Amino Acid Metabolsim

Question 1

Give an overview of the Amino Acid Metabolism

Answer 1

Question 2

What are the roles of amino acids?

Answer 2

1. building blocks of proteins.
2. precursors of biologically important compounds.
3. under certain conditions (starvation) energy can be extracted from amino acids.
4. utilised during extremely prolonged (2-6 hrs) endurance activity.

Question 3

Why are some amino acids essential?

Answer 3

1. 20 amino acids in the human body.
2. non-essential amino acids can be synthesised.
3. essential amino acids are obtained from the diet.

Question 4

Describe amino acid metabolism.

Answer 4

amino acids contain nitrogen -> they exist as NH3 which is degraded and converted to urea for disposal -> carbon skeletons of alpha-keto acids may be metabolised to CO2 + H2O, glucose, fat Acetyl CoA, and ketones.

Question 5

What are the sources and fate of amino acids?

Answer 5

3 sources: body protein turnover, dietary proteins and synthesis of non-essential amino acids.

they enter amino acid pool: circulatory system, liver, and skeletal muscle are principal compartments - each compartment is in equilibrium with the other, so short term deficiencies can be made up from other compartments.

results are:
1. body protein synthesis
2. glucose & glycogen
3. ketones, fatty acids, and steroids.
4. carbon dioxide
5. synthesis of porphyins, creatine, neurotransmitters, purines, pyrimidines.

Question 6

why excess amino acids cannot be stored?

Answer 6

surplus amino acids are converted to metabolic fuel (not as effective as fat or glycogen)
the nitrogen containing amine group must first be removed.
degradation of amino acids - 2 stage process.

Question 7

Describe the first stage of amino acid degradation.

Answer 7

1. transamination = transfer of an amino acid group from the amino acid (AA1) to a keto acid (generally alpha-ketoglutarate) leaving a new alpha-keto-acid derived from AA1.
   - transamination occurs in the liver.
   - amino acceptor is alpha-keto glutarate + amino = glutamate (amino acid)
   - alpha-keto glutarate is an intermediate in the Krebs cycle.
   - efficient if all amino acids are transaminated to form glutamic acid as don’t need separate pathways for all 20 amino aicds.

Summary: NH2 amino acids 1 + alpha-ketoglutarate -> glutamate (NH2) + ketao acid 1

Question 8

Describe the second stage of amino acid degradation.

Answer 8

2. oxidative deamination - occurs in the mitochondrial matrix.
   - glutamate is deaminated back to alpha-ketoglutarate and electrons are transferred to NAD -> NADH
   - provides intermediate (alpha-ketoglutarate) for Krebs cycle and NADH can be used to generate ATP.

Question 9

What are the sources of ammonia?

Answer 9

1. from AA - tissues (primarily liver) - via transamination and oxidative deamination.
2. from bacterial degradation of urea to ammonia in the intestinal lumen - absorbed -> blood -> liver -> urea
3. from amines in diet and breakdown of amine hormones and neurotransmitters.
4. from purines and pyrimidines.

Question 10

Describe the transports of ammonia.

Answer 10

1. Urea formation (liver) - most important in NH3 disposal.
2. present in only in low levels in blood.
3. many tissues transport ammonia in the form of certain AA (glutamine has two amino groups).

Question 11

Give an overview of the Urea cycle.

Answer 11

Question 12

Summary of amino acid degradation

Answer 12

1. supplies energy
2. supplies intermediates for the synthesis of other compounds.
3. removes excess amino acids
4. where glucose supply is adequate, pyruvate, amalate and oxaloacetate can be converted to fatty acids.
5. carbon skeletons funnelled - ends up as pyruvate or TCA cycle intermediates & excess nitrogen excreted via urea.

Question 13

What is the fate of alpha-keto acid (carbon skeletons)

Answer 13

• after removal of the nitrogen group most amino acid residues appear as pyruvate or as a Krebs Cycle intermediate
• the carbon atoms can be converted to glucose (gluconeogenic amino acids) or to ketones or to acetyl-CoA (ketogenic amino acids)

During fasting and starvation, catabolism of amino acids is very important for maintaining blood glucose levels, essential for brain and kidney function.

Question 14

Describe gluconeogenic amino acids

Answer 14

1. gluconeogenesis when liver cannot store sufficient glycogen to maintain BG - as glycogenolysis falls the liver increases the rate of gluconeogenesis.
2. prolonged exercise creates a condition analogous to fasting.
3. expensive way of gaining energy because: proteins are an expensive component of the diet
4. if the protein is derived from body tissue the cost is even dearer!