Biochemistry Chapter 14: Fat as Fuel

Biological Functions of Lipids

• Components of cell membranes (phospholipids and cholesterol)
• Precursors of hormones (cholesterol and arachidonic acid)
• Long-term fuels (triglycerides)

Efficiency of Triglycerides as Fuel

• Compact storage - triglycerides stored as large fat droplets in adipose tissue
• Large body stores - 70 kg adult has 11 kg fat (as TG) and 120 g glycogen in liver
• Efficiency on weight basis - 1 g fat yields 38 kJ, 1 g protein yields 21 kJ, and 1 g carbohydrate yields 17 kJ

Structure of Triglyceride Fat

• Triglyceride fat (triacylglycerols) composed of glycerol and three fatty acids
• Common fatty acids: palmitic acid (C 16:0), stearic acid (C 18:0), oleic acid (C 18:1), and linoleic acid (C 18:2)

Breakdown of Stored Triglyceride Fat

• Lipase breaks down triglyceride fat into fatty acids and glycerol
• Adrenaline and glucagon activate lipase
• Fatty acids travel in plasma bound to albumin
• Glycerol diffuses in blood stream to all tissues

Metabolism of Glycerol

• Glycerol is water soluble and taken up by all tissues
• Enters glycolysis pathway for conversion to pyruvate, then into TCA cycle for oxidation to CO2

Fatty Acid Metabolism by β-Oxidation Pathway

• All reactions occur in the mitochondrial matrix
• Intermediates present as CoA thioesters
• Biological energy of fatty acid molecule conserved as transfer of 2 H atoms to cofactors NAD+ and FAD
• Series of four enzyme reactions results in removal of two carbon units as acetyl CoA

Activation of Long Chain Fatty Acids

• Fatty acyl-CoA synthetase in cytosol adds CoA to fatty acid
• Coenzyme A forms thioester bonds with carboxylic acids

Transport of Fatty Acyl-CoA into Mitochondria

• Carnitine shuttle system transports fatty acyl-CoA into mitochondria
• Two stages: Stage 1 - Fatty acyl-CoA diffuses across the outer mitochondrial membrane, and Stage 2 - Carnitine is switched back for CoA by carnitine acyltransferase II

β-Oxidation Pathway

• Overview: Fatty acyl-CoA with 10 carbons undergoes β-oxidation producing acetyl-CoA and a fatty acyl-CoA that is 2 carbons shorter
• One round of β-oxidation produces acetyl-CoA and a fatty acyl-CoA that is 2 carbons shorter
• Four reactions: 1) Removal of 2 H atoms, 2) Addition of water, 3) Removal of 2 H atoms, and 4) Removal of 2 C units

Regulation of Fat Metabolism

• Release of fatty acids from adipose tissue activated by adrenaline and glucagon
• Rate of entry into mitochondria via carnitine shuttle
• Rate of reoxidation of cofactors NADH and FADH2 by Electron Transport Chain

Metabolism of Odd Numbered Fatty Acids

• β-Oxidation results in the production of propionyl-CoA
• Propionyl-CoA-carboxylase converts propionyl-CoA to methylmalonyl-CoA
• Methylmalonyl-CoA mutase converts methylmalonyl-CoA to succinyl-CoA

Ketone Body Formation

• Ketogenesis occurs when fat metabolism is the main source of energy
• Fatty acid oxidation in hepatocytes leads to high concentrations of Acetyl CoA - exceeds capacity of the TCA cycle
• Excess Acetyl CoA is converted into ketone bodies (acetoacetate and β hydroxybutyrate) in liver
• Ketone bodies can be utilised for energy by most tissues

This quiz covers the topic of fat as fuel in biochemistry and molecular biology, specifically chapter 14. It assesses understanding of the role of fat in metabolic processes.