Flashcards in Protein Breakdown (Catabolism) Deck (73):
Amino acid catabolism creates ____.
Urea, carbon dioxide, and water
The amino acid pool can be used to make ___.
porphyrins, purines and pyrimidines, nitric oxide, mealnin, creatine, hormones and neurotransmitters
The stomach secretes ____.
HCl and Pepsinogen
HCl is secreted by ___.
parietal cells in the stomach
Acholrhydria can cause ____.
Pepsinogen is secreted by ___.
Chief cells in the stomach.
How does pepsinogen function?
Autocatalytic activation occurs by a conformational change at lower pH. (Goes from pepsinogen to pepsin.) Pepsinogen acts as an endopeptidase, cleaving peptide bonds (prefers CO group provided by an aromatic or acidic amino acid).
Smaller peptides and some free amino acids are produced.
The pancreas produces ___.
Trypsinogen, chymotripsinogen, proelastase, procaboxypeptidase
What zymogens are activated by trypsin?
Chymotripsinogen --> chymotrypsin,
Preoelastase ---> elastase,
Procarboxypeptidases --> carboxypeptidase A or B (exopeptidase)
What enzyme activates trypsinogen?
Enteropeptidase activates trypsinogen by proteolysis into trypsin
Zymogens are activated by ___.
Amino acids are absorbed by semi-specific ___ transport proteins
There are at least ___ types of carriers for amino acids.
Hartnup and cystinuria are caused by ___.
defects of sodium transport proteins (for amino acid transport)
Autosomal recessive gene that presents with ataxia, emotional instability and pellagra rash caused by a mutation in the sodium-dependent and chloride-independent neutral amino acid transporter, expressed predominately in the intestine and kidneys.
Also called amidoaciduria?
Urine levels of neutral aa are high due to failure to reabsorb amino acids in kidney.
Tryptophan is a precursor for niacin synthesis and most symptoms of hartnup are believed to be niacin deficiency related
Affects Neutral amino acid transport
It affect dibasic amino acid transport and the transport of cytstine. Cystine stones appear in the bladder and kidney
What carries out intracellular proteolysis?
Lysosomes (acid hydrolases, pH 5 is optimal) and the 26S proteasome in the cytosol (for poly-ubiquitin-tagged proteins)
What carries out extracellular proteolysis?
Serine proteases (elastase from neutrophils contributes to lung damage in chronic obstructive pulmonary disease) and matrix metalloproteases (collagenases)
What is a byproduct of amino acid breakdown?
What amino acids are used to transport nitrogen in the blood?
Alanine and glutamine
Where does ammonium detoxification occur?
In the liver. (It is the only organ with all the enzymes for urea synthesis.)
Ammonium is converted to ___ because of these properties: ___.
It is soluble and not charged
____ should be obtained for any chile with unexplained vomiting, lethargy, or other evidence of an encephalopathy.
A plasma ammonia level
Ammonia is very toxic to the ___. It is converted to two nontoxic compounds: _____ and ____.
Urea and glutamine
What reactions are involved in the first step of amino acid catabolism?
Removal of the amino groups through transamination or deamination
moves amino groups into glutamate (either use or deaminate, releasing ammonium ion)
carried out by aminotransferase
Aminotransferase reactions require pyridoxal phosphate
Direct release of ammonium ion, different methods for different amino acids:
glutamate dehydrogenase, hydrolytic deamination (e.g. glutaminase), non-oxidative (PLP dependent)
What does aminotransferase do?
It catalyzes the transfer of an alpha-amino group from an amino acid to an alpha-ketoacid (uses PLP)
What is an example of an aminotransferase?
Aspartate aminotranferase (converts aspartate and alpha-ketoglutarate to oxaloacetate and glutamate) or alanine aminotransferase (converts alanine and alpha-ketolglutarate to pyruvate and glutamate) are listed
True or false: Transamination reactions are irreversible.
False (They are reversible)
What is used to evaluate liver damage?
Serum ALT and AST
___ is required for aminotransferase reactions.
Pyridoxal phosphate (derived from vitamin B6)
The aldehyde carbon forms and Schiff base (C=N)
In the liver, most amino acids transer the ___ amino group to ____ to form ___.
Glutamate releases ___ via ____.
Glutamate dehydrogenase is located in ___.
Glutamate dehydrogenase can use ___ or ___ to convert glutamate to alpha-ketoglutarate.
NAD or NADPH
What regulates glutamate dehydrogenase?
GTP and ATP are allosteric inhibitors of glutamate dehydrogenase.
GDP and ADP activate glutamate dehydrogenase
True or false: Glutamate dehydrogenase catalyzes a reversible reaction.
Hydrolytic deamination occurs on the ____.
side chain amide groups of asparagine and glutamine
Deamination reaction of glutamine
Glutamine and water are converted to glutamate and ammonium by glutaminase
(Asparagine is the same but with asapar- prefix instead of gluta-prefix)
Nonoxidative deamination occurs with what amino acids?
Serine and Threonine
What is the deamination process starting with serine?
Serine is converted to pyruvate and ammonium using serine dehydratase. Pyridoxal phosphate is required.
What is the deamination process starting with threonine?
Threonine is converted to alpha-ketobutyrate and ammonium using threonine dehydratase. Pyridoxal phosphate is required.
A mutation in glutamine synthetase can cause ___.
buildup of glutamate in the brain, which leads to newborns with sever brain malformations
What does glutamine synthetase do?
It fixes ammonium in a carbon compound.
It converts glutamate and ammonium to glutamine (using ATP).
When does positive-nitrogen balance occur (in nitrogen homeostasis)?
During physical trauma or for a growing child, nitrogen is needed to increase protein synthesis and excreted nitrogen is less than consumed nitrogen
When does negative-nitrogen balance occur?
During protein malnutrition or starvation. The body degrades protein to synthesize essential proteins and the excreted nitrogen is greater than consumed nitrogen.
What is the nitrogen balance in a normal healthy adult?
The protein content exceeds requirement and excreted nitrogen is equivalent to consumed nitrogen.
What is the overall reaction for the urea cycle?
NH4+ + CO2 + aspartate + 3 ATP + 2 H2O ---> Urea + fumarate + 2 ADP + AMP + PPi + 2 Pi
_#_ high energy bonds are used for each molecule of ureas
True or false: The urea cycle is reversible.
False (There is a large, negative delta G)
Ammonia toxicity is called ___.
Ammonia is toxic especially to the ___.
Brain and CNS
Draw Urea Cycle
See slide 20
Where is carbamoyl phosphate synthase I located and what is its function?
In mitochondria, urea synthesis
Where is carbamoyl phosphate synthase II located and what is its main function?
In the cytosol, pyrimidine synthesis
What does carbamoyl phosphate synthase require?
N-acetylglutamate (NAG). It is an allosteric activator.
What is NAG?
N-acetylglutamate is an allosteric activator that is synthesized in the mitochondria by N-Acetylglutamate synthase. (Glutamine and acetyl CoA form NAG). It is positively regulated by the presence of arginine.
A CPS-1 dimer is composed of ___.
Two CPS-1-NAG monomers. The monomer is catalytically active.
What regulates the urea cycle?
In the short term (minutes) CPS-1 is regulated.
It is allosterically activated by NAG. A protein-rich meal results in increased NAG synthesis.
In the long term (days)
Dietary protein levels regulate the synthesis of urea cycle enzymes. Starvation raises protein degradation in tissues and synthesis of urea cycle enzymes, where as regular dietary protein leads to increased urea enzyme synthesis and increase urea excretion.
The Km of NAG synthase for glumatate is ___ and it is ____ under physiological conditions.
The equivalent of __ ATPs are consumed to synthesize one molecule of urea.
4 (The extra ATP comes from where aspartate would have entered the TCA cycle)
Connect the urea cycle to the TCA cycle in a drawing
See slide 27. Basically show how argininosuccinate converts aspartate to fumarate, and how oxaloacetate is converted to aspartate using glutamate and aminotransferase (which produces alpha-keto-glutarate).
What is the defective enzyme and the products accumulated in Hyperammonemia type I?
The enzyme is CPS-1, the products accumulated are ammonium (NH4+), glutamine, and alanine
What is the defective enzyme and the products accumulated in Hyperammonemia type II?
The enzyme is ornithine transcarbamoylase,
The products accumulated are ammonium, glutamine, and orotic acid
What is the defective enzyme and the products accumulated in Citrullinemia?
Argininosuccinate synthetase is the enzyme.
What is the defective enzyme and the products accumulated in Argininosuccinic aciduria?
Argininosuccintate lyase is the enzyme,
What is the defective enzyme and the products accumulated in Argininemia?
Arginase is the defective enzyme.
Arginine builds up.
What are the mechanisms of ammonium toxicity?
A shift in the equilibrium of glutamate dehydrogenase reaction toward the direction of glutamate formation could deplete alpha-ketoglutarate (essential for TCA) resulting in decreased oxidation and ATP production (which makes the brain particularly vulnerable to hyperammonemia)
high levels of glutamine in the brain lead to brain swelling because of an osmotic increase in water in astrocytes. It also causes a decrease in neurotransmitters glutamate and GABA
How can defects in the urea cycle be treated?
Acute hyperammonemia can be treated using hemodialysis or exchange transfusion to reapidly lower ammonia.
Diet: limit protein intake. A low protein, high carbohydrate diet with frequent meals
Removing excess ammonia using levulose, avoid and treat infections, kill intestinal ammonia producing bacteria with antibiotics, and administer benzoate and phenylacetate or phenylbutyrate (use this one now) to reduce glycine and glutamine levels
Replace missing intermediates
Supllement with arginine or citrulline
Other ways nitrogen could be excreted
Amino groups can be removed by scavenging amino acids for excretion. Benzoic acid removes glycine, phenylbutyrate removes glutamine
What is ornithine transcarbamylase deficiency?
It is an x-linked trait that acts like a dominant trait in heterozygous females, so abnormal OTC gene is more visible than others.
Not as noticeable in females as in males