LECTURE 10 Introduction to lipid metabolism and oxidation of fatty acids I V. SRIDEVI 28.02.2012

OBJECTIVES Brief overview of the metabolism of lipids Transport of fatty acids into mitochondria ß Oxidation of fatty acids and energetics of ß oxidation of fatty acids.

Brief overview of the metabolism of lipids The major aspects of lipid metabolism are Fatty Acid Oxidation to produce energy and Synthesis of lipids (lipogenesis) Lipid metabolism is closely connected with carbohydrate metabolism which may be converted to fat.

Brief overview of the metabolism of lipids… Enzymic degradation of dietary lipids by pancreatic enzymes in the small intestine to produce glycerol and fatty acids.

Triacylglycerol degradation: Triacylglycerol molecules are acted upon by an esterase, pancreatic lipase, which preferentially removes the fatty acids at carbons 1 and 3. The primary products of hydrolysis are a mixture of monoacylglycerol and free fatty acids

Dietary lipids (Triacylglycerol) Monoacylglycerol Fatty acids

Fate of free fatty acids: Free fatty acids can be taken up by adipocytes. Adipocytes can re-esterify free fatty acids to produce triacylglycerol molecules, which are stored until the fatty acids are needed by the body. Alternatively, the free fatty acids are taken up by cells. Most cells can oxidize fatty acids to produce energy.

Fate of glycerol: Glycerol or monoacylglycerol which is released from triacylglycerol is used almost exclusively by the liver Glycerol is used to produce glycerol 3-phosphate Glycerol 3-phosphate is converted to dihydroxyacetone phosphate which can enter glycolysis or gluconeogenesis.

Fate of Glycerol

Oxidation of fatty acid 1. Hydrolysis of triacylglycerol by lipase (lipolysis) Occurs in the cell cytosol of adipose cells. The hydrolysis of triacylglycerol produces glycerol and free fatty acids. The FFA travel in the blood bound to albumin and are taken up by muscle or liver cells for oxidation. 2. Activation of fatty acids Before they can be oxidized, fatty acids are activated by attachment to CoA to form acyl CoA molecules; this takes place in the cell cytosol. 3. Transport into mitochondria β-oxidation occurs in the mitochondrial matrix. The acyl CoA molecules are transported into the mitochondria by the carnitine shuttle. 4. β-oxidation Fatty acids are degraded by a cyclical sequence of four reactions oxidation, hydration, oxidation, thiolysis.

Fatty Acid Breakdown The major pathway for the catabolism of fatty acids is a mitochondrial pathway called β-Oxidation. Occurs in mitochondrial matrix. Fatty acids are converted to their acyl coenzyme A (CoA) derivatives and then degraded by successive removal of acetyl CoA (two-carbon) units from the end of the fatty acid. Saturated fatty acids of even numbered carbon chains. Pathway produces Reoxidation of FADH2 Reoxidation of NADH. Oxidation of acetyl coA in the TCA cycle

I . Activation of fatty acids. Fatty acid CoA ATP(2) Thiokinase (Acyl coA synthase) AMP + PPi Acyl CoA II. Transport of fatty acids into mitochondria. Cytosol Inner Mitochondrial mitochondrial matrix Acyl CoA Carnitine Carnitine Acyl CoA carnitine Carrier carnitine acyl transferase I protein acyl transferase II CoA Acyl carnitine Acyl carnitine CoA Carnitine acyl carnitine translocase

Carnitine shuttle

Carnitine shuttle Carnitine acyl transferase I(CAT- I) otherwise called as Carnitine palmitoyl transferase I(CPT-I) Carnitine acyl transferase II(CAT- II) otherwise called as Carnitine palmitoyl tranferase II(CPT-II). Carnitine can be obtained from diet -primarily from meat products. Also can be synthesized from amino acid lysine and methionine by an enzymatic pathway found in liver and kidney but not in skeletal and heart muscle.

Reaction Sequence in -Oxidation

Beta Oxidation Acyl coA dehydrogenase Acyl coA dehydrogenase

Beta Oxidation Enoyl coA hydratase

Hydroxy acyl coA dehydrogenase Beta Oxidation Hydroxy acyl coA dehydrogenase

Beta Oxidation Keto acyl coA thiolase

Energetics of palmitic acid oxidation: calculate the ATP production when (16 carbon) is completely oxidized. Mechanism ATP yield 1. -oxidation 7 cycles I step liberates 2 ATP(2x7) 14 III step gives 3 ATP(3x7) 21 2. From 8 acetyl CoA Oxidized by citric acid cycle, each acetyl CoA 96 provides 12 ATP Total energy from one molecule of palmitate 131 Energy utilized for activation in the I step -2 Net yield of oxidation of one molecule of palmitate 129 Palmitic acid C16H32O2                                  

Beta Oxidation

Beta Oxidation

Beta Oxidation

Lipid Metabolism

Use the criteria for understanding and describing all pathways Names: Functions: Substrates: Product: Control Enzymes: Regulation: Compartment(s): Tissues of interest:

Learning Outcomes By the end of lecture, students should be able to: Explain the role of carnitine in the transport of fatty acids into the mitochondrion for oxidation. Describe how fatty acids can be used as fuels via oxidative conversion (especially β-oxidation) to acetyl CoA.