Protein Turnover & Amino Acid Catabolism
16 important questions on Protein Turnover & Amino Acid Catabolism
What happens to excessive amino acids?
The amino groups go through the urea cycle and their carbon skeletons are transformed into acetyl CoA, acetoacetyl CoA, pyruvate or one of the intermediates of the citric acid cycle.
The carbon skeletons are converted into glucose, glycogen and fats.
What does pyridoxal phosphate do?
Which 3 enzymes are involved in ubiquitination and what do they do?
- E2: switches places with E1 (so ubiquitin is now bound to E2)
- E3: determines specificity. Binds to E2 and the substrate. Catalyzes the transfer of ubiquetin to substrate.
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Which are the 2 pathways by which proteins can be degraded?
2. Ubiquitin/proteasome system: for short-lived nuclear & cytosolic proteins and improperly folded proteins.
What is the signal for ubiquitination?
* K48-linked --> proteasomal degradation
What are the signals for ubiquitination?
- Destruction box: sequence
- PEST sequence
- Hydrophobic sequences: no consensus sequence. The hydrophobic sequence is exposed to the outside (usually on the inside).
Where are amino acids degraded?
Branched-chain amino acids (Leucine, Isoleucine and Valine) are also degraded in the muscle
What is the first step of amino acid degradation?
- Starts with a transamination reaction in which tha a-amino group of an amino acid is transferred to a-ketogluterate to form glutamate, which is then oxidatively deaminated to yield NH4+ and a-ketogluterate.
1. Transamination: A-amino group + a-ketogluterate --> glutamate + a-ketoacid
2. Oxidative deamination: Glutamate --> NH4+ + a-ketogluterate
What does glutamate dehydrogenase do?
* Glutamate dehydrogenase can utilize both NAD+ and NADP+ as electron acceptor.
* Glutamate dehydrogenase is present in the liver and is localized in mitochondria. This compartmentalization ensures that that toxic NH4+ is efficiently removed.
Which amino acids can be deaminated directly and how?
- Done by dehydratases
Serine --> pyruvate + NH4+
Threonine --> a-ketobutyrate + NH4+
How do peripheral tissues (muscles) transport nitrogen to the liver?
- Muscles lack the enzymes for the urea cycle
- NH4+ is converted into Alanine and Glutamine.
Alanine and Glutamine serve as non-toxic transport forms of nitrogen between muscle and liver --> glucose-alanine cycle
- Alanine aminotransferase:
Glutamate + Pyruvate --> a-ketogluterate + Alanine
- Glutamine synthetase:
NH4+ + Glutamate + ATP --> Glutamine + ADP + Pi
How does to urea cycle go?
1. NH4+ + HCO3- + 2ATP --> Carbamoyl-phosphate + 2ADP + Pi
2. Ornithine + carbamoyl-phosphate --> Citrulline
- Cytoplasm:
3. Citrulline + Aspartate + ATP --> Arginosuccinate + AMP + PPi
4. Arginosuccinate --> Arginine + Fumarate
5. Arginine + H2O --> Ornithine + Urua
How is the first step of the urea cycle (synthesis carbamoyl phsphate) regulated?
- Acetyl CoA + Glutamate --> N-Acetylglutamate (stimulated by Argenine)
- Feed-forward activation: Arginine stimulates the conversion of Acetyl CoA and glutamate into N-Acetylglutamate. Argenine is also an intermediate of the urea cycle and product of protein degradation. So, high argenine concentration signals that nitrogen is produced.
How is the urea cycle linked to gluconeogenesis?
Into which 7 molecules are the carbon skeletons of the degraded amino acids converted? And what is their main goals?
2. Acetyl CoA
3. Acetoacetyl CoA
4. A-ketogluterate
5. Succinyl CoA
6. Fumarate
7. Oxaloacetate
Glucose or citric acid cycle
What are ketogenic aminoacids?
They are for the formation of ketone bodies and fatty acid synthesis.
- Leucine and Lysine
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