Amino Acid Metab and Urea Cycle Flashcards
** concentration of free amino acids** is higher in the cell or out of the cell?
intracellular concentration is ocnsiderably higher
this gradient is maintained by active transport of amino acids into cells
the intracellular/extracellular free AA gradient varies with different AA, the greatest with
glutamate and glutamine.
** what are the most abundant amino acids in serum.
Glutamine and alanine
In the body there is about 100g of free amino acids.
how much is glutamate and glutamine?
how much is essential AA?
50% – glutamate, gluatmine
10% – essential AA
*********
What are the essenital AA?
“Pvt. Tim Hall”
Phenylalnine
Valine
Trptophan
Threonine
Isoleucine
Methionine
Histidine
Arginine
Leucine
Lysine
____ becomes essential if **methionine is low **
Cysteine
_____ becomes essential if phenylalanine is low.
tyrosine
essential AA for growth?
arginine
inputs into the AA pool are from
dietary protein and proteolysis of cellular protein
Approximately half the endogenous protein (6-7kg) is associated with
the skeleton and other supporting tissues
(Of which the major protein component is collagen)
Gastrointestinal inputs
70-100g of dietary protein and 35-200g of endogenous protein from sloshed off____ absorbed daily.
intestinal cells
Disorders associated with defects in amino acid transport lead to
increased levels of these amino acids in the urine,
Disorders associated with defects in amino acid transport
prognosis?
these disorders are benign or cause only minor health problems, since amino acids are also absorbed from the intestines as small peptides
___ separate transporters of AA from the gut into intestinal epithelial cells
FIVE
transport of amino acids from the gut into intestinal epithelial cell all are ___ symporters
NA or PROTON SYMPORTERS
High degree of redundancy with ___ since they overlap considerably in their specifity
AA transporters from gut to intestinal epithelial cells
what is the defect involved in Hartnup’s Disease??
A defect in the transport system for neutral and aromatic amino acids from the gut and the renal tubules
Hartnup’s Disease sx?
4D’s: Diarrhea, Dermatitis, Dementia, Death)
Hartnup’s Disease tx?
administration of niacin
how is Hartnup’s Disease dx?
diarrhea, dermatitis, dementia, death with HIGH LEVELS OF NEUTRAL OR AROMATIC AA IN URINE
what is the defect in Cysinuria?
A defect in the transport system for basic amino acids and cystine (a disulfide- linked dimer of cysteine) from the gut and the renal tubules.
Cystinuria sx?
crystals formed by cystine can lead to UTI AND KIDNEY STONES
Cystinuria tx?
fluids, and administration of the drug penicillamine (reacts with cystine to form a significantly more soluble compound)

where is trancytosis along the brush border most pronounced?
in infants where it enables them to acquire antibodies from breast milk.
Cystic Fibrosis Because of a defect in the chloride channels in the pancreatic secretory ducts, what do these pts need to be given?
secretory ducts harden, so need pancreatic enzymes
when is body protein being broken down to free AA?
continuously
The rate of this proteolysis varies widely between proteins from half lives of a few minutes to years.
The urinary levels of ____ can provide a measure of muscle protein breakdown.
3-methyl histidine
it is liberated when actomyosin is broken down. histidine residues of muscle are methylated posttranslationally
Metabolic breakdown of amino acids to urea and CO2 is a continuous drain. This is reduced during
starvation, but is never turned off.
___ and ___ provide complementing essential amino acids.
Grains and beans
Normal nitrogen balance (in a healthy person)
total daily N intake = totai daily N loss (in urine, skin feces)
If nitrogen losses are less than intake, the subject is in ____ nitrogen balance
positive
examples of ppl in positive N balance?
children and convalescing adults
If nitrogen losses are more than intake, the subject is in ____ nitrogen balance
negative
examples of people in negative N balance?
wasting or starvation
Prolonged negative balance can be fatal if loss of body protein reaches about ____of total body protein.
one-third
Insufficient quantities of how many essential amino acids is adequate to turn an otherwise nml individual into one with a negative nitrogen balance.
just one
In general, ___ proteins are deficient in lysine, methionine, and tryptophan
plant
the absence of lysine in low-grade cereal proteins, used as a dietary mainstay in many underdeveloped countries can lead to
kwashiorkor syndrome
Free ammonia ion is highly poisonous so the body can move remove free NH + in three ways.
- Glutamate Dehydrogenase
- Glutamine Synthase
- Carbamoyl Phosphate Synthase I and II
how Glutamate Dehydrogenase can remove NH4?
combine NH4 with a-ketoglutarate —> puts NH4 in glutamate

how Glutamine Synthase removes NH4?
NH4 + glutamate —> glutamine

how Carbamoyl Phosphate Synthase I and II removes NH4?

difference between Carbamoyl Phosphate Synthase I and II
I. Located in mitochondria for urea cycle
II. Located in the cytoplasm for pyrimidine nucleotide biosynthesis
II does not use NH4
what is Transamination?
The transfer of an amino group from an AA to an α-keto acid to form a new AA and a new keto acid
what is formed with transamination?
- a new AA
- a new keto acid
____ an essential cofactor for all transaminations.
Pyridoxal phosphate (vitamin b6):
_____ functions as a cofactor in a number of reactions including: a) transaminations
b) decarboxylations
c) dehydration of ß-hydroxyamino acids
d) racemizations of α-amino acids e) removal of H2S from cysteine
Pyridoxine
All ____ appear to have a common enzyme mechanism utilizing pyridoxal phosphate
transaminases
The α-amino groups of at least 12 amino acids are removed by pyridoxal phosphate transaminase
alanine, arginine, asparagine, aspartate, cysteine, isoleucine, leucine, lysine, phenylalanine, tryptophan, tyrosine and valine.
____ is an efficient way for cells to send 3 carbon groups back to the liver to be easily converted into glucose while disposing of an NH4 group.
alanine, which can be turned into pyruvate
Most transaminases converge on the amino acid ___.
glutamate
why is glutamate one of the most abundant AA inside cells
Most transaminases converge on the amino acid glutamate.
where does Oxidative Deamination by Glutamate Dehydrogenase?

in the mitocondria
Glutamate dehydrogenase can use what are a cofactor
either oxidized NAD or NADP
Under normal conditions in the liver the ratio of NADPH to NADP is ___
high
the ratio of NADH to NAD is low,
so that there is always cofactor available to participate in the Oxidative Deamination by Glutamate Dehydrogenase in either the forward or reverse direction.
Glutamate dehydrogenase is activated by? inhibited by?
activated – adp and gdp
inhibited by – atp and gtp
Glutamate is deaminated mainly in the___
liver.
difference in when liver genederates NADH?
If the liver needs energy it will generate NADH for electron transport.
If it is energy rich, it will generate NADPH for biosynthetic reaction.
where are Amino acid oxidases found? what are they found to?
found in the kidneys and liver
tightly bound to flavins
what do Amino acid oxidases convert?
an amino acid and a water molecule into its corresponding α-keto acid and ammonia.
direct deamination by dehydratases is facilitated in the aa ____ bc of the chemistry of the hydroxyl side
serine and threonine,
what is The cofactor for both serine and threonine dehydratase is
pyridoxal phosphate
Homocysteine desulfhydrase is also a pyridoxal phosphate containing enzyme that removes both ___ and ____ from homocysteine.
ammonia and sulfur
3 alternate mechanisms for AA deamination
- AA oxidases
- Direct deamination by dehydratases
- Desulfhydrases
how is α-ketobutyrate disposed of?

what fatty aci dpathway does vit. b12 catalyze?
b12 catalyzes the last step of the α-ketobutyrate –> succinyl CoA pathway

___release during catabolism of amino acids is toxic and must be constantly removed from the body.
Ammonia
why does the body converts the free ammonia to urea
- the constant excretion of ammonia in the urine would lead to drastic changes in blood pH
- urea, a readily soluble and easily excreted compound
ammonia and CO2 —>
Carbamoylphosphate
ammonia and CO2 —> Carbamoylphosphate
Enzyme-
carbamoylphosphate synthetase
ammonia and CO2 —> Carbamoylphosphate
Energy requirement-
Two molecules of ATP
carbamoylphosphate and ornithine —> Citrulline
enzyme?
ornithine transcarbamoylase (OTC)
ornithine transcarbamoylase is an X-linked gene
what rxn is affected with a defect in this pathway
carbamoylphosphate and ornithine —> Citrulline
via this enzyme
citrulline and aspartate —>
Argininosuccinate
citrulline and aspartate —> Argininosuccinate
Enzyme-
argininosuccinate synthetase.
citrulline and aspartate —> Argininosuccinate
Energy requirement?
Energy requirement- one molecule of ATP goes to AMP
citrulline and aspartate —> Argininosuccinate
what is formed?
PPi is generated
it is degraded by pyrophosphatase
citrulline and aspartate —> Argininosuccinate
where does asparate come from?
transamination of oxaloacetate by glutamate.
cleavage of argininosuccinate—>
Arginine and Fumarate
cleavage of argininosuccinate—> Arginine and Fumarate
Enzyme-
Enzyme- argininosuccinate lyase (argininosuccinase)
cleavage of arginine.—>
Urea and Ornithine
cleavage of arginine.—> Urea and Ornithine
Enzyme-
arginase
Urea acid cycle
____ <——> Urea + 2ADP + 1 AMP + PPi + Fumarate + 2Pi
CO + NH + + 3ATP + Aspartate + 2H O
CO + NH + + 3ATP + Aspartate + 2H O <——> ____
Urea + 2ADP + 1 AMP + PPi + Fumarate + 2Pi
what urea cycle enzymes are in the Mitchondria?
carbamoylphosphate synthetase
ornithine transcarbamoylase
what urea cycle enzymes are in the Cytosol?
argininosuccinate synthetase argininosuccinate lyase arginase
Urea carries ___ammonia groups
two ammnoia
Urea Soluble to ___10M (640g/l)
>
Why Urea? (4)
- Carries two ammonia groups
- Soluble to >10M (640g/l)
- Non protonatable-10% solution pH 7.2
- Low reactivity (inert)
KEY STEPS OF THE UREA CYCLE ARE CARRIED OUT IN THE MITOCHONDRIA?
- NH4+ + CO2 + 2 A TP —> CARBAMOYL PHOSPHA TE
- ORNITHINE —> CITRULLINE
Protein free diet- urea excretion accounts for only ___% of the total urinary nitrogen as
compared to 80% in a normal diet.
60
with a ___ diet, levels of urea cycle enzymes decline
protein free diet
High Protein diet- or in starvation, urea cycle enzymes
increase several fold.
since gluconeogenesis from amino acids is high
symptom of most of the diseases caused by defective urea cycle enzymes?
An accumulation of ammonia in the blood
___ deficiency is the most common urea cycle disorder
OTC
* due to its presence on the X chromosome
Mental Retardation is seen with what urea acid disorder?
Citrullinemia
defect in Argininosuccinate Synthetase
Argininosuccinic aciduria
defect is in what enzyme?
what is seen in the blood?
defect – Argininosuccinase
blood – NH4 and Argininosuccinic Acid
urine – Arginosuccinic Acid
Hyper- ammoniemia
defect is in what enzyme?
what is seen in the blood?
enzyme – Carbamoyl Phosphate Synthase
blood – High NH4
Arginiemia
defect is in what enzyme?
what is seen in the blood?
defect?
defect – Arginase
blood – High NH4 and Arginine
urine – Arg, L ys, Orn
Hyper- ornithinemia
defect is in what enzyme?
what is seen in the blood?
urine?
defect –Ornithine Transcarbamylase
blood – High NH4, orotate and Ornithine
urine – Ornithine
Treatment of urea cycle disorders usually includes
low protein diet that is supplemented with arginine or citrulline since arginine usually becomes essential in these patients.
administration of sodium benzoate and sodium phenylacetate