Download Amino acid catabolism I

Protein
synthesis
break down
Amino acids
de novo
Oxidation
synthesis
Muscle
Transamination
Amino acids
Blood
Schematic protein turnover and metabolic fates
Alanine
Glutamine
Glutamate
Muscle will not improve with protein feeding alone!
Fed-state gains and fasted state losses
in muscle protein balance
Skeletal muscle mass is maintained by normal protein
feeding.
Feeding refreshes muscle protein to improve muscle
function, permitting more physical activity.
Overfeeding of protein increases insulin resistancemuscle proteolysis
Fed state gains are enhanced, fasted state losses are less
Improvement in
immune function
stimulation of
protein synthesis
Degradation of amino acids
rich protein diet:
Catabolised for energy,
postprandial gluconeogenesis
stored as liver glycogen
PEPCK Glucose-6P-ase
biosynthetic
reactions
excreted
directly in the urine
AMP
purine nucleotide
cycle
Starvation, catabolic
states:
Gluconeogenesis
IMP
PEPCK Glucose-6P-ase
(liver)
blood
kidney
urine
Amino acid catabolism I: Fate of the nitrogen
Central role of glutamate in nitrogen metabolism
Ser
Thr
amino acid
a-keto-glutarate
+
oxidative deamination
NH4
transamination
keto acid
His
Asp
Glutamate
Gln
- Amino group from the majority of amino acids is collected by glutamate (by transamination)
in the hepatocytes.
- Liberation of the amino group in the formation of NH+4 by GDH.
L-glutamate dehydrogenase reaction
Glutamate in amino acid synthesis, degradation and interconversion
glutamate
glutamine
NH4+
other amino acids
intestinal bacteria
Allosteric regulation of glutamate dehydrogenase
Catabolism of L-amino acids
Transaminases (aminotransferases)
GOT
Glutamate + oxaloacetate (OAA) <---> a-ketoglutarate + aspartate
ASAT
GPT
Glutamate + pyruvate <---> a-ketoglutarate + alanin
ALAT
Coupled transamination reaction
PRP
Pyridoxal phosphate (PRP) and PRP in aldimine
linkage to the lysine residue of the transaminase
(Schiff-base)
Different forms of pyridoxal phosphate during a transamination reaction
R1-
R2
E
R1
R2pyridoxamine phosphate
Specific pathways for the deamination of amino acids
(minor routes)
Serine dehydratase
Metabolism of serine for gluconeogenesis
cystein desulphhydrase
D-amino oxidases (FAD), L-amino acid oxidase (FMN)
Transport of ammonia
Concentration of ammonia in the systemic blood is very low (25-50μmol/L),
toxic to the brain.
Transport: glutamine and alanine (muscle)
glutamine ( brain)
Glutamine: non-toxic carrier 0.5-0.8mM in arterial plasma, 20-25%
of circulating free amino acids
precursor for synthesis of many nitrogen containing compounds
metabolic fuel for rapidly dividing cells
generates glutamate and GABA in the brain
Glutamine transport , interorgan metabolism of glutamine
de novo synthesis: L-Glutamate + NH4+ + ATP
L-Glutamine + ADP+ Pi
compartmentalised
glutaminase
glutamine synthetase
Glutamine in diet
low glutaminase
Glutamine - principle non-toxic carrier of nitrogen
Intracellularly – muscle pool – released in response to stress, hypercatabolic states
brain – glutamine-glutamate cycle- GABA
liver – catabolised - substrate for ureagenesis and gluconeogenesis
kidney – catabolised - ammoniagenesis and gluconeogenesis
muscle, lung, adipose - major sites of glutamine release to blood
glutamine synthetase
Muscle
Lung/adipose
Muscle
release
Plasma Glutamin
uptake
Liver
NH+
4
Gut
urea
carbon sceleton:
glycogen
glucose
Brain
Kidney
Glutamateglutamine
cycle
acid-base
balance
Glutamin
ornithine,citrulline, alanine
NH3
portal vein
glutamine+H2O
glutaminase
glutamate + NH
3
proline,
liver
The liver receives both amino acids and ammonia from circulation
Scource of ammonia in different tissues:
1. degradation of amino acids
transdeamination (transamination+GDH)
minor patways
2. deamination of other compounds
N-containing side chains of nucleotides
neurotransmitters
3. ammonia production in the large intestine by bacteria
portal vein, direct transport of ammonia.
Urea cycle
Function: 1. prevents ammonia levels from rising too high when large amounts of amino acids are catabolized
2. urea cycle enzymes: extrahepatic arginine synthesis
Biosynthesis of urea in the liver
55-100g protein/day
ORNT1
ORNT1
The liver receive both ammonia and amino acids from the circulation
GDH and major aminotransferases catalyze reactions close to equilibrium
Quantitative aspects of nitrogen incorporation, regulation?
Regulation of the urea cycle
1. Short term: NAG an allosteric regulator of CPSI and glutaminase activity
increased amino acid catabolism
+
increase in NAG
increased flux with constant
ammonia concentration
increase in glutamate, more NAG
glutaminase
Arginine
+
mitochondria
2. Long term: high protein diet: transcriptional regulation. Hepatic glycogen syntesis
Caloric restriction: increased protein catabolism – CPSI induction (cAMP responsive element), glucose need.
ORNT1 - increased transcription.
Hyperammonemias
deffect: carbamoyl phosphate synthetase
CPSD
CP
cytosol, pyrimidine synthesis, orotic acid
deffect: ornithine transcarbamoylase
OTCD
NH4+
Inherited urea cycle diseases
(+liver failure)
Hyperammonemia
Brain edema, convulsions, coma
Having no urea cycle, brain relies on glutamine synthetase for the removal of exes ammonia
NH3
Change in astrocyte morphology: cell swelling astrocytosis
acute
hyperammonemia
chronic
hyperammonemia
Changes in expression of glutamate transporters in astrocytes.
Scriver et.al.The metabolic and Molecular Bases of Inherited Deseases,2001
Hyperammoniemic encephalopathy
Brusilov: Rev. in Mol. Medicine,2002
Computer axial tomography scan of the head of hyperammonemic encephalopathy
in the composite case of ornythine transcarbamoylase deficiency.
A. CT within normal limits upon admission
B. CT scan after tonic seizure with bilateral hemispheric edema with effacement of
cerebrospinal fluid spaces.
The actrocyte demonstrating its relationship with other structures in the brain
Brusilov: Rev.in Mol. Medicine,2002
The glutamate synapse, effect of NH3 on the the
Glutamate-glutamine cycle
intracellular Glu depletion
glutaminase
NH3
glutamine synthetase
extracellular accumulation
Ca2+
NO
Brain injury
Felipo et.al.:Progress in Neurobiology,2002
Treatment: - limited nitrogen diet
- arginine becomes an essential amino acid
- detoxification reactions as alternatives to the urea cycle, ATP dependent
Hepatic metabolism of glutamine, zonal distribution of glutaminase and
glutamine synthetase
bulk
remaining
detoxify
glutaminase
glutamine synthetase
high affinity
high capacity
low affinity
spare aminonitrogen
in starvation
at metabolic acidosis:
net producer of glutamine
Sequential synthesis of urea and glutamine – efficient to ensure systemic/nontoxic level of ammonia
Ammonium ion - feed-forward activator of synthesis of glutamate and N-acetyl glutamate
Hepatic synthesis of glutamine – acid-base balance. Decrease pH – activation of glutamine synthetase –sparing of glutamine
Interorgan metabolism of glutamine during metabolic acidosis
Acut response: plasma glutamine
Renal extraction of glutamine
glutamine synthetase
release
uptake
pH
incresased ammonia excretion
increased gluconeogenesis
PEPCK
The urea cycle – part of the metabolism centered around L-arginine
L-arginine is semiessential amino acid,
synthesized in collaboration. The intestinal – renal axis.
Arg
Bioavailability of arginine is complex
1.Exogenous supply
2.Endogenous release
3.Arginine resynthesis
4.Arginine catabolism, arginase
5.Arginine transport
CAT-1
AS, AL
EC, nerve cells,
macrophages
urea
arginase
Arg
CAT-1
Arg
NO+citrulline
circulation
Insufficient Arg:
strict carnivors
small bowel, kidney disease
conditions with elevated
amino acid catabolism:
inflamation, sepsis, recovery.
Arginine availability: arginases and NOS use a common substrate
Fate of citrullin: intercellular citrulline-NO cycle
- cell proliferation
repair
Citrulline
recycled to Arg, in kidney
+other tissues
inflammatory stimuli
Arginine is the largest scource for NO production
NO,(EDRF): labile, common gas
NO-cGMP-mediated effects: smooth muscle cell relaxation
in EC: cGMP-prostacyclin mediated decrease in platelet aggregation
decrease in leukocyte adhesion and migration
NO functionality – vascular health/vasculopathy - production of NO – depends on NOS activity
“Arginine paradox”
Km for eNOS: 1.4-2.9 μmol/L
Intracellular L-arginine: 0.5-2mmmol/L
eNOS should be saturated with substrate
Despite high cellular arginine, and low Km of eNOS:
arginine, citrulline supplementation “in vivo” improves NO function: increased vasodilation
decreased leukocyte adhesion
decreased platelet adhesion
Possible reasons: altered arginine transport
increased arginase activity
compartmentalisation of arginine
Supplementation:
Arg: low bioavailability, increased arginase
Cit: Arg synthesis, increased NO levels
Gln: major vehicle of transport, Glu-gluthatione
reduction of oxidative stress
Gly: restores NO balance at increased nutrient
demands
Meth,
Homocys: increased cardiovascular risk
Lys: decreases Arg transport