Amino Acid and Protein Metabolism

Question 1

Explain the importance of dietary protein?

Answer 1

• only some amino acids can be synthesised in the body, other need to be ingested
• amino acids are required to make proteins and for neurotransmitters, nucleotides, creatine
• they act as a source of blood glucose in fasting and starvation

Question 2

What is the amino acid pool?

Answer 2

All the amino acids in the extracellular fluid.

They are free amino acids and they can mix and exchange with other free amino acids distributed throughout the body.

Question 3

What is the average turnover of amino acids in the body per day?

Answer 3

300g-400g

Question 4

Most proteins have a half life of a few days. Which protein is an exception?

Answer 4

Collagen

Question 5

Can we store protein in the body?

What happens to surplus amino acids?

Answer 5

No storage form of protein

Surplus is rapidly catabolised and the nitrogen excreted as urea

Question 6

How many amino acids do we rely on the diet to give us?

Answer 6

Half (10)

Question 7

What are 3 essential amino acids we need to make?

Answer 7

• alanine
• aspartate
• glutamate

Question 8

What is a healthy nitrogen balance in an adult?

Answer 8

Total nitrogen intake should be equal to the total nitrogen excretion.

Question 9

Give details on positive nitrogen balance

Answer 9

N intake > N excretion
Protein synthesis exceeds the rate of breakdown
During normal growth in children
In convalescence after serious illness
After immobilisation after an accident 
In pregnancy

Question 10

Give details on negative nitrogen balance

Answer 10

N intake < N excretion
In starvation 
During serious illness
In late stages of some cancers
In injury and trauma 
If not corrected and becomes prolonged, there will be irreversible loss of essential body tissue and will ultimately lead to death

Question 11

What happens in the degradation of most cellular proteins?

Foreign proteins?

Answer 11

cellular = recognised as old and removed by ubiqutin breakdown system giving a mixture of the 20 amino acids

Foreign exogenous protein - taken into vesicles by endocytosis or autophagocytosis, the vesicle fuses with lysosomes and proteolytic enzymes degrade proteins into amino acids

Question 12

What is formed from amino acid degradation?

Answer 12

Pyruvate

Question 13

What happens in oxidative deamination?

Answer 13

Water and a coenzyme is added to an amino acid and this forms a keto acid, ammonia and the coenzyme.

Question 14

When transamination occurs, an amino acid is added to 2-oxaglutarate to form what?

Answer 14

an oxo acid and glutamate

Question 15

What do all aminotransferases contain?

Answer 15

A tightly bound prosthetic group which is derived from vitamin B6 and it acts as a carrier of the amino group

Question 16

What is the fate of oxo-acids?

Answer 16

Most of the amino acids become keto acids. They can be metabolised by the TCA pathway to carbon dioxide and water providing ATP. in starvation, 13 of the amino acids can be converted to glucose by the liver (glucogenic)

Question 17

What can leucine and lysine be converted to?

Answer 17

Acetyl CoA (ketogenic)

Question 18

What are the 3 fates of amino acids in rhe body?

Answer 18

• excreted as urea
• turned into special compounds such as neurotransmitters
• turned to keto-acids to be used in TCA cycle

Question 19

What is the role of the liver in nitrogen metabolism?

Answer 19

• removal of amino acids, glucose and fats from blood supply
• absorbed amino acids used for synthesis of cellular proteins
• synthesis of plasma proteins (albumin, clotting factors)
• synthesis of haem and nucleotides
• degradation of excess amino acids by trans deamination
• conversion of ammonia to urea for excretion

Question 20

What tissue degrades proteins to amino acids?

How are the amino groups (NH2) transported in the blood stream?

Answer 20

Skeletal muscle

As glutamine

Question 21

If water is added to glutamine along with glutaminase, what happens?

Answer 21

Ammonia is released and glutamate is formed.

Question 22

What 4 amino acids are important in the inter-organ transport of nitrogen and explain one

Answer 22

• alanine
• glutamate
• glutamine
• aspartate

glutamine - safe carrier of ammonia in the blood. It can carry 2 ammonia equivalents to the lover for urea formation and can deliver ammonium ions to the kidney for pH regulation

Question 23

Review the urea cycle

Answer 23

Look up a diagram

Question 24

What are the end products of nitrogen metabolism?

Answer 24

• urea (protein breakdown)
• creatine (creatine phosphate breakdown)
• uric acid (DNA and RNA breakdown)
• ammonia (control of body pH)

Question 25

What can happen with an impaired conversion of ammonia to urea?

Answer 25

liver failure

genetic defects