Amino Acid Deamination and the Urea Cycle Flashcards
Amino acids from degraded proteins can be used to
1) Form new proteins
2) Form glucose during starvation
3) Be used as a source of energy
Before entering TCA cycle/ketogenesis, amino acids need to be altered by
Removal of alpha-amino group of the amino acid
Steps to remove alpha-amino group
T or F: amino acids can be stocked (stored)
False they cant
1st step of removing alpha-amino group
Overall scheme of converting amino acids to be used in the urea cycle
1st step: transamination
Exchange an amino group with a carbonyl group (=O) by amino transferase/transaminase (rxn occurs between a keto acid and amino acid)
Transamination process
Transaminases are bound to
PLP coenzyme (contains pridoxine = vitamin B6)
Transamination (1st stage step 1)
Transfer alpha-amino group of amino acid onto transminase (amino group is carried by coenzyme PLP)
-Reaction is reversible
Transamination (1st stage step 2)
Transaminase-PLP complex is converted to pyridoxamine phosphate (PMP) which role is just to hold the amino group for the next reaction
Transamination (2nd stage Step 1)
Transfer amino group from pyridoxamine phosphate (PMP) complex to alpha-ketoglutarate, forming glutamate
Transmaination (2nd stage step 2)
Once amino group us transferred from PMP to alpha-ketoglutarate, transaminase-PLP complex is formed again and the process can restart
4 facts summarizing Transamination
1) PLP-dependent transamination reactions are reversible
2) Any amino acids can be used as substrate for the first stage (forward) reaction.
3) α-ketoglutarate is the major α-keto acid substrate in the second stage (reverse) reaction.
4) This reaction happens where the protein is degraded or when proteins are ingested/digested and reach the liver
Transport and Oxidative Deamination
4 facts of transferring ammonia to the liver
1) All organs degrade amino acids and produce ammonia (NH3).
2) Ammonia accumulation has toxic consequences
3) Liver is in charge of converting ammonia into urea
4) Ammonia is transported in the blood in the form of glutamine or alanine.
Name 3 ways ammonia is transported to the liver
1) Amino acids: Ingested proteins/cellular proteins
2) Alanine: Muscle
3) Glutamine: Most tissues including muscle
Production of alanine in the muscle steps (Glucose-Alanine Cycle (Cahill cycle))
1) Transaminase reaction using pyruvate generated by glycolysis as a terminal transamination substrate and is catalyzed by alanine transaminase
2) alpha-ketoglutarate is recycled and alanine is generated
3) Alanine will be transported to the liver and converted back to pyruvate
Reversal of alanine and alpha-ketoglutarate reaction is
yielding pyruvate and glutamate
What is the alpha-ketoacid corresponding to alanine
Pyruvate
Glucose-Alanine Cycle (Cahill cycle) in the liver
NOTE this cycle is like the Cori Cycle
Yield of glucose alanine cycle in comparison to cori cycle
Transport of ammonia to the liver using glutamine
1) Glutamate through transmaination is converted to glutamine by glutamine synthetase (invest 1 ATP)
2) Glutamine is carried to liver to be converted back to glutamate
3) In liver glutamine is converted into glutamate by mitochondrial glutaminase
4) Ammonia produced will enter the Urea cycle
Oxidative deamination of glutamate in the liver
Glutamate dehydrogenase catalyzes the deamination which occurs in the mitochondria to produce NH4+ (ammonia) which enters the urea cycle
-Reaction is reversible
-One NADH is generated (1NADPH is used in the reverse reaction)
After oxidative deamination of glutamate, what is the next step
Urea cycle
After oxidative deamination of glutamate, what is the next step
Urea cycle/Krebs-Henseleit Urea cycle
The urea cycle starts and ends with
Ornithine
Ornithine
A carrier on which are assembled the carbon and nitrogen atoms that will constitute urea (carries urea)
Urea Cycle Overview
Urea Cycle Mitochondrial Phase
Urea Cycle Mitochondrial Phase Reaction 1
Condensation of bixarbonate (HCO3-) with NH3 catalyzed by carbamoyl phosphate synthetase 1(CPS1)
-Requires 2 ATP
Urea Cycle Mitochondrial Phase Reaction 2
Ornithine is converted to citrulline catalyzed by ornithine transcarbamylase
Citrulline needs to move out of the mitochondrial matrix to the cytoplasm, so it uses
ORC1 (Ornithine Carrier 1) = bidirectional
Ornithine carrier 1 (ORC1) transports
1) Ornithine
2) Lysine
3) Arginine
4) Citrulline
Urea cycle Cystolic Phase
Reaction 3 Urea cycle (cystolic phase)
Where does the aspartate come from in this photo
Malate-aspartate shuttle (link between krebs and urea cycle)
Reaction 4 Urea cycle (Cystolic Phase)
Argininosuccinase
Reaction 5 Urea cycle (Cystolic Phase)
Arginase
Krebs Bicycle
Urea cycle is regulated at 2 levels
1)Long term regulation through rate of gene synthesis - All 4 enzymes of urea cycle and CPS1 are synthesized at higher rates under starving conditions because protein is a source of fuel and production of ammonia has increased
2)Short term regulation through allosteric control
T or F: Nitrogen flux varies based on your diet
T
2 types of short term regulation for allosteric control of urea cycle
1) N-acetylglutamate is an allosteric activator of carbamoyl phosphate synthetase I (CPS1) synthesized using glutamate and acetyl-CoA and is catalyzed by N-acetylglutamate synthase (requires a high concentration of glutamate)
2) Arginine is an allosteric activator of N-acetylglutamate synthase.
regulating pathway flux by N-acetylglutamate
Lots of arginine means a lot of glutamate means a lot of ammonia
Urea cycle is confined to the
liver
T or F: The above raction soccur only in the mitochondria (ignore)
False, mitochondria and cytosol
Ornithine transport from cytosol to mitochondria is carried out by
translocase
Amino groups of urea are derived from [blanks] while the carbon atom is derived from [blank]
NH3 and asparate…..CO2
T or F: Carbon skeleton products can be used in gluconeogeneis
True
In a study conducted some years ago, cats were fasted overnight then given a single meal complete in all amino acids except arginine.
Within 2 hours, blood ammonia levels increased from a normal level of 18 mg/L to 140 mg/L, and the cats showed the clinical symptoms of ammonia toxicity.
A control group fed a complete amino acid diet or an amino acid diet in which arginine was replaced by ornithine showed no unusual clinical symptoms.
N.B. Cats cant synthesize arginine. They need to get it through their diet.
a) What was the role of fasting in the experiment?
b) What caused the ammonia levels to rise in the experimental group?
c) Why did the absence of arginine lead to ammonia toxicity?
d) Why can ornithine be substituted for arginine?
a) Want the cat to solely rely on the amino acid as a form of energy (no glycogen or glucose left so they rely only on amino acid) = lower blood glucose and catabolism of AA
b) They cant get rid of thee high levels of ammonia (cant break it down) = see image goes into c)
c) Lack of arginine slowed conversion of ammonia into urea because cats synthesize ornithine solely from arginine, so there are insufficient amounts of ornithine to feed the urea cycle
d) Ornithine can be substituted because it is an intermediate of the urea cycle
Most amino acids can be converted to one of 7 metabolic intermediates
Most amino acids can be converted to one of 7 metabolic intermediates
Glucogenic AA can be converted to
pyruvate, α-ketoglutarate, succinyl-CoA or oxaloacetate and are therefore, glucose precursors.
Ketogenic AA can be converted to
acetyl-CoA or acetoacetate to create ketone bodies (Leucine and Lysine)
