CATABOLISM OF AMINO ACIDS AND UREA CYCLE

Question 1

State 5 examples of glucogenic non essential amino acids.

Answer 1

1. Glutamate
2. Glutamine
3. Cysteine
4. Serine
5. Alanine

Question 2

State 3 examples of glucogenic essential amino acids.

Answer 2

1. methionine
2. threonine
3. valine

Question 3

What is the difference between ketogenic and glucogenic amino acids

Answer 3

Ketogenic amino acids are amino acids that form acetyl CoA or acetoacetylCoA. It forms precursors for ketone bodies. It is important for ketogenesis. Ketogenic amino acids are lysine and leucine.

While glucogenic amino acids are amino acids that can be converted into glucose via gluconeogenesis. It forms glucose precursors. It is important in gluconeogenesis. It includes most essential and non- essential amino acids.

Question 4

List the characteristics of ketogenic amino acids

Answer 4

• amino acids that form acetyl CoA or acetoacetylCoA
• precursors for ketone bodies
• important for ketogenesis
• such as lysine and leucine

Question 5

List the characteristics of glucogenic amino acids

Answer 5

• amino acids that form glucose via gluconeogenesis.
• precursors for glucose
• important in gluconeogenesis
• such as glutamate, glutamine, cysteine, serine and alanine.

Question 5

Explain the removal of amino group from amino acids

Answer 5

• The catabolism of amino acids involved the removal of alpha amino groups followed by the breakdown of the resulting carbon skeleton.
• The carbon chain are either converted to fats or carbohydrates.
• The removal of amino acids via 2 ways which are transamination or deamination.
• Transamination is the transferring of amino group to the alpha ketoglutarate.
• While deamination is the removal of amino group from the amino acids
• The amino acids are removed via 4 enzymes which are transaminases, oxidative deaminases, non oxidative deaminases and L and D amino acids deaminases.
• Transaminase such as SGOT and SGPT
• Oxidative deaminases such as glutamate dehydrogenase
• Non oxidative deaminase such as dehydratase

Question 6

State the 4 enzymes that can removed the amino acids

Answer 6

1. SGOT
2. SGPT
3. glutamate dehydrogenase
4. dehydratase

Question 7

State how the amino acids are removed

Answer 7

1. Transamination
2. Deamination

Question 8

Why the level of ammonia in the blood is low?

Answer 8

The level of ammonia in the blood is low because

1. rapid removal of ammonia as urea by the liver
2. many tissues especially muscle release amino acid in the form of glutamine, alanine, or asparagine rather than free ammonia.

Question 9

Explain the catabolism process of glutamine and state the type of amino acid

Answer 9

• In most tissue, glutamine sequesters ammonia in a non toxic form.
• Glutamate which is catalyzed by the glutamine synthetase is converted to glutamine through amination process in most tissues
• Glutamine will act as a carrier for amino group in the liver.
• In the liver, the glutamine is catalysed by the glutamate dehydrogenase enzyme is converted to glutamate back through deamination process.
• In this deamination process, ammonia is released and transferred to the urea cycle.
• This is to ensure the ammonia is eliminated as urea by the liver.
• Glutamine is a glucogenic non- essential amino acid.

Question 10

Explain the catabolism process of alanine and state the type of amino acids.

Answer 10

• Pyruvate which is catalysed by the alanine aminotransferase is converted to alanine through transamination process.
• The amino donor would be glutamate or aspatate.
• If the amino donor is glutamate, then the product would be a-ketoglutarate.
• If the amino donor is aspartate, then the product would be oxaloacetate.
• The alanine produced just now will be transferred from the muscle to the liver.
• In the liver, the alanine will undergoes transamination process to be converted to pyruvate once again with the help from alanine aminotransferase.
• The pyruvate formed will be converted to glucose in the liver.
• This is to ensure that the glucose can be used by the muscle in glucose-alanine process.
• Alanine is a non- essential glucogenic amino acids.

Question 11

Free ammonia in the body is liberated by….. (clue: enzyme)

Answer 11

1. Glutamate dehydrogenase (oxidative deaminase)
2. Amino acid dehydratase (non oxidative deaminase)
3. D- and L- amino acid oxidase enzymes

Question 12

Explain the catabolism of phenylalanine and state the type of amino acids.

Answer 12

• Phenylalanine which is catalysed by the phenylalanine hydroxylase is converted to tyrosine with the help from the tetrahydrobiopterin and oxygen which act as cofactors.
• tetrahydrobiopterin and oxygen are reduced to dihydrobiopterin and water.
• The tyrosine then undergoes multiple reaction and produce fumarate and acetoacetate.
• Phenylalanine is an essential glucogenic and ketogenic amino acids.

Question 13

Where does urea synthesis occur?

Answer 13

Urea synthesised occur in mitochondria and partly in cytosol of liver, released into the blood and cleared by kidneys.

Question 14

What does urea synthesised required?

Answer 14

• 4 molecules of ATP
• 1 NH4+
• 1 CO2
• a- amino nitrogen from aspartate
• carbamoyl phosphate synthetase 1 is the rate limiting enzyme

Question 15

Explain glutamine and asparagine as buffer.

Answer 15

• Glutamine and asparagine provide a non toxic storage and transport form of ammonia.
• Glutamine formation occurs mainly in muscle and liver but more importantly in the nervous system where it is the main method for removal of ammonia from the brain.
• Circulating glutamine is hydrolyzed in kidneys by glutaminase to free glutamic acid + NH3.
• Similarly, asparagine is hydrolysed by the asparaginase to aspartic acid + NH3.
• NH3 picks up a H+ and is excreted as NH4+ thus buffering blood pH.

Question 16

Explain the urea cycle.

Answer 16

• Firstly, urea cycle occur at the matrix mitochondria and cytosol of liver.
• Carbomyl phosphate (CP) is formed when ammonium (NH4+) is condensed with bicarbonate (CO2) and this process required 2 ATPs.
• The process for formation of carbomyl phosphate is catalysed by the carbomyl phosphate synthetase I (CPS1).
• Ornithine combine with CP will produce citrulline and this process is catalysed by the enzyme ornithine transcarbomylase.
• Then, the citrulline is transferred from the mitochondria to the cytosol of the liver.
• In the cytosol of liver, the citrulline is converted to argininosuccinate which is catalysed by the argininosuccinate synthase and this process hydrolyses the ATP.
• The a-amino of aspartate act as the 2nd nitrogen in this process.
• Next, the argininosuccinate is cleaved to fumarate and arginine which is catalysed by the enzyme argininosuccinate lyase.
• The fumarate is transferred back to the mitochondria so it will reenter the TCA cycle.
• While the arginine will cleave to ornithine and urea. This process is catalysed by the enzyme arginase.
• Urea will be excreted out through urine by the kidney
• While ornithine will be transferred back to the mitochondria.

Question 17

How the urea cycle is regulated?

Answer 17

• The urea cycle is regulated by the allosteric regulation of carbomyl phosphate synthetase I (CPS1) by NAG.
• It also regulated by induction of the urea cycle enzymes due to the increased protein metabolism such as fasting and high protein diet
• Lastly, it is regulated by the substrate availability where high ammonia will produce high urea.

Question 18

Explain regarding the allosteric regulator of urea cycle

Answer 18

• Argininosuccinate will cleave to arginine and fumarate.
• The arginine will stimulate the N- acetylglutamate (NAG) which going to activate the carbamoyl phosphate synthetase I (CPS1)
• The high amount of NAG can be seen when there is high amount of protein diets as the diets supply glutamate as well as arginine.