Post-absorption processing of proteins

Question 1

What does the human body do with amino acids?

Answer 1

Protein synthesis, glucose/glycogen, energy (ATP), fatty acids, ketone bodies, nitrogen-containing metabolites.

Question 2

What can histidine be synthesised to?

Answer 2

Histamine

Question 3

Which amino acid is used in the synthesis of serotonin?

Answer 3

Tryptophan

Question 4

Which amino acid is used in the synthesis of dopamine, noradrenaline and adrenaline?

Answer 4

Tyrosine

Question 5

For what compound are both glycine and arginine required?

Answer 5

Creatinine

Question 6

What is Kwashiorkor?

Answer 6

A form of severe protein–energy malnutrition characterized by edema, irritability, anorexia, ulcerating dermatoses, and an enlarged liver with fatty infiltrates.

Question 7

What is the aetiology of kwashiorkor?

Answer 7

Extreme lack of protein causes an osmotic imbalance in the gastro-intestinal system causing ascites and fatty liver. Exchange between lymphatic system and bloodstream is stunted due to inability of the body to effectively overcome the hydrostatic pressure gradient. Proteins are responsible for creating the colloid osmotic pressure (COP) observed in the blood and tissue fluids. The difference in the COP of the blood and tissue is called the oncotic pressure. The oncotic pressure is in direct opposition with the hydrostatic pressure and tends to draw water back into the capillary by osmosis. However, due to the lack of proteins, no substantial pressure gradient can be established to draw fluids from the tissue back into the blood stream. This results in the pooling of fluids, causing the swelling and distention of the abdomen.

Question 8

What waste metabolite is produced from energy reactions involving proteins?

Answer 8

Ammonia

Question 9

What are the symptoms of hyperammonaemia?

Answer 9

Tremor, vomiting, cerebral oedema, coma and death.

Question 10

What can cause hyperammonaemia?

Answer 10

Genetic defects, liver disease etc..

Question 11

Where does disposal of ammonia mainly occur?

Answer 11

Liver

Question 12

What are the three steps in the ammonia disposal pathway?

Answer 12

Transamination, deamination and urea synthesis

Question 13

What is transamination?

Answer 13

Transfer of amino group from AA to ketoglutarate forming glutamate (in most tissues).

Question 14

What is deamination?

Answer 14

Release of ammonia from glutamate (mainly in the liver).

Question 15

How is urea synthesised?

Answer 15

Via the urea cycle in the liver.

Question 16

How is ammonia transported safely to the liver?

Answer 16

As glutamate or glutamine.

Question 17

What happens once glutamate/glutamine reaches the liver?

Answer 17

It is deaminated and ammonia released.

Question 18

What other product is released in the transaminase reaction?

Answer 18

Keto acid

Question 19

Aside from ammonia, what other products are released from the deamination reaction?

Answer 19

NADPH (NADP required) and alpha-ketoglutarate

Question 20

What is pyridoxal phosphate the active form of?

Answer 20

Vit B6

Question 21

Aside from transamination, what else is Vit B6 required for?

Answer 21

Decarboxylation reactions required for neurotransmitter synthesis, haem synthesis, some aspects of energy metabolism and lipid synthesis.

Question 22

What does lack of B6 lead to?

Answer 22

Anaemia (lack of haem), neurological symptoms (lack of neurotransmitters), poor growth, skin lesions, poor immune response (lack of protein synthesis).

Question 23

Where does the urea cycle take place?

Answer 23

Inside the mitochondrion and the cytosol.

Question 24

What is the control step in the urea cycle?

Answer 24

Carbamoyl group transferred to ornithine to form citrulline (inside the mitochondria). This the entry point to the urea cycle.

Question 25

What urea cycle reactions occur in the cytosol?

Answer 25

Second amino group added from aspartate - arginine formed and urea released.

Question 26

How is carbamoyl phosphate synthetase activated?

Answer 26

Allosterically - by N-acetylglutamate, which is formed when glutamate levels are high (ie signalling high levels of protein catabolism).

Question 27

What activates the urea cycle?

Answer 27

Protein catabolism

Question 28

From what is uric acid derived?

Answer 28

Purine nucleotides

Question 29

What can excess uric acid in the blood (hyperuricaemia) lead to?

Answer 29

Deposition of sodium urate crystals in kidneys. Gout.

Question 30

What is the aetiology of hyperuricaemia?

Answer 30

Either overproduction of urate or poor excretion via kidneys.

Question 31

Given examples of high purine foods.

Answer 31

Red meat, seafood, yeast-containing foods inc. beer.

Question 32

What might the carbon skeletons of AAs be used for?

Answer 32

Kreb’s cycle, fatty acid and ketone synthesis, gluconeogenesis.

Question 33

Which amino acids cannot be made into glucose?

Answer 33

Lysine and leucine (ketogenic only).

Question 34

What is a glucogenic amino acid?

Answer 34

Can be degraded to glucose precursors.

Question 35

What is a ketogenic amino acid?

Answer 35

Can be degraded to precursors of fatty acids and ketone bodies.

Question 36

What is gluconeogenesis and where does it mainly take place?

Answer 36

Making glucose from non-carbohydrate intermediates during fasting or exercise. Main site is the liver.

Question 37

What are the possible substrates for gluconeogenesis?

Answer 37

Keto acids (derived from AAs), lactate (from anaerobic glycolysis) and glycerol (from triacylglycerol breakdown).

Question 38

How long do glycogen stores last?

Answer 38

18 hours

Question 39

What are the three main reactions in gluconeogenesis?

Answer 39

Pyruvate -> phosphoenolpyruvate (via several steps)

Fructose-1,6-bisphosphate -> fructose-6-phosphate

Glucose-6-phosphate -> glucose

Question 40

How many ATP molecules are required for gluconeogenesis?

Answer 40

6

Question 41

How is gluconeogenesis controlled?

Answer 41

Low [glucose] and/or low energy = gluconeogenesis

Question 42

What is the Cori cycle?

Answer 42

Converts lactate from rapidly respiring muscle back to glucose.

2lactate -> 2pyruvate -> glucose

Requires 6 ATP

Question 43

What is the function of the glucose - alanine cycle?

Answer 43

Sustains transamination in peripheral tissue.

Question 44

How does alcohol inhibit gluconeogenesis?

Answer 44

Ethanol metabolism produces large amounts of NADH. Tends to cause conversion of pyruvate and alanine to lactate by shifting the position of equilibrium. This prevents the conversion of gyceraldehyde-3-phosphate to glucose.