Protein Breakdown and Urea formation Flashcards
What is the equation of growth?
Growth = Synthesis – breakdown
This applies for all aspects of the body e.g individual proteins, single cells, or whole tissues
Describe metabolism of amino acid - storage, the components and what is done with the carbon skeleton?
Amino acids are used or broken down but not stored.
They have two parts: a carbon skeleton and nitrogen.
The carbon skeleton is involved in energy metabolism and biosynthetic pathways
Describe the removal of nitrogen
- Nitrogen is toxic so has to be removes safely.
- In mammals the nitrogen is converted to the non-toxic neutral compound urea and excreted in the urine.
- Amino acid nitrogen is transferred to urea in three steps: transamination, formation of ammonia and formation of urea.
- (note urea cannot be formed in muscle as the enzyme is not present, the carbon skeleton can be obtained however and used for energy).
- Urea contains 48% nitrogen by weight, protein contain 16% therefore 1g of urea is formed from 3g of protein.
Describe Transamination - first step of urea formation
Step 1: Transamination
• In transamination, the nitrogen as part of the α-amino group is transferred to an α-keto-acid to become a new amino acid.
• α-ketoglutarate, pyruvate and oxaloacetate are α-keto acids.
• The enzymes that do this are transaminases, there are quite a lot of different types of transaminases. The most important are the alanine (ALT) and aspartate (AST) transaminases. As explained above, they transfer an amino group from an AA to an α-keto acid.
• ALT allows:
o Alanine + α-ketoglutarate pyruvate and glutamate
• AST allows:
o Aspartate + α-ketoglutarate Oxaloacetate and glutamate
• α-ketoglutarate, pyruvate and oxaloacetate can be oxidised or converted to make glucose (supplementing gluconeogenesis).
• Glutamate is a way the body can transport potentially toxic nitrogen.
• In the context of urea formation although the reactions of these two enzymes are almost fully reversible the equilibrium of the ALT is towards the formation of pyruvate and glutamate. The equilibrium of the AST however is over towards the left and the generation of aspartate and α-ketoglutarate.
• The transaminases are primarily liver enzymes so can be used diagnostically, high levels of AST and ALT in the blood are indicative of liver damage (as normally shouldn’t be found in plasma).
• So, if we input the alanine and α-ketoglutarate into the diagram above we get ->
• Alanine donates its α-amino group to α-ketoglutarate to give glutamate and pyruvate. This reaction requires vitamin B6.
Describe Step 2: Formation of Urea and why this reaction occurs and is important
So, what happens to this glutamate?
• Glutamate can release the ammonia by action of a second enzyme, glutamate dehydrogenase that is present in the mitochondrial matrix (transamination occurs in cytosol).
• It will yield back α-ketoglutarate.
• The reaction is fully reversible and either NAD or NADP can be used, however it is usual for NADH to be used for degradation and NADPH for synthesis.
• Other amino acids transfer their alpha amino group to alpha ketoglutarate this then passes it on to glutamate.
• This process is called oxidative deamination.
• 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.
• All this combined simply, appears as below ->
- In this process you have the transamination to glutamate and then the oxidative deamination back to α-ketoglutarate, but what is significant is what happens in this process itself, rather than the simply the fact its converted.
- The reason it is 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.
Describe Step 3: Elimination of free ammonia
- Glutamate formed as a result of transamination can be used to transport another nitrogen which is part of an ammonium ion.
- Glutamate + NH4+ + ATP Glutamine + ADP
- Glutamine synthase
- Glutamine is like another transport molecule, a way in which the body can transport the potentially toxic N to the liver
- This is a reaction that often takes place in the periphery, glutamine synthase is widely distributed, esp. in blood vessels with a lot of protein breakdown including those blood vessels of the liver itself.
- Note the reaction goes both ways so we can resynthesise the glutamate from the glutamine.
Give an overview of the urea cycle
- (Glutamate, glutamine and ammonia come together to form the urea).
- The urea cycle is a metabolic pathway that is the means for excreting nitrogen. It is restricted in its distribution, being predominantly in the liver. It does not exist in the muscle.
- It takes place in the mitochondria and cytoplasm of hepatocytes. The urea cycle uses as its substrate’s bicarbonate, aspartate and ammonium ions (released from glutamine or glutamate).
- The bicarbonate comes from the breakdown of the carbon skeleton (i.e. CO2 as a by-product of metabolising the carbon skeleton is formed into bicarbonate which is then used to obtain the CO2 needed).
- Here is urea, it has two nitrogen atoms. One of them 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.
- Both the urea cycle and TCA cycle are linked. This is shown below ->
How is urea finally formed?
- The 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.
- Carbamoyl phosphate then reacts with Ornithine to produce Citrulline.
- Citrulline reacts with Aspartate to form Arginino-succinate.
- Arginino-succinate then is metabolised to Arginine and Fumarate.
- The Arginine is acted upon by the enzyme arginase which is how ultimately it is formed.
- The fumarate is converted to Malate which is transported back into the mitochondria and converted into oxaloacetate. The process then continues.
- The starting point could be thought to be Aspartate, formed by the transamination of α-amino acids, when reacting oxaloacetate (another type of keto-acid) with an α-amino acid. And the formation of the Carbamoyl phosphate.
Describe how the muscle gets rid of amino acids
- The 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.
- Carbamoyl phosphate then reacts with Ornithine to produce Citrulline.
- Citrulline reacts with Aspartate to form Arginino-succinate.
- Arginino-succinate then is metabolised to Arginine and Fumarate.
- The Arginine is acted upon by the enzyme arginase which is how ultimately it is formed.
- The fumarate is converted to Malate which is transported back into the mitochondria and converted into oxaloacetate. The process then continues.
- The starting point could be thought to be Aspartate, formed by the transamination of α-amino acids, when reacting oxaloacetate (another type of keto-acid) with an α-amino acid. And the formation of the Carbamoyl phosphate.
Describe the fate of the carbon skeleton
- There are two sorts of amino acids, the keto-genic amino acids and the gluco-genic amino acids. The keto-genic AAs will form ketone bodies, while the gluco-genic AAs can be used by the liver to produce glucose.
- Some AAs are in both categories.
