adenosyl
homocysteine
S-adenosyl
methionine
“SAM”
„CH3-”
Activated methyl cycle (Met and SAM metabolism)
adenosyl
transferase
Methyltransferase reactions II
S-adenosyl-homocysteine
methyltransferase
•SAM: methyl group donor
in synthetic reactions,
methylation of
DNA, RNA, proteins,
biosynthesis of
phosphatidylcholine
creatine.
diet
Methyl-H4folate-H4folate
Methionine synthase (MS)
Homocysteine-methionine
hydrolase
Methionine metabolism: remarkable vitamin dependence, folate, vitamin B12, B6, FAD
Vitamin
FAD
Vitamin B6
Conversion of homocysteine to methionine is essential to: conserve methionine
detoxify homocysteine
produce SAM
Regulation of homocysteine formation by SAM level, SAM „switch”
Protein (choline, methionine, betaine)- “labil” methyl groups
provide methyl group to SAM)
Diet: 1-1.5g protein/kg
43% of homocysteine
Remethylated
57% transsulfuration
High SAM
- Enhance the flow of homocysteine
out of the methionine cycle
Low SAM
- Conserve homocysteine within the
methionine cycle
Homocysteine causing oxidative stress
Glutathione peroxidase
Homocysteine potentiates oxidative injury in vascular diseases: coronary artery disease, cerebrovascular
events, and in brain degenerative deseases (AD, Parkinson).
Genetic predisposition to hyoerhomocysteinemia:
most common inherited form of hyperhomocysteinemia: alteration in the gene encoding the
enzyme methylene tetrahydrofolate reductase (MTHFR), leading to moderate
hyperhomocysteinemia.
less often the cause is cystathionine -synthase (CBS) deficiency, with very high
homocysteine levels.
Aquired mild hyperhomocysteinemia
Dietary defficiencies of folate, vitamin B12 and/orB6.
Cofactors for the optimal function of MTHF and CBS.
Vascular diseases
Folate recommandation: 400 to 600 μg per day.
Insufficient folate – neural tube deffect, spina bifida, anencephaly.
Plants vary in their folate level, wheat and rice contain extremely low level - biofortification?
Increased utilisation of SAM due to oxidative stress also results in
accumulation of homocysteine
„methyl balance” maintain
adequate level of SAM
unstable intermediate
, aging
Methionine metabolism in the cellular assimilation of folate, the “folate trap”
Functions of methionine synthase: 1. methionine conservation
2. cellular folate assimilation by conversation of 5-methyl-THF to THF,
to support DNA synthesis.
Impaired MS activity (brought about by B12 defficiency): functional folate defficiency.
Vitamin B12 is the only acceptor of methyl-THF. There is also only one acceptor for methyl-B12 - homocysteine in a
reaction catalyzed by methionine synthase. A defect in homocysteine methyltransferase or a deficiency of B12 can lead
to a methyl-trap of THF and a subsequent deficiency.
Inhibition of nucleotide synthesis – effecting erythropoiesis – megaloblastic anaemia.
(Immature large cells released from the bone marrrow to try to compensate for anemia.)
Folate-Diet
Gluco - ketoplastic
Tryptophan dioxygenase N-formyl-kynurenin
few% hydroxylase
formyl
(THB)
kynurenin formamidase
Serotonin
(5-hydroxy-tryptamine)
Regulation of sleeping, psychic processes
THF
mood. Serotonin effects accelerated by MAO
accumulation in
inhibitors
kynurenin Kynureninate
schizophrenia
kynureninase
B6
alanine
3-hydroxyantranylate
nicotinate, NAD+
acetoacetyl-CoA
Tryptophan, and 5-HT (serotonine) and central phatigue
5-HT - arousal, lethargy, sleepiness, mood
peripheral
5-HT induced central
fatigue
Metabolism of Phe and Tyr
Gluco-and ketoplastic
NH3+
CH2
CH
COO
phenylalanine
Phenylalanine
Hydroxylase
O2 + tetrahydrobiopterin
H2O + dihydrobiopterin
NH3+
HO
CH2
CH
COO
tyrosine
Mixed function oxidation
one O atom of O2 is reduced to H2O the other is incorporated
into amino acid.
Tyrosine: precursor of dopamine, epinephrine, norepinephrine.
Genetic deficiency of
Phenylalanine
Hydroxylase,or defective
production of
cofactor(cofactor PKU)
tetrahydrobiopterine (THB)
phenylketonuria(PKU)
Transaminase
Phenylalanine
Phenylpyruvate
(Phenylketone)
Phenylalanine THB
Deficient in
Hydroxylase
Phenylketonuria
Tyr hydroxilase
Tyrosine
Multiple
Reactions
DOPA
Melanins
dopamine
transaminase,dyoxigenase
Fumarate + Acetoacetate
Phenylpyruvate, phenylacetate and phenyllactate accumulate in blood, urine,
damage myelin of nerve cells. 1:10 000 live birth.
Mental retardation
Treatment: limiting phenylalanine (essential aa) intake. No sweetener aspartame! (aspartate+phenylalanine)
Tyrosine, an essential nutrient for individuals with phenylketonuria, must be
supplied in the diet.
Transaminase
Phenylalanine
Phenylpyruvate
(Phenylketone)
Phenylalanine Deficient in
Hydroxylase
Phenylketonuria
THB
Tyrosine hydroxylase
THB
Tyrosine Melanins
DOPA
tyrosinase
Multiple
Reactions
dopamine
melanin
Tyrosine transaminase
Fumarate + Acetoacetate
Tyrosine: precursor for synthesis of melanins and of cathecolamins.
THB is also a cofactor of tyrosine hydroxilase, treatment of cofactor PKU is
complicated.
High phenylalanine inhibits tyrosinase, on the pathway for synthesis of the
pigment melanin from tyrosine.
Albinism: deffect of tyrosinase gene
Leucine increasing mTOR signaling in the hypothalamus and regulating food intake
L-leucin regulation in the cells of arcuate nucleus (ARC), hypothalamus
Amino-acid leucine
Leptin
Refeeding
Glucose, FFA
L-leucine
(glutamate synthesis)
mTOR
~ AMPK
cholecistokinine
Feeding off signal
Food intake
CNS control of energy balance and glucose homeostasis
Leucine: - not synthetized, not metabolized
in the liver
- its level reflects ingested protein
- transported rapidly to neuron and to glia
with L- transporters
- can selectively stimulate mTOR
in the hypothalamus