ML6: Stage I & II catabolism

Question 1

What are the classes of lipids?

Answer 1

• Fatty acid derivatives
  
  • Fatty acids – fuel molecules
  • Triacylglycerols – fuel storage and insulation
• Hydroxy-methyl-glutaric acid derivatives
  
  • Ketone bodies (C4) – water soluble fuel molecules
• Vitamins
  
  • A, D, E, and K

Question 2

What are triacylglycerides (TAGs)? Are they (overall) hydrophobic, amphipathic, or hydrophilic?

Answer 2

Major dietary and storage lipids

Consist of three fatty acids esterified to glycerol

Overall hydrophobic

Question 3

What happens to dietary lipids?

Answer 3

1. Lipids are emulsified in the small intestine
   Bile salts
2. Digestion of lipids by lipases
   TAGs – digested by pancreatic lipase
   Cholesterol esters – digested by cholesterol esterase
   Phospholipids – digested by phospholipases
3. Absorption by intestinal mucosal cells
4. Transportation to tissues
   Consumer tissues – fatty acid oxidation
   Adipose tissue – storage

Question 4

Give an overview of fatty acid catabolism (β-oxidation).

Answer 4

• Occurs in mitochondria
• FA cycles through sequence of reactions
• C₂ removed at each cycle
• 1 NADH and 1 FADH₂ produced in each cycle
• All intermediates linked to coenzyme-A

Question 5

Give the stages of fatty acid catabolism (β-oxidation).

Answer 5

1. Activation of fatty acids
   Occurs in the cytoplasm
   FA actiated by linkage to coenzyme-A
   Catalysed by acyl CoA synthetase
   fatty acid + coenzyme A → fatty acyl CoA
2. Transport of fatty acyl CoA into mitochondria
   Fatty acyl group linked to carnitine
   Transported across the membrane by translocase protein
   Fatty acyl CoA reformed in the matrix
   Rate limiting step for faty acid oxidation
3. Oxidation of fatty acyl CoA
   A series of four reactions
   1 NADH, 1 FADH₂ and 1 acetyl-CoA are generated in each round
   Each round shortens the chain by two carbon atoms
   Cycle continues until only acetyl-CoA remains

Overall: released 106 molecules of ATP

Question 6

What are essential amino acids? Name them.

Answer 6

Amino acids that cannot be synthesised

His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val

Question 8

What are the stages of digestion and absorption of proteins?

Answer 8

• Dietary protein digestion begins in the stomach
  
  • Denaturation
  • Pepsin
• Further digestion in the lumen of the intestine
  
  • Chymotrypsin, trypsin, carboxypeptidase – pancrease aminopeptidase
• Transport into intestinal cells and into blood

Question 9

What happens to excess amino acids? Why and how?

Answer 9

They cannot be stored so have to be degraded

• Transfer of amino group to α-ketoglutarate
  Uses aminotransferases
• Dehydrogenation to produce ammonium ion
  Uses glutamate dehydrogenase

e. g. aspartate + α-ketoglutarate produces oxaloacetate and glutatamate
e. g. alanine + α-ketoglutarate produces pyruvate + glutatamate

Question 10

How are ammonium ions removed?

Answer 10

The urea cycle

Toxic NH₄⁺ is converted to urea and excreted

Question 11

Are proteins produced for a specific function or synthesised as a store of excess amino acids?

Answer 11

They are produced for a specific function

Question 12

How can energy be provided during starvation?

Answer 12

By breaking down muscle

Question 13

Explain the nitrogen balance.

Answer 13

Positive nitrogen balance
Intake of nitrogen (protein) > loss
During active growth, pregnancy, tissue repair

Negative nitrogen balance
Intake of nitrogen (protein) < loss
During starvation