Post on 16-Jan-2016
description
Amino Acid Catabolism II: Fate of Carbon Skeleton
proteinrich
Extra glucose in the fed state
Long-term protein overfeeding accelerates the status insulin resistance
Quantitative aspects of amino acids catabolism
1. Amino acids only undergo partial oxidation in the liver2. Partial oxidation ATP in the fed state3. Hepatic gluconeogenesis 4.Urea synthesis and gluconeogenesis from dietary amino acids on the same pathway.
-
each endproduct can yield a new oxaloacetate
can yield FA or ketone body
Glucogenic and ketogenic amino acids
The 3-C -keto acid pyruvate is produced from alanine, cysteine, glycine, serine and partly from tryptophan (indolylalanine). Glucoplastic
Alanine via transaminase directly yields pyruvate.
a l a n i n e - k e t o g l u t a r a t e p y r u v a t e g l u t a m a t e
A m i n o t r a n s f e r a s e ( T r a n s a m i n a s e )
C O O
C H 2
C H 2
C
C O O
O
C H 3
HC
C O O
N H 3+
C O O
C H 2
C H 2
HC
C O O
N H 3+
C H 3
C
C O O
O + +
glutathion
creatine
hem
bile acids
Major pathways of serine in humans
Serine
Serine ethanolamine
H2O
HCO3 3 S-adenosyl-methionine
Serine pyruvate
choline
neurotransmitter synthesis
Glycine
active C1 transfer
HCO3+NH+4 glycine cleavage
glyoxalate oxalate, transaminase deffect: kidney stones
O P O
O
O
H2C
CH
H2C
OCR1
O O C
O
R2
CH2 CH2 N CH3
CH3
CH3
+
phosphatidylcholine
transamination
transaminase
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
Sulfur containing amino-acids I. Role in protein structure
Cystine
MAT: Meth adenosyltransferaseSAHH: S-adenosylhomocysteine hydrolaseCBS: Cystathionine synthaseCGL: Cystathionine lyaseMTHFR: Methylenetetrahydrofolate reductaseMS: Methionine synthaseBHMT: Betaine:homocysteine methyltransferaseSHMT: Serine hydroxymethyltransferase
Sulfur-containing amino acid (cysteine, methionine) II. Major pathways
Transsulfuration pathwayirreversible
conjugates with bileantioxidant
Met and Cys incorporate intoproteins, homocysteine and taurinedo not.
Transmethylation pathway
remethylationpathway
(limited expression: liver, kidneyintestine,pancreas)
(ubiquitous in all cells)
Met- essential a.a.
1.
2.
3.
4
5.
6
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.
a s p a r t a t e - k e t o g l u t a r a t e o x a l o a c e t a t e g l u t a m a t e
A m i n o t r a n s f e r a s e ( T r a n s a m i n a s e )
C O O
C H 2
C H 2
C
C O O
O
C O O
C H 2
HC
C O O
N H 3+
C O O
C H 2
C H 2
HC
C O O
N H 3+
C O O
C H 2
C
C O O
O + +
The 4-C TCA cycle intermediate succinyl-CoA is produced from isoleucine, valine, threonin (branched chain amino acids (BCCA) and methionine.
Leu IleVal
BCCA – branched-chain amino acids: leucine, isoleucine, valine
- 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
Role in protein structure I.
Why BCCA?
- Coiled-coiled -helices: fibrinogen, myosine, keratin, transcription factors Leucine-zippers: permit formation of homodimers/heterodimers of transcription factors
BCCA and protein structure II.
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
BCAA in the diet are metabolized extrahepatically, major site - muscle
Ile, Val Gln synthesis
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.
- genetic deffect of BCKDH: maple sirup urine disease (odor of keto acids)Val Ile Leu
Propionyl-CoA - carboxylated to methylmalonyl-CoA.
Racemase - L-isomer. Methylmalonyl-CoA Mutase - molecular rearrangement-linear chain of succinyl-CoA.
C C H 3
C S-C o A
O
C C H 3
C S-C oA
O
C O O
C
C S-C oA
O
C O O
C C
C O O
C
C
O
H
H
CoA-S H
HH HH
H
H
H
H
H CO 3
A T P A D P
+ P i
p r o p i o n y l - C o A D - m e t h y l m a l o n y l - C o A L - m e t h y l m a l o n y l - C o A s u c c i n y l - C o A
P r o p i o n y l - C o A M e t h y l m a l o n y l - C o A M e t h y l m a l o n y l - C o A C a r b o x y l a s e ( B i o t i n ) R a c e m a s e M u t a s e ( B 1 2 )
Coenzyme B12 (vitamin B12+ATP, adenosylcobalamine)- cofactor of Methylmalonyl-CoA Mutase.
Ile, Val
The 5-C TCA Cycle intermediate -ketoglutarate is produced from arginine, glutamate, glutamine, histidine and prolin.
a s p a r t a t e - k e t o g l u t a r a t e o x a l o a c e t a t e g l u t a m a t e
A m i n o t r a n s f e r a s e ( T r a n s a m i n a s e )
C O O
C H 2
C H 2
C
C O O
O
C O O
C H 2
HC
C O O
N H 3+
C O O
C H 2
C H 2
HC
C O O
N H 3+
C O O
C H 2
C
C O O
O + +
creatine NO transport of amino acid N
Gla (- carboxyglutaminic acid)vitamin KGABA (amino butyrate)
histamin glutamatesemialdehyde
glutamateglutamate
Histidine
N-formiminoglutamate is converted to glutamate by transfer of the formimino group to THF - N5-formimino-THF.
HC C CH2
HC COO
NH3+N NH
CH
OOCHC CH2 CH2 COO
HN NHCH
OOCHC CH2 CH2 COO
NH3+
THF
N 5-formimino-THF
NH4+
H2O
H2O
histidine
N-formimino-glutamate
glutamate
histidine lyase
urokanase
What is folate?THF+ derivatives
C1 unit transfer in amino acid catabolism
- 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
NH
HNN
HN
H2N H
H
H
CH2
HNO C
O
NH
CH
COO
CH2
CH2
COO
Tetrahydrofolate (THF)
pteridine -aminobenzoate glutamate
HH
H
87
65
H
9
10
Interconversion of derivatized THF, role in amino acid metabolismC1 attached to THF
cal
S-Adenosyl-Methionine as methyl donor and its metabolic versatility
synthesis in plants
,Creatine aminoisopropyl group
methyl
biotinelipoic acid
S+
The methyl group’s transfer at N-5 of THF is insufficient, S-Adenosylmethionine is preferred for methyl transfer
methionine adenosyl transferase
Liver: SAM is a precursor for glutathione
CH2 NH3+CHHO
HOOH
CH2
HNCHHO
HOOH
CH3
S-adenosylmethionine
S-adenosylhomocysteine
norepinephrine
epinephrine
Bulk of SAM is used in methyltransferase reactions I :
- 0.6-1.6% of all genes code for methyltransferases at present 25% identified …….
- creatine synthesis
- phosphatidylcholine synthesis
Transfer of one carbon atom units (C1)
histidine glycine serine tryptophan tetrahyrofolic acid (THF4)
purine ring thymidilate synthase dTMP formation S-adenosyl S-adenosyl homocysteine methionine “SAM” „CH3-”
C1 C1C1 C1
C1
C1
C1
B12
Vitamin
Methyl cycle
adenosyltransferase
Activated methyl cycle (Met and SAM metabolism)
S-adenosyl-homocysteinemethyltransferase
hydrolaseMethionine synthase (MS)
diet
•SAM: methyl group donor in synthetic reactions, methylation ofDNA, RNA, proteins, biosynthesis of phosphatidylcholinecreatine.
Methyl-H4folate-H4folate
Homocysteine-methionine
Methyltransferase reactions II
Methionine metabolism: remarkable vitamin dependence, folate, vitamin B12, B6, FAD
Vitamin B6
Vitamin
FAD
Conversion of homocysteine to methionine is essential to: conserve methionine detoxify homocysteine produce SAM
Protein (choline, methionine, betaine)- “labil” methyl groupsprovide methyl group to SAM)
Regulation of homocysteine formation by SAM level, SAM „switch”
Diet: 1-1.5g protein/kg43% of homocysteineRemethylated57% transsulfuration
High SAM- Enhance the flow of homocysteine out of the methionine cycleLow SAM- Conserve homocysteine within the methionine cycle
Glutathione peroxidase
Homocysteine causing oxidative stress
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.
Dietary defficiencies of folate, vitamin B12 and/orB6. Cofactors for the optimal function of MTHF and CBS.
Aquired mild hyperhomocysteinemia
Vascular diseasesFolate 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
, aging
„methyl balance” maintain adequate level of SAM
unstable intermediate
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
Tryptophan
Serotonin
dioxygenaseN-formyl-kynurenin
formyl
THFkynurenin
3-hydroxyantranylate
acetoacetyl-CoAnicotinate, NAD+
alaninekynureninase B6
kynurenin formamidase
(5-hydroxy-tryptamine)
hydroxylase(THB)
Kynureninate accumulation inschizophrenia
few%
Regulation of sleeping, psychic processesmood. Serotonin effects accelerated by MAOinhibitors
Gluco - ketoplastic
Tryptophan, and 5-HT (serotonine) and central phatigue
5-HT induced centralfatigue
exer
cise
peripheral
5-HT - arousal, lethargy, sleepiness, mood
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.
CH2 CH COO
NH3+
CH2 CH COO
NH3+
HO
phenylalanine
tyrosine
O2 + tetrahydrobiopterin
H2O + dihydrobiopterin
Phenylalanine Hydroxylase
Metabolism of Phe and Tyr
Gluco-and ketoplastic
Phenylpyruvate, phenylacetate and phenyllactate accumulate in blood, urine, damage myelin of nerve cells. 1:10 000 live birth.
Mental retardationTreatment: 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
Tyrosine Melanins
Multiple Reactions
Fumarate + Acetoacetate
Genetic deficiency of Phenylalanine Hydroxylase,or defective production of cofactor(cofactor PKU) tetrahydrobiopterine (THB) phenylketonuria(PKU)
transaminase,dyoxigenase
DOPAdopamine
Tyr hydroxilase
THB
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
Transaminase Phenylalanine Phenylpyruvate (Phenylketone) Phenylalanine Deficient in Hydroxylase Phenylketonuria
Tyrosine Melanins
Multiple Reactions
Fumarate + Acetoacetate
DOPAdopamine
melanin
Tyrosine hydroxylaseTHB
THB
Tyrosine transaminase
tyrosinase
mTOR ~ AMPK
Amino-acid leucine
Leptin Refeeding
Food intake
Leucine increasing mTOR signaling in the hypothalamus and regulating food intake
cholecistokinine
Feeding off signal
CNS control of energy balance and glucose homeostasis
L-leucin regulation in the cells of arcuate nucleus (ARC), hypothalamus
Glucose, FFAL-leucine(glutamate synthesis)
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