Amino acid catabolism Flashcards
Most AA are metabolized in the __. So what has to be done?
Most AA are metabolized in the liver. So they have to be transported in order to be utilized
What are the 2 products of AA metabolism? What are their destinies?
- Ammonia – recycled (in the liver) or excreted
- Carbon skeleton – TCA cycle or Gluconeogenesis or ketogenesis
Where do transamination reactions occur?
Transamination reaction can occur in all cells, including liver cells
What is the general amino acceptor?
α-Ketoglutarate
Are aminotransferases specific?
Aminotransferases are AA specific (e.g. alanine aminotransferase) PLP acts as cofactor for these enzymes (recall: Glycogen phosphorylase)
What is the net lost of amino groups in transamination?
There is NO net loss of amino group in transamination reactions- amino groups are simply TRANSFERRED from AA to alpha-ketoglutarate
What is the role of PLP?
PLP acts as cofactor for aminotransferases enzymes (recall: Glycogen phosphorylase)
Most amino acids are metabolized in the __
Most amino acids are metabolized in the liver
Describe what happens to amino groups of most amino acids in the liver. Where exactly in the liver?
In the cytosol of liver cells (hepatocytes), amino groups from most amino acids are transferred to α-ketoglutarate to form glutamate, which enters mitochondria and gives up its amino group to form NH4+ .
What are the four amino acids play central roles in nitrogen metabolism? How?
glutamate, glutamine, alanine, and aspartate
These particular amino acids are the ones most easily converted into citric acid cycle intermediates:
glutamate and glutamine to α-ketoglutarate
alanine to pyruvate
aspartate to oxaloacetate.
Glutamate and glutamine are especially important, acting as a kind of general collection point for amino groups.
What usually happens to excess amino groups in skeletal muscles?
In skeletal muscle, excess amino groups are generally transferred to pyruvate to form alanine, another important molecule in the transport of amino groups to the liver.
Alanine is transported to the liver where it is converted into pyruvate by the removal of amino group
What happens to excess ammonia in tissues?
Excess ammonia generated
in most other tissues is converted to the amide nitrogen of glutamine, which passes to the liver, then into liver mitochondria
What is transamination?
Amino group of AA acid is transferred to alpha-ketoglutarate, so only carbon backbone is left
Amino group on alpha-ketoglutarate produces glutamate
This is done by amino transferase. Each AA has its specific amino transferase
___ acts as a buffer system that accepts all the amino groups
Glutamate acts as a buffer system that accepts all the amino groups
What can carbon skeleton of AA be used for?
Carbon skeleton can go into new AA synthesis or TCA cycle
Liver cells have a lot of __ which can accept amino groups
Liver cells have a lot of alpha-ketoglutarate which can accept amino groups
Other tissues send their amino group in the form of __ which arrives to hepatocytes
Other tissues send their amino group in the form of glutamine which arrives to hepatocytes
How is glutamine converted to glutamate?
Glutamine donates a amino group and becomes glutamate
__ is a universal amino group donor
Glutamate is a universal amino group donor
How’s amino acid turned into Keto acid?
Amino group of amino acid is transferred by Pyridoxal phosphate (Vit B6) to a-ketoglutarate turning it into Glutamate
Amino acid then becomes a keto acid
What is PLP
PLP (Pyridoxal phosphate) is a B6 vitamin - it is also a glycogen phosphorylase cofactor
Transfers amino group
Transamination has to be followed by __ so the ___
Transamination has to be followed by deamination so the amino group is excreted
Describe how does deamination occur? Is it reversible
Glutamate dehydrogenase enzyme removes the amino group from glutamate, so the amino group can be excreted in the urea cycle
Glutamate is converted to alpha-ketoglutarate
Reversible
Where does deamination occur?
Mitochondria
What can alpha-ketoglutarate
generated by deamination be used for?
Used for transamination, TCA cycle or gluconeogenesis
Describe the transport, conversion and excretion of amino group
All tissue send their Amino groups in the form of glutamine which is degraded in the liver->amino group is excreted through liver
All those cells can convert glutamate to glutamine via glutamine synthetase enzyme
This conversion of glutamate in glutamine occurs in all tissues
This glutamine is transported to the liver where there’s an enzyme glutaminase that converts glutamine to glutamate so this amino group that came from other tissue can go to excretion or recycle pathway and other pathways
Where’s glutaminase synthetase and glutaminase found?
Glutaminase synthetase is found in all tissues glutaminase is found on liver mitochondria
What is so specific about muscles in terms of AA metabolism?
Muscles have additional mechanism in the form of alanine
They can use pyruvate- only muscles can do it
Convert pyruvate to alanine by getting the amjno group from glutamate (instead of converting glutamate into glutamine)
Describe glucose alanine cycle
Glutamate can be converted to glutamine for transport to the liver or it can transfer its amino group to pyruvate, a readily available product of muscle glycolysis, by the action of alanine aminotransferase, forming Alanine
Alanine can be send out from muscles-> it will be brought back to the liver
In the cytosol of hepatocytes, alanine aminotransferase transfers the amino group from alanine to a-ketoglutarate, forming pyruvate and glutamate. Glutamate can then enter mitochondria, where the glutamate dehydrogenase reaction releases NH4+, or can undergo transamination with oxaloacetate to form aspartate, another nitrogen donor in urea synthesis,
Urea cycle begins in the __ and continues in the __
Urea cycle begins in the mitochondria and continues in the cytoplasm
__ is the main way of transport of amino group in the body
Glutamine is the main way of transport of amino group in the body
How is ammonia generated in intestines? How and where is it delivered from there?
Bacterial digestion in large and small intestines generates some amount of ammonia gas which arrive to the liver cells through portal system
__ is the main amino group donor
Glutamine is the main amino group donor
What is the initiation reaction of urea cycle?
Amino group has to be first carbonated to form carbamoyl phosphate- requires a lot of energy - 2 ATP (1 for enzymatic reaction and one for addition of phosphate group)- this energy is used by carbamoyl phosphate synthetase I
Aspartate acts similar to glutamate- it accepts an amino group (oxaloacetate + amino group = aspartate)
What is are the initial reactants of urea cycle and their sources?
NH4+
The first amino group to enter the urea cycle is derived from ammonia in the mitochondrial matrix. The liver also receives some ammonia via the portal vein from the intestine, from the bacterial oxidation of amino acids.
Describe the prep step of urea cycle
Prep stepAmino group (NH4+) is first carboxylated to form carbamoyl phosphate in the matrix by carbamoyl phosphate synthetase I, a regulatory enzyme- requires a lot of energy - 2 ATP (1 for enzymatic reaction and one for addition of phosphate group)
Which reactions of the urea cycle occur in the matrix?
- Carboxylation of NH4+ by carbamoyl phosphate synthetase I to form carbamoyl phosphate
Describe the urea cycle
Carbamoyl phosphate enters the matrix
1. Carbamoyl phosphate donates its carbamoyl group to ornithine to form citrulline, with the release of Pi. The reaction is catalyzed by ornithine transcarbamoylase.
2. The citrulline produced in the first step of the urea cycle passes from the mitochondrion to the cytosol. The next two steps bring in the second amino group.
3. The source is aspartate generated in mitochondria by transamination and transported into the cytosol. A condensation reaction between the amino group of aspartate and carbonyl group of citrulline forms argininosuccinate. This cytosolic reaction, catalyzed by argininosuccinate synthetase, requires ATP
4. The argininosuccinate is then cleaved by argininosuccinase to form free arginine and fumarate, the latter being converted to malate before entering mitochondria to join the pool of citric acid cycle intermediates. This is the only reversible step in the urea cycle.
In the last reaction of the urea cycle the cytosolic enzyme arginase cleaves arginine to yield urea and ornithine. Ornithine is transported into the mitochondrion to initiate another round of the urea cycle.
ornithine + __ = __
ornithine + carbamoyl group = citrulline
Which reactions of urea cycle occur in the mitochondria
- Prep phase- carboxylation of amino group to form carbamoyl phosphate
- Transfer of carbamoyl group to ornithine to form citrulline
How many amino groups participate in urea cycle?
2
What is the source of the 2nd amino group in urea cycle?
aspartate generated in mitochondria by transamination and transported into the cytosol
What carries the 2 amino groups in urea cycle?
Arginine
Arginine - 2 _ __ = __
Arginine - 2 amino groups = ornithine
Each amino cycle __ amino groups are excreted in the form of urea
Each amino cycle 2 amino groups are excreted in the form of urea
Which enzyme of the urea cycle is a regulatory enzyme?
carbamoyl phosphate synthetase I- mitochondrial isoform
How’s carbamoyl phosphate synthetase I regulated?
Allosterically activated by high Acetyl-CoA and glutamate - positive regulation
When does expression of urea cycle enzymes increase?
- High protein diet: high levels of AA metabolism
- Starvation: body metabolizes protein in the muscles
- Uncontrolled diabetes: plenty of glucose around is not sensed by cells (lack of insulin) -> body breaks down its own protein
Tumor: body metabolizes protein in tissues
How’s urea cycle and TCA connected
Urea cycle cannot continue without TCA cycle happening, especially in the cytoplasm
This is because urea cycle requires aspartate which comes from oxaloacetate (TCA intermediate)
oxaloacetate is taken away from TCA
What is the problem with the transport of fumarate from cytosol to mitochondria? How is it solved?
There is no transporter to directly move the fumarate generated in cytosolic arginine synthesis back into the mitochondrial matrix. However, fumarate can be converted to malate in the cytosol.
Only malate can be transported to mitochondria, thus fumarate produced in the cytoplasm during urea cycle is converted by malate dehydrogenase to malate
What is the fate/transport of asparate
Aspartate formed in mitochondria by transamination between oxaloacetate and glutamate can be transported to the cytosol, where it serves as nitrogen donor in the urea cycle reaction catalyzed by argininosuccinate synthetase.
Describe fumarase and malate dehydrogenase
There are 2 enzymes important for this process: fumarase and malate dehydrogenase
Malate dehydrogenase converts malate in oxaloacetate and back- reversible. Oxaloacetate can then be converted to aspartate again- recycling of aspartate needed for Urea cycle
Fumarase converts fumarate to malate and back- also reversible
These 2 enzymes are present both in the mitochondria and cytoplasm
How does recycylign of TC Ametabolites mostly occur?
Recycling of metabolites happens mainly through arginosuccinate when it is converted to arginine-> fumarate is released
Fumarate can be converted into malate which comes back to TCA
Amino acids yielding __ are ketogenic
amino acids yielding __ are glucogenic
Amino acids yielding acetyl-CoA are ketogenic
amino acids yielding other end products are glucogenic
Name AA which are both ketogenic and glucogenic
Isoleucine Phenylalanine Threonine Tryptophan Tyrosine
Metabolism of any AA lead to one of the __-
Metabolism of any AA lead to one of the TCA intermediates
__ and __ are the only AA that are only ketogenic
Lysine and Leucine are the only AA that are only ketogenic
Name AA that are used as fuel in muscle/adipose/brain/cardiac tissues What are they called
Leucine, Isoleucine and Valine
They are called Branched chain amino acids
What is the 1st step of AA catabolism?
First step of AA catabolism is transfer of the NH3 usually to ⍶- ketoglutarate to yield L-glutamate
What can AA be degraded to?``
Amino acids are degraded to pyruvate, acetyl-CoA, α- ketoglutarate, succinyl-CoA, and/or oxaloacetate
Describe how is glutamate dehydrogenase regulated
Positive regulation- ADP (sign of low energy)
Negative regulation: GTP
How are all 4 enzymes ore urea and carbamoyl phosphate synthase I regulated
by nutritional status (diet and starvation)
Is high energetic requirements of urea cycle detrimental?
High energy (ATP) used by urea cycle is offset by NADH (=2.5 ATP) from malate oxaloacetate conversion reaction in TCA
How many amino groups does urea contain?
2
Which substances can AA yield?
Pyruvate, a-ketoglutarate, succinyl-coa, oxaloacetate, fumarate,
Branched chain aminotransferase is only expressed in
Extra-hepatic cells ( outside liver)
