Protein Breakdown and Urea Formation Flashcards
What is nitrogen balance?
Nitrogen balance is a measure of nitrogen input minus nitrogen output.
What are the two parts of an amino acid?
- the carbon skeleton
- nitrogen
The carbon skeleton is broken down by energy metabolism and biosynthetic pathways.
Why do we need to remove nitrogen, and what is it converted into to be removed?
Nitrogen is toxic, so it has to be removed safely.
In mammals, the nitrogen is converted to the non-toxic, neutral compound urea and excreted in urine.
What are the three steps in which amino acid nitrogen is transferred to urea?
- transamination
- formation of ammonia
- formation of urea
Describe transamination.
It’s a chemical reaction that transfers an amino group to a keto acid to form new amino acids.
Transaminase (aka aminotransferases) is the enzyme involved in this reaction. It catalyses a transamination reaction between an amino acid and a α-keto acid.
The nitrogen group of one amino acid is transferred to a particular keto acid to give us a second amino acid.
The synthesised molecules can be metabolised more readily.
Give some examples of α-keto acids.
- α-ketoglutarate
- pyruvate
- oxaloacetate
α-keto acids are important metabolic intermediates. They can be oxidised or converted to glucose.
Name two important aminotransferases and the chemical reaction that they catalyse.
- ALANINE (ALT)
Alanine will react with α-ketoglutarate to give pyruvate and glutamate. In the context of urea formation, this reaction predominates. - ASPARTATE (AST)
Aspartate will react with α-ketoglutarate to give oxaloacetate and glutamate. In the context of urea formation, the opposite of this reaction predominates.
Both generate glutamate, and both reactions are fully reversible. These reactions require pyridoxal phosphate derived from Vit B6.
How can the levels of transaminases be used diagnostically?
Transaminases are primarily liver enzymes, so high levels of ALT and AST in the blood can be indicative of liver damage (since they’re not meant to be released into circulation).
What happens to the glutamate after transamination?
Glutamate can release ammonia directly by the action of Glutamate Dehydrogenase.
The reaction is fully reversible and can use either NAD or NADP; however, it is usual for NAD to be used for degradation and NADPH for synthesis.
Glutamate is a very useful molecule because it is freely interchangeable with the α-keto acids as well as the ability to donate and accept ammonium ions.
What is the significance of having the transamination to glutamate and then the oxidative deamination back to α-ketoglutarate of amino acids?
The reason it’s very important is because it allows conversion of many amino acids from their original state into glutamate, which can be transported (note it is not often transported as glutamate) and then re-converted back into something the body can use for energy (or transamination again) while re-synthesising the ammonia which can be fed into the urea cycle.
How do we eliminate free ammonia?
Free ammonia generated in tissue combines with Glutamate to give Glutamine.
Glutamate + NH4+ + ATP –> Glutamine + ADP
This reaction is catalysed by Glutamine Synthase.
What is the importance of glutamine in the transport of nitrogen?
Glutamine is the main transporter of nitrogen.
It’s formed from glutamate which, in addition to having already accepted amino-groups to α-ketoglutarate, accepted more nitrogen to form glutamine.
Glutamine can donate nitrogen for the biosynthesis of amino acids, nucleotides, amino sugars and NAD+.
Describe the structure of urea.
Urea is made up of two amine groups joined to a C=O. One amine group is donated from aspartate, while the other comes from glutamine/glutamate.
The carbon C=O comes from the carbon skeleton, through using CO2 that has been produced from its breakdown.
Hence, the detrimental products of amino acid degradation can be used to combine to form urea, a non-toxic, soluble compound that can be readily excreted.
The urea cycle and the TCA are interlinked. With that in mind, describe the urea cycle.
CO2 comes from the bicarbonate and reacts with the ammonium ion that has come from glutamine/glutamate (formed by transamination of α-ketoglutarate and α-amino acid). They form carbamoyl phosphate (in the mitochondria).
Carbamoyl phosphate then reacts with Ornithine to produce Citrulline.
Citrulline reacts with Aspartate to form Argininosuccinate.
Argininosuccinate then is metabolised to Arginine (urea cycle) and Fumarate (TCA).
The Arginine is acted upon by the enzyme arginase which is how, ultimately, urea is formed. The urea cycle continues.
The Fumarate is converted to Malate which is transported back into the mitochondria and converted into oxaloacetate. The TCA then continues.
How are muscles involved in the breakdown of amino acids?
Muscles don’t have the enzymes needed to form urea, so the urea cycle doesn’t take place in muscles. However, muscles do break down amino acids for energy during prolonged exercise or starvation.
