Protein Breakdown and Urea Formation

Question 1

What is positive nitrogen balance?(3)

Answer 1

• Amino acids pool is driven toward body protein synthesis.
• Growth, pregnancy
• Exercise and tissue hypertrophy, and response to anabolic hormones

Question 2

What is negative nitrogen balance?(3)

Answer 2

Increased production of protein breakdown and urea production
• Response to either an increase catabolic or a reduction of anabolic hormones (e.g. in diabetes)
• Wasting diseases, burns, trauma

Question 3

What is the difference between carbohydrates and amino acids?

Answer 3

amino acids are either used or broken down but not stored

Question 4

What is the carbon skeleton used for?(2)

Answer 4

• Energy metabolism

* Biosynthetic pathways

Question 5

What is the nitrogen is for?(2)

Answer 5

– Nitrogenous compounds

– Urea

Question 6

Why do we need to remove nitrogen, and what is it converted into to be removed?(2)

Answer 6

→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.

Question 7

What are the three steps in which amino acid nitrogen is transferred to urea?

Answer 7

→transamination
→ formation of ammonia
→formation of urea

Question 8

Describe transamination(6)

Answer 8

→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.
6. • Process requires vitamin B6.

Question 9

What are some alpha keto acids and what can they be oxidized to?(4)

Answer 9

→ α-ketoglutarate
→pyruvate
→oxaloacetate
→α-keto acids are important metabolic intermediates. They can be oxidised or converted to glucose.

Question 10

What are two important aminotransferases and what are the chemical reaction that they catalyse?

Answer 10

→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 pyruvate .

→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.

Question 11

What happens to OAA and alpha-ketoglutarate in the liver?

Answer 11

• In the liver OAA and α-ketoglutarate can be used to make glucose

Question 12

What happens to pyruvate in the TCA cycle?

Answer 12

• In the muscle pyruvate can be used in the TCA cycle and ETC to make ATP

Question 13

How can the levels of transaminases be used diagnostically?(2)

Answer 13

→Transaminases are primarily liver enzymes, so high levels of ALT and AST in the blood can be indicative of liver damage

→they’re not meant to be released into circulation

Question 14

Which amino acids do not undergo transamination and why?

Answer 14

1. threonine and lysine and proline

2. If lysine were transaminated it would cyclize into a toxic non metabolite

Question 15

What happens to the glutamate after transamination?(4)

Answer 15

• Glutamate can form ammonia directly by the action of glutamate dehydrogenase in the mitochondrial matrix.
• This is known as oxidative deamination too produce ammonia and a-ketoglutarate
• This pathway can be linked directly to the transamination reaction.
4. fully reversible and can use either NAD or NADP however it is usual for NAD to be used for degradation and NADP for synthesis

Question 16

why is having the transamination to glutamate and then the oxidative deamination back to α-ketoglutarate of amino acids important?(3)

Answer 16

→ conversion of many amino acids from their original state into glutamate

→ can be transported and then re-converted back into something the body can use for energy (or transamination again)

→ re-synthesizing the ammonia that is fed into the urea cycle

Question 17

How is free ammonia generated?(2)

Answer 17

1 .Free ammonia generated in non-hepatic tissue combines with glutamate to give glutamine and ADP.
2. The enzyme is glutamine synthase

Question 18

How is free ammonia eliminated?(3)

Answer 18

→Free ammonia generated in tissue combines with Glutamate to give Glutamine.

→Glutamate + NH4+ + ATP –> Glutamine + ADP

→This reaction is catalysed by Glutamine Synthase.

Question 19

What is the importance of glutamine in the transport of nitrogen?(3)

Answer 19

→Glutamine is the main transporter of nitrogen.

→ 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+.

Question 20

Which amino acids transport nitrogen?

Answer 20

Alanine and glutamine

Question 21

Describe the structure of urea(4)

Answer 21

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.

→detrimental products of amino acid degradation can be used to combine to form urea, a non-toxic, soluble compound that can be readily excreted.

Question 22

Describe the urea cycle(6)

Answer 22

1. The CO2 from the bicarbonate and ammonium ions from the glutamine react and form carbamoyl phosphate
2. Carbomyl phosphate in the mitochondria reacts with ornithine to form citrulline.
3. The citrulline moves out of mitochondria to cytosol and combines with aspartate to form arginino-succinate
4. Argininosuccinate then is metabolised to Arginine (urea cycle) and Fumarate (TCA)
5. Ornithine is also formed and used again with another carbamoyl.
6. The aspartate is made from the transamination of a-amino acids and OAA. The transferase is a mitochondrial enzyme
7. Fumarate is also formed which is converted to malate then OAA in the mitochondria which reacts with an amino acid in a transaminase react to give aspartate and a-keto-acid.
8. The Arginine is acted upon by the enzyme arginase which is how urea is formed.

Question 23

How are muscle involved in the breakdown of amino acids?

Answer 23

→Muscles don’t have the enzymes needed to form urea, so the urea cycle doesn’t take place in muscles.

→muscles do break down amino acids for energy during prolonged exercise or starvation.

Question 24

Describe the glucose-alanine cycle between the muscle and the liver(6)

Answer 24

→In the muscle, branched amino acids are taken and broken down.

→The carbon skeleton is used for energy production.

→ the NH4 can be used to convert to pyruvate to Alanine. Alanine is then exported into the blood and travels to the liver.

→ alanine is then converted to glutamate via transamination (reacting with α-ketoglutarate) also producing a pyruvate.

→The pyruvate can enter the gluconeogenic pathway to form glucose, and the glucose can be transported in the blood back to the muscle where it can be used for energy.
6. Glutamate loses ammonium for the urea cycle to occur in the liver mitochondria.

Question 25

Describe the transport of ammonia from peripheral tissue to the liver to produce glucose

Answer 25

1. Glutamate picks up free ammonia from tissue and, with ATP and glutamine synthase, become glutamine.
2. Glutamine is transported to the liver and reacts with water (glutaminase) to produce ammonia for urea cycle.
3. Glutamate is also produced, and glutamate dehydrogenase produces ammonia and a- ketoglutarate.
4. a-ketoglutarate undergoes transamination with alanine to produce pyruvate.
5. Pyruvate undergoes gluconeogenesis to produce glucose.

Question 26

What happens to the branched amino acids in the muscle?(2)

Answer 26

1. the carbon skeletons are used for oxidative phosphorylation.
2. the ammonium ions released react with pyruvate to form alanine which is transported in the blood to the liver where it is converted glucose and sent back in the blood to the muscle.

Question 27

What are the two ways in which remaining amino acids are dealt with in the muscle?

Answer 27

→1) Nitrogen is transferred to alanine (via glutamate and pyruvate)

→2) Circulating/intracellular glutamate can be made into glutamine (and return to the liver)