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