Lipid metabolism Flashcards
Why do lipids produce more energy than carbohydrates?
Are more reduced (i.e. contain less O and more H per C atom) so release more energy when oxidised.
Name the 3 classes of lipids.
- Fatty acid derivatives
- Hydroxy-methyl-glutaric acid derivatives (C6 compound)
- Vitamines
Give examples (inc their role) of fatty acid derivatives.
- Fatty acids (fuel molecules)
- Triacylglycerols (fuel storage and insulation)
- Phospholipids (membrane components and plasma lipoproteins)
- Eicosanoids (local mediators)
Give examples (inc their role) of hydroxy-methyl-glutaric acid derivatives.
- Ketone bodies (C4) (water-soluble fuel molecules)
- Cholesterol (C27) (membranes and steroid hormone synthesis)
- Cholesterol esters (cholesterol storage)
- Bile acids and salts (C24) (lipid digestion)
Give examples of lipid-soluble vitamins.
Vitamins A, D, E and K
What is the major dietary and storage lipid in the body? Where are they stored?
- Triaglycerols
- Hydrophobic so stored in anhydrous form in a highly specialised storage tissue = adipose tissue
Describe the structure of triaglycerols.
Consist of 3 fatty acids (usually long chain of about 18C) esterified to glycerol (can be broken down via lipolysis).
What are triaglycerol fuel stores mainly used for?
Prolonged aerobic exercise, stress situations such as starvation and during pregnancy
How are triaglycerol stores controlled?
Hormonal control
- Storage promoted by insulin.
- Storage reduced by glucagon, adrenaline, cortisol, growth hormone, thyroxine
What happens to triglycerides in the GI tract after ingestion?
Hydrolysed by pancreatic lipase in the small intestine to release glycerol + fatty acids. Process also requries bile salts and colipase.
What happens to the glycerol produced from triaglycerol hydrolysis in the GI tract?
Dissolves into bloodstream and is transported to liver where it is metabolised (e.g. reused for triaglycerol synthesis or used in glycolysis via DHAP intermediate)
Why are some fatty acids essential dietary components, and others non-essential?
- Saturated fatty acids (no C-C double bonds) = non-essential components as can be synthesised from carbohydrates and certain amino acids.
- Unsaturated fatty acids (>1 C=C double bonds) = essential components as cannot be synthesised in body - mammals cannot introduce a double bond beyond C9.
E.g. arachidonic acid = important polyunsaturated fatty acid as is starting point for synthesis of eicosanoid local mediators, inc prostaglandins.
How are fatty acids produced by lipolysis in the GI tract transported to adipose tissues? And then from adipocytes to tissues to be used as energy?
- converted to lipoproteins - chylomicrons - in gut epithelial cells to allow diffusion in blood.
- bound non-covalently to albumin (= non-esterified FAs or free FAs).
Which tissues can or can’t use fatty acids as an energy source?
- Many tissues inc. liver, heart muscle and skeletal muscle can.
- RBCs can’t as don’t possess mitochondria to perform FA beta-oxidation.
- WBSc can’t as mitochondria are involved in oxidative burst for ROS generation.
- CNS cells (brain and spinal cord) can’t as FAs do not readily cross the blood-brain barrier.
Why does low extracellular [glucose] result in fatty acid release as an alternative fuel?
- Fat store in adipose tissue is dynamic - triglyceride/FA cycle: triglycerides continuously broken down (lysis) to FAs and recombined with glycerol-1-P (esterification) to reform triglyceride.
- Released glycerol moves to liver so need a constant supply of glucose to regenerate glycerol-1-P store.
- So low extracellular [glucose] in starvation reduces [glycerol-1-P] and ability to synthesise triglycerides.
- So FA released into circulation (bound to albumin).
