BIOC Lecture 7: Nitrogen II

Question 1

When does amino acid catabolism occur?

Answer 1

• Excess protein is consumed
• Insufficient dietary protein
• Insufficient dietary energy

Question 2

What happens in the situations where amino acid catabolism occurs?

Answer 2

Carbon skeletons enter the main energy pathways and amino groups are processed to urea in the liver for excretion

Question 3

What do aminotransferase enzymes catalyse?

Answer 3

Transaminase reactions to transfer amines

Question 4

What are aminotransferase enzymes sometimes called?

Answer 4

Transaminases

Question 5

How do aminotransferases work?

Answer 5

• Generate a keto acid from original AA
• Pyridoxamine phosphate is now in N bound form
• It then transfers N to another keto acid
• New AA is formed

Question 6

How do we make new AA’s?

Answer 6

An interchanging of N groups to other keto acids

Question 7

What organ has the highest concentration of aminotransferase enzymes?

Answer 7

Liver

Question 8

The receiving keto acid (the one that picks up the nitrogen) is generally one of what three keto acids?

Answer 8

• α-Ketoglutarate
• Oxaloacetate
• Pyruvate

Question 9

Where is Pyridoxamine phosphate derived from?

Answer 9

Vitamin B6

Question 10

Where is Pyridoxamine phosphate found?

Answer 10

In the active center of the transaminase enzyme

Question 11

What is the amino acid - keto acid pair for a-ketoglutarate?

Answer 11

Glutamate

Question 12

What is the amino acid - keto acid pair for oxaloacetate?

Answer 12

Aspartate

Question 13

What is the amino acid - keto acid pair for pyruvate?

Answer 13

Alanine

Question 14

The deamination of most amino acids leads to the production of what?

Answer 14

Glutamate, aspartate or alanine

Question 15

What AA is produced the most?

Answer 15

Glutamate as a-ketoglutarate is the most prominent

Question 16

Aminotransferases are specific for the donor amino acid but mostly only accept?

Answer 16

a-ketoglutarate or oxaloacetate as the donor keto acid

Question 17

Are aminotransferases reactions reversible?

Answer 17

Yes - dependent on concentration of substrates

Question 18

What do aminotransferases require?

Answer 18

the co-enzyme pyridoxal-phosphate (derived from the vitamin B6) which is involved in transfer of the amino group

Question 19

What do aminotransferases allow for?

Answer 19

Generation of AA’s in short supply and safe removal of excess amino groups

Question 20

What can glutamate form?

Answer 20

Glutamine

Question 21

What is the glutamate-glutamine reaction catalysed by?

Answer 21

Glutamine synthetase (GS)

Question 22

What are glutamate and glutamine very important in?

Answer 22

Brining the nitrogen back from tissues

Question 23

Why is glutamine special?

Answer 23

Because it can carry two amine groups, therefore is very efficient in terms of transferring nitrogen

Question 24

How does the reaction of glutamate become glutamine?

Answer 24

• Catalysed by GS
• Fixes another ammonia group onto glutamate structure
• Requires ATP

Question 25

When is the glutamate-glutamine reaction highly driven?

Answer 25

When there is high levels of ammonia - you don't want free ammonia in the tissues

Question 26

What is the reaction that converts glutamine back to glutamate catalyzed by?

Answer 26

Glutaminase

Question 27

What happens in the reaction from glutamine to glutamate?

Answer 27

- Glutamine loses an amino group - Release of ammonia - Ammonia is then used to make urea in the liver

Question 28

What is the loss of an amino group called?

Answer 28

Oxidative deamination

Question 29

What is the reaction of glutamate losing its amino group catalyzed by?

Answer 29

Glutamate dehydrogenase (GDH)

Question 30

Glutamate dehydrogenase is one of very few enzymes that can use...

Answer 30

NAD or NADP

Question 31

What ways can an amino group be removed?

Answer 31

1. Transaminations 2. Oxidative deamination 3. Hydrolysis

Question 32

What is hydrolysis?

Answer 32

a molecule is split into two parts by the addition of a water molecule

Question 33

What is oxidative deamination?

Answer 33

the removal of an amino group from an amino acid, producing a keto acid and free ammonia

Question 34

Where are glutamine and alanine prominent?

Answer 34

In plasma

Question 35

How is Nitrogen transported in most tissues?

Answer 35

Most tissues in terms of nitrogen metabolism, the nitrogen for transfer to the liver will be contained in glutamate - The glutamine synthetase reaction will occur to give you glutamine

Question 36

What happens to glutamine after it is produced in most tissues?

Answer 36

It will then go back to the liver

Question 37

What happens to glutamine in the liver?

Answer 37

- It will undergo the glutaminase reaction which will release one ammonia group - Glutamate can also then undergo the glutamate dehydrogenase reaction to release its second ammonia - That ammonia will be used to make urea

Question 38

What is the glucose-alanine cycle?

Answer 38

a metabolic pathway that facilitates the exchange of metabolites between muscle and liver

Question 39

What does the glucose-alanine cycle allow?

Answer 39

The muscles to transfer the amino group from amino acid catabolism to the liver, where it can be safely converted into urea and excreted

Question 40

What is the main amino acid coming out of muscle?

Answer 40

Alanine

Question 41

Why does lots of alanine come out of the muscle?

Answer 41

Skeletal muscle relies a lot on glycolysis for energy, there is high levels of pyruvate being formed, therefore high levels of alanine being produced

Question 42

What happens in the muscle during the glucose-alanine cycle?

Answer 42

- Amino acids undergo transamination to form glutamate - Glutamate donates its amino group to pyruvate forming alanine - Alanine is then transported to the liver

Question 43

What happens when alanine is transported to the liver?

Answer 43

- Alanine undergoes transamination again to form glutamate and pyruvate. - Glutamate can then undergo oxidative deamination, releasing ammonia (NH₃) and regenerating α-ketoglutarate - ammonia is converted to urea and excreted

Question 44

What happens to the pyruvate generated by alanine in the glucose-alanine cycle?

Answer 44

used to synthesize glucose through gluconeogenesis.