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Protein. synthesis. break down. Amino acids. de novo synthesis. Oxidation. Transamination. Muscle. Amino acids. Blood. Alanine Glutamine Glutamate. Schematic protein turnover and metabolic fates. Muscle will not improve with protein feeding alone!. - PowerPoint PPT Presentation
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Protein
Amino acidsbreak down synthesis
Amino acidsMuscle
Blood
de novo
synthesis
Oxidation
Transamination
AlanineGlutamineGlutamate
Schematic protein turnover and metabolic fates
Fed-state gains and fasted state lossesin muscle protein balance
Skeletal muscle mass is maintained by normal protein feeding.Feeding refreshes muscle protein to improve muscle function, permitting more physical activity.
Fed state gains are enhanced, fasted state losses are less
Improvement inimmune functionstimulation ofprotein synthesis
Muscle will not improve with protein feeding alone!
Overfeeding of protein increases insulin resistance-muscle proteolysis
blood kidney urine
Degradation of amino acids
purine nucleotide cycle
IMP
P
biosyntheticreactions
excreted directly in the urine
(liver)
rich protein diet:Catabolised for energy,postprandial gluconeogenesis stored as liver glycogen
Starvation, catabolicstates: Gluconeogenesis
PEPCK Glucose-6P-ase
PEPCK Glucose-6P-ase
Amino acid catabolism I: Fate of the nitrogen
amino acid
keto acid
transamination
-keto-glutarate
Glutamate
NH4oxidative deamination
Ser
Thr
His
Asp
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.
Central role of glutamate in nitrogen metabolism
L-glutamate dehydrogenase reaction
glutamate
glutamine
intestinal bacteria
other amino acidsNH4+
Glutamate in amino acid synthesis, degradation and interconversion
Allosteric regulation of glutamate dehydrogenase
Glutamate + oxaloacetate (OAA) <---> -ketoglutarate + aspartate
Glutamate + pyruvate <---> -ketoglutarate + alanin
GOT ASAT
GPT ALAT
Catabolism of L-amino acidsTransaminases (aminotransferases)
Coupled transamination reaction
Pyridoxal phosphate (PRP) and PRP in aldimine linkage to the lysine residue of the transaminase (Schiff-base)
PRP
Different forms of pyridoxal phosphate during a transamination reaction
R1-
R1
pyridoxamine phosphate
R2-
E
R2
Specific pathways for the deamination of amino acids (minor routes)
cystein desulphhydrase
D-amino oxidases (FAD), L-amino acid oxidase (FMN)
Serine dehydratase
Metabolism of serine for gluconeogenesis
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 - 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 in diet
low glutaminase
glutaminase glutamine synthetasecompartmentalised
de novo synthesis: L-Glutamate + NH4+ + ATP L-Glutamine + ADP+ Pi
Glutamine transport, interorgan metabolism of glutamine
Plasma Glutamin
carbon sceleton:
glycogenglucose
acid-basebalance
Glutamin proline,
ornithine,citrulline, alanine
GutNH+
4 urea
NH3
portal vein liver
glutamine+H2O glutamate + NH3 glutaminase
Muscle
Muscle Lung/adipose
release
uptake
glutamine synthetase
Kidney
Liver
Glutamate-glutaminecycle
Brain
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
The liver receives both amino acids and ammonia from circulation
Biosynthesis of urea in the liver
ORNT1
ORNT1
55-100g protein/day
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?
1. Short term: NAG an allosteric regulator of CPSI and glutaminase activity
Regulation of the urea cycle
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.
mitochondria
Arginine +
increased amino acid catabolism
increase in NAG
increased flux with constantammonia concentrationincrease in glutamate, more NAG
glutaminase +
Hyperammonemias deffect: carbamoyl phosphate synthetase
deffect: ornithine transcarbamoylase
NH4+
CPSD
OTCD
CP cytosol, pyrimidine synthesis, orotic acid
Inherited urea cycle diseases (+liver failure)
Having no urea cycle, brain relies on glutamine synthetase for the removal of exes ammonia
Hyperammonemia Brain edema, convulsions, coma
Change in astrocyte morphology: cell swelling astrocytosis
acutehyperammonemia
chronichyperammonemia
Changes in expression of glutamate transporters in astrocytes.
NH3
Scriver et.al.The metabolic and Molecular Bases of Inherited Deseases,2001
Hyperammoniemic encephalopathy
Computer axial tomography scan of the head of hyperammonemic encephalopathyin the composite case of ornythine transcarbamoylase deficiency.A. CT within normal limits upon admissionB. CT scan after tonic seizure with bilateral hemispheric edema with effacement of cerebrospinal fluid spaces.
Brusilov: Rev. in Mol. Medicine,2002
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
Felipo et.al.:Progress in Neurobiology,2002
NH3
intracellular Glu depletion
extracellular accumulation
Ca2+
NOBrain injury
glutamine synthetaseglutaminase
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
detoxify
high capacitylow affinity
bulk remaining
glutaminase
glutamine synthetasehigh affinity
Sequential synthesis of urea and glutamine – efficient to ensure systemic/nontoxic level of ammoniaAmmonium ion - feed-forward activator of synthesis of glutamate and N-acetyl glutamateHepatic synthesis of glutamine – acid-base balance. Decrease pH – activation of glutamine synthetase –sparing of glutamine
at metabolic acidosis:net producer of glutamine
spare aminonitrogenin starvation
Interorgan metabolism of glutamine during metabolic acidosis
Acut response: plasma glutamine Renal extraction of glutamine
uptake
release
glutamine synthetase
incresased ammonia excretion
increased gluconeogenesis PEPCK pH
The urea cycle – part of the metabolism centered around L-arginine
L-arginine is semiessential amino acid,synthesized in collaboration. The intestinal – renal axis.
urea
circulation
EC, nerve cells,macrophages
Arg
AS, AL
NO+citrulline
arginase
CAT-1
Arg
CAT-1
Arg
Bioavailability of arginine is complex1.Exogenous supply2.Endogenous release3.Arginine resynthesis4.Arginine catabolism, arginase5.Arginine transport
strict carnivorssmall bowel, kidney diseaseconditions with elevatedamino acid catabolism:inflamation, sepsis, recovery.
Insufficient Arg:
Arginine availability: arginases and NOS use a common substrate
Citrulline recycled to Arg, in kidney +other tissues
- cell proliferation repair
Fate of citrullin: intercellular citrulline-NO cycle
inflammatory stimuli
“Arginine paradox”
Km for eNOS: 1.4-2.9 μmol/LIntracellular 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
Arginine is the largest scource for NO production
NO,(EDRF): labile, common gasNO-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
Supplementation:Arg: low bioavailability, increased arginaseCit: Arg synthesis, increased NO levelsGln: major vehicle of transport, Glu-gluthatione reduction of oxidative stressGly: restores NO balance at increased nutrient demands
Meth, Homocys: increased cardiovascular riskLys: decreases Arg transport