Amino Acid Metabolism, Transaminases, Urea Cycle Flashcards
What happens to the amine groups from AA?
Removed and packed into urea
Where is urea formed?
In the liver
What is major form of getting rid of nitrogen?
Through urea
Even though NH4+ is toxic to us, why is it still a nitrogenous excretory products?
because it is made in the kidney and is excreted right there. It doesnt have to travel through the blood
How are proteolytic enzymes first produced?
as zymogens (inactive enzyme precursors)
Why are proteolytic enzymes inactive when first generated?
They would break down the proteins in our cells
What is the earliest point in the GI tract that protein digestion can begin?
In the stomach with HCL - hydrolysis of peptide bonds
Where do most protein digestion occurs?
In the small intestine with the help of activated zymogens
Where does the trypsin protease cleaves?
What kind of protease is it?
at AA with positive charges (Arg, Lys)
Endo protease
Where does the pepsin protease cleaves?
What kind of protease is it?
Phe, Tyr, Glu, Asp
Endo protease
Where does the amino peptidases cleave?
What kind of protease is it?
At the amino terminus (N terminus)
Exo protease
Where does the carboxy protease cleave?
What kind of protease is it?
At the carboxy terminus (C - terminus)
Exo protease
How do AA enter cells?
Via Na-coupled transporters
How do Na-AA coupled transporters work?
Do they require energy?
They work via the Na+ gradient
There is more Na+ outside the cell so it travels into the cell and carries along AAs
It doesn’t require energy/ATP
How does peptide absorption work?
Does it require energy?
Peptides are carried from the lumen of the small intestine to the enterocyte via peptide carriers in the brush border membrane
Energy is required
Peptides are metabolized into free AA in the enterocyte
Only free AA are absorbed into the blood
What is kwashiorkor disease
Protein deficiency in the diet
symptoms: swollen belly, delayed growth, frequent infections
What is Hartnup disease?
hereditary defect in transport of nonpolar AA (especially Trp)
Symptoms: dermatitis, failure to thrive, photosensitivity, intermittent ataxia, nystagmus, tremor
In the following peptide: Met - Cys - Pro - Met - Leu - Glu -/- Gln - Pro - Leu
The “/” represents a cleavage site for which enzyme?
A. Trypsin
B. Pepsin
C. Carboxypeptidase A
D. Aminopeptidase
B.
AA on the amino terminus end is the important one ie Glu. Since Glu is negative then it is recognized by pepsin
What are 3 pathways that produce ammonia?
- Deamination of AAs
- Purine nucleotide cycle
- Bacteria in the gut
Which system is very sensitive to high ammonia levels?
CNS
Where is nitrogen transported from and in what form?
Where is it transported to?
From peripheral tissues
as amino acid
Liver
How is Nitrogen excreted?
as urea
What enzymes or pathways are involved in the processing of nitrogen?
transaminases glutamate dehydrogenases glutamine synthases glutaminase urea cycle (critical)
What is transamination?
Transfer of amino group to an a-keto acceptor
Which AAs do not undergo transamination
Lysine and threonine
What is the cofactor needed in transamination?
pyridoxal phosphate (pyridoxine, vit B6)
What are two enzymes involved in transamination and where are they?
Alanine aminotransferase (ALT) Aspartate aminotransferase (AST) high in the liver
Why does increases ALT and AST in the blood indicate liver damage?
Because these enzymes are in the liver. If they’re found in the blood then some damaged happened to the liver and they leaked out
Is transamination reversible
highly reversible
Where does glutamate dehydrogenase reaction occurs?
Mitochondria of most cells
Is glutamate dehydrogenase reaction reversible?
yes
What is the main purpose of glutamate dehydrogenase?
major source of free ammonia for the urea cycle
What is another source of N for the urea cycle?
Aspartate
How is N transported to the liver via glutamine?
- Glutamine is produced in skeletal muscle from glutamate
- It is converted back to glutamate in the liver and NH3 is produced
- GluDH releases NH3 from Glu, leaving a-KG
How is N transported to the liver via alanine?
Alanine is produced in skeletal muscle from pyruvate
- it is converted back to pyruvate in the liver, NH3 produced
- NH3 goes to urea cycle, pyruvate goes to gluconeogenesis etc
What are the important AAs for the urea cycle?
- Glutamate - directly gives N group
- Glutamine/Aspartate
3, Alanine
What is the importance of the urea cycle?
Produces urea .Major means of detoxifying/disposing of ammonia
Where does the urea cycle occurs?
Liver (mitochondria/cytosol)
What are the reactions of the urea cycle?
- Synthesis of carbamoyl phosphate
- Formation of citrulline
- Formation of arginosuccinate
- Formation of arginine
- Formation of urea and ornithine
Where does the first N comes from in the urea cycle?
Where does the second N comes from?
Free ammonia
Asp in the cytosol
What is the rate limiting step in the urea cycle
NH4+ + HCO3- –> carbamoyl phosphate
catalyzed by CPSI
How is the urea cycle regulated?
- Substrate availability (ammonia and other substrates)
- allosteric activation of CPSI by N-acetylglutamate
- induction/repression of the synthesis of urea cycle enzymes
How does N-acetylglutamate regulated by CPSI
It is an allosteric activator
Which state, Fed or fasting, is urea highest?
fasting
A poisoning victim with severe liver damage might present with elevated blood levels of which of the following metabolites?
A. Carbamoyl phosphate
B. Ammonium
C. Urea
D. ATP
B
Elevated blood urea nitrogen (BUN) levels
kidney disease
b/c high protein leads to increased AA metabolism, excess N produces elevated urea in blood
In kidney failure, kidneys are unable to remove urea from blood
Decreased BUN levels
low protein diet
malnutrition
liver failure
B/c low protein/malnutrition leads to overall decreased AA metabolism, less N produces low urea in the blood
In the liver, decreased urea cycle, liver does not package NH3 into urea
Urea cycle disorders
hyperammonemia (elevated NH4 in blood)
symptoms include lethargy, slurred speech, seizures, coma
Effects of high concentration of ammonia
would drive glutamine synthase
glutamate + ATP + NH3 –> glutamine + ADP + Pi
This would deplete glutamate - a neurotransmitter and precursor for synthesis of the neurotransmitter GABA
What would affect glutamate dehydrogenase reaction to reverse?
depletion of glutamate and high ammonia level
glutamate + NAD (P)
What is the result of depleting a-ketoglutarate
impair energy metabolism in the brain
