Metabolism of lipids: tryacylglycerols, fatty acids, cholesterol and phospholipids metabolism. Ketogenesis and ketolysis. Regulation and pathology of lipid metabolism. Atherosclerosis.
Lipids are water-insoluble organic biomolecules that can be extracted from cells and tissues by nonpolar solvents, e.g., chloroform, ether, or benzene.
Classification of lipids, based on their backbone structures: Simple lipids: Acylglycerols, steroids, waxes. Complex lipids: phospholipids glycerophospholipids, sphingophospholipids. glycolipids glycosylglycerols, glycosphingolipids.
Triacylglycerols (Triglycerides) Fatty acid esters of the alcohol glycerol are called acylglycerols or glycerides; they are sometimes referred to as "neutral fats," a term that has become archaic. When all three hydroxyl groups of glycerol are esterified with fatty acids, the structure is called a triacylglycerol: Triacylglycerols are the most abundant family of lipids and the major components of depot or storage lipids in plant and animal cells. Triacylglycerols that are solid at room temperature are often referred to as "fats" and those which are liquid as "oils."
Storage and Mobilization of Fatty Acids TGs are delivered to adipose tissue in the form of chylomicrones and VLDL, hydrolyzed by lipoprotein lipase into fatty acids and glycerol, which are taken up by adipocytes. Then fatty acids are reesterified to TGs. TGs are stored in adipocytes. To supply energy demands fatty acids and glycerol are released – mobilisation of TGs.
TG hydro-lysis is inhibited by insulin in fed state At low carbohydrate and insulin concentrations (during fasting), TG hydrolysis is stimulated by epinephrine, norepinephrine, glucagon, and adrenocorticotropic hormone. TG hydro-lysis is inhibited by insulin in fed state
Lipolysis - hydrolysis of triacylglycerols by lipases. A hormone-sensitive lipase converts TGs to free fatty acids and monoacylglycerol Monoacylglycerol is hydrolyzed to fatty acid and glycerol or by a hormone-sensitive lipase or by more specific and more active monoacylglycerol lipase
Oxidation of Glycerol Glycerol is absorbed by the liver. Steps: phosphorylation, oxidation and isomerisation. Glyceraldehyde 3-phosphate is an intermediate in: glycolytic pathway gluconeogenic pathways
Isomerase
ATP Generation from Glycerol Oxidation glycerol – glycerol 3-phosphate - 1 ATP glycerol 3-phosphate - dihydroxyaceton phosphate 2.5ATP (1 NADH) glyceraldehyde 3-phosphate – pyruvate 4,5 ATP (1NADH + 2 ATP) pyruvate – acetyl CoA 2.5 ATP (1 NADH) acetyl CoA in Krebs cycle 10 ATP (3NADH + 1 FADH2 + 1GTP) Total 19,5-1 = 18,5 ATP
Reaction sequence in the b-oxidation
Connections to Electron Transport and ATP Connections to Electron Transport and ATP. One turn of the fatty acid spiral produces ATP from the interaction of the coenzymes FAD (step 1) and NAD+ (step 3) with the electron transport chain. Total ATP per turn of the fatty acid spiral is Step 1 - FAD into e.t.c. = 2 ATP Step 3 - NAD+ into e.t.c. = 3 ATP Total ATP per turn of spiral = 5 ATP Example with Palmitic Acid = 16 carbons = 8 acetyl groups Number of turns of fatty acid spiral = 8-1 = 7 turns ATP from fatty acid spiral = 7 turns and 5 per turn = 35 ATP. NET ATP from Fatty Acid Spiral = 35 - 1 = 34 ATP
Fatty Acid Synthesis Occurs mainly in liver and adipocytes, in mammary glands during lactation Occurs in cytoplasm FA synthesis and degradation occur by two completely separate pathways
Three stages of fatty acid synthesis: A. Transport of acetyl CoA into cytosol Acetyl CoA from catabolism of carbohydrates and amino acids is exported from mitochondria via the citrate transport system Cytosolic NADH also converted to NADPH Two molecules of ATP are expended for each round of this cyclic pathway B. Carboxylation of acetyl CoA C. Assembly of fatty acid chain
Sources of NADPH for Fatty Acid Synthesis 1. One molecule of NADPH is generated for each molecule of acetyl CoA that is transferred from mitochondria to the cytosol (malic enzyme). 2. NADPH molecules come from the pentose phosphate pathway.
B. Carboxylation of Acetyl CoA Enzyme: acetyl CoA carboxylase Prosthetic group - biotin A carboxybiotin intermediate is formed. ATP is hydrolyzed. The CO2 group in carboxybiotin is transferred to acetyl CoA to form malonyl CoA. Acetyl CoA carboxylase is the regulatory enzyme.
C. The Reactions of Fatty Acid Synthesis Five separate stages: (1) Loading of precursors via thioester derivatives (2) Condensation of the precursors (3) Reduction (4) Dehydration (5) Reduction
The elongation phase of fatty acid synthesis starts with the formation of acetyl ACP and malonyl ACP. Acetyl transacylase and malonyl transacylase catalyze these reactions. Acetyl CoA + ACP acetyl ACP + CoA Malonyl CoA + ACP malonyl ACP + CoA
Condensation reaction. Acetyl ACP and malonyl ACP react to form acetoacetyl ACP. Enzyme - acyl-malonyl ACP condensing enzyme.
Reduction. Acetoacetyl ACP is reduced to D-3-hydroxybutyryl ACP. NADPH is the reducing agent Enzyme: -ketoacyl ACP reductase
Dehydration. D-3-hydroxybutyryl ACP is dehydrated to form crotonyl ACP Enzyme: 3-hydroxyacyl ACP dehydratase
Reduction. The final step in the cycle reduces crotonyl ACP to butyryl ACP. NADPH is reductant. Enzyme - enoyl ACP reductase. This is the end of first elongation cycle (first round).
In the second round butyryl ACP condenses with malonyl ACP to form a C6--ketoacyl ACP. Reduction, dehydration, and a second reduction convert the C6--ketoacyl ACP into a C6-acyl ACP, which is ready for a third round of elongation.
Final reaction of FA synthesis Rounds of synthesis continue until a C16 palmitoyl group is formed Palmitoyl-ACP is hydrolyzed by a thioesterase
The overall equation for palmitic acid biosynthesis starting from acetyl-S-CoA: 8 Acetyl—S—CoA + 14NADPH + 14H+ + 7ATP + H2O palmitic acid + 8CoA + 14NADP+ + 7ADP + 7P.
Ketogenesis The ketone bodies are acetoacetate b-hydroxybutyrate acetone
Ketogenesis is the process by which ketone bodies are produced as a result of fatty acid breakdown
The ways of formation of active form of glycerol The ways of formation of active form of glycerol. There are two ways of formation of active form of glycerol. 1. Phosphorilation of glycerol through the action of glycerol kinase: ATP + glycerol glycerol 3-phosphate + ADP 2. Reduction of dihydroxyacetone phosphate which is the product of the aldolase reaction of glycolysis. Dihydroxyacetone phosphate is reduced to glycerol 3-phosphate by the NAD-linked glycerol-3-phosphate dehydrogenase of the cytosol: Dihydroxyacetone phosphate + NADH + H+ glycerol 3-phosphate + NAD
Biosynthesis of triacylglycerols The first stage in triacyglycerol formation is the acylation of the free hydroxyl groups of glycerol phosphate by two molecules of fatty acyl-CoA to yield first a lysophosphotidic acid and then a phosphatidic acid:
Lysophosphotidic acid Phosphatidic acid
The activity of acetyl-CoA carboxylase depends on its phosphorylation status . In its inactive form acetyl-CoA carboxylase is phosphorylated in serine, whereas the active form is not phosphorylated.
The phosporylation of acetyl CoA carboxylase is catalyzed by an AMP-dependent protein kinase (AMPK). High AMP levels induce the phosphorylation and inactivation of acetyl-CoA carboxylase.
Pathway of cholesterol biosynthesis
Bile acids perform such functions: eliminating cholesterol from the body; driving the flow of bile to eliminate catabolites from the liver; emulsifying lipids and fat soluble vitamins in the intestine; and aiding in the reduction of the bacteria flora found in the small intestine and biliary tract.
Obesity is a major risk factor for coronary heart disease, which can lead to heart attack.
Atherosclerosis Atherosclerosis - the process in which deposits of fatty substances, cholesterol, cellular waste products, calcium and other substances build up in the inner lining of an artery. It usually affects large and medium-sized arteries. Plaques can grow large enough to significantly reduce the blood's flow through an artery. But most of the damage occurs when they become fragile and rupture. Plaques that rupture cause blood clots to form that can block blood flow or break off and travel to another part of the body. If either happens and blocks a blood vessel that feeds the heart, it causes a heart attack. If it blocks a blood vessel that feeds the brain, it causes a stroke. And if blood supply to the arms or legs is reduced, it can cause difficulty walking and eventually lead to gangrene.