Lipid metabolism Pavla Balínová Lipids Lipids dissolve well in organic solvents but they are insoluble in water. Biological roles of lipids: ● lipids are important source of energy – they serve as metabolic fuel ● amphipathic lipids are building blocks of cellular membranes ● some of them are substrates for synthesis of other compounds (eicosanoids, bile acids) ● lipids are excellent insulators Classification of lipids I. Simple lipids ● Triacylglycerols TAG (fats) ● Waxes II. Complex lipids ● Phospholipids ● Sphingophospholipids ● Glycolipids III. Isoprenoids and steroids Isoprenoids: vitamins A, D, E, K Steroids: sterols, bile acids, steroid hormones Figure is found on http://en.wikipedia.org/wiki/Triacylglycerol Fatty acids (FA) The figure was adopted from: J.Koolman, K.H.Röhm / Color Atlas of Biochemistry, 2nd edition Degradation of fats in adipose tissue Adipose tissue (fat cells) = fat storage Degradation of TAG in adipose tissue (lipolysis) is catalyzed by hormone sensitive lipase (HSL). This enzyme is activated by epinephrine and glucagon and inhibited by insulin. Figure is found on http://web.indstate.edu/thcme/mwking/fatty-acid-oxidation.html Utilization of FA within the cell Tissues take up FA from the blood to rebuild fats or to obtain energy from their oxidation. Metabolism of FA is especially intensive in the liver. „Free“ fatty acids (FFA) are transferred with albumin in the blood. FA in blood → enter to the cell → in the cytoplasm FA are converted to their CoA derivatives by enzyme acyl- CoA-synthetase (ATP is consumed) → acyl-CoAs Fatty acid + ATP + CoA ---> Acyl-CoA + PPi + AMP Transfer of acyl-CoAs from cytoplasm to the mit. matrix is performed by a carnitine transporter Figure is found on http://web.indstate.edu/thcme/mwking/fatty-acid-oxidation.html β-oxidation of fatty acids • substrate: acyl-CoA • product: n acetyl-CoA, n NADH + H+, n FADH2 • function: gain of energy from fatty acids • subcelullar location: matrix of mitochondria • organ location: liver, skeletal muscles and other tissues with expection to CNS • regulatory enzyme: carnitine acyltransferase I Summary For complete degradation of palmitic acid 7 cycles are required. Degradation of palmitic acid (16 C) gives 106 ATP in total. Regulation of β-oxidation of FA: Regulatory enzyme is carnitine acyltransferase I – it is inhibited by malonyl-CoA (intermediate of FA synthesis). Synthesis of ketone bodies (ketogenesis) • substrate: acetyl-CoA • product: acetoacetate, 3-hydroxybutyrate, acetone • function: energy substrate for extrahepatal tissues • subcelullar location: matrix of mitochondria • organ location: liver Excessive production of ketone bodies is typical during starvation or diabetes mellitus: ↑ lipolysis → ↑ FA → β-oxidation of FA → excess of acetyl-CoA → ↑ ketogenesis Synthesis of ketone bodies (ketogenesis) Figure is found at http://themedicalbiochemistrypage.org/fatty-acid-oxidation.html#ketogenesis Use of ketone bodies by the extrahepatal tissues • acetoacetate and 3-hydroxybutyrate are reconverted to acetyl-CoA (→ citric acid cycle) • is located in matrix of mitochondria of the peripheral tissues • is significant in skeletal muscles, heart and also in the brain if lack of Glc occurs Use of ketone bodies by the extrahepatal tissues Liver lacks this enzyme therefore is unable to use ketone bodies as fuel Figure is found at http://themedicalbiochemistrypage.org/fatty-acid-oxidation.html#ketogenesis Fatty acid synthesis • substrate: acetyl-CoA, NADPH + H+ • product: palmitate (= endproduct of FA synthesis) • function: de novo synthesis of FA which are stored as TAG • subcelullar location: cytosol • organ location: mainly liver and adipose tissue and also other tissues • regulatory enzyme: acetyl-CoA carboxylase The committed step in FA synthesis Formation of malonyl-CoA from acetyl-CoA and HCO3- is catalyzed by enzyme acetyl-CoA carboxylase (a key regulatory enzyme). Citrate is an allosteric stimulator and palmitoyl-CoA inhibits this enzyme. Hormonal regulation: glucagon and epinephrine - inhibition insulin - stimulation The growing fatty acids are linked to a phosphopantetheine group of an acyl carrier protein (ACP) of FA synthase. Acetyl-CoA is carboxylated by HCO3- to yield malonyl-CoA → condensation between the acetyl-ACP and the malonyl-ACP → acetoacetyl-ACP is formed. FA synthesis is performed through a cycle of four reactions: Figure is found on http://18.104.22.168/bio/Courses/biochem2/FattyAcid/FASynthesis.html Biosynthesis of TAG Biosynthesis of TAG occurs in cytoplasm and ER of liver and fat cells but also in other tissues. Figure is found on http://web.indstate.edu/thcme/mwking/lipid-synthesis.html#phospholipids Complex lipids (phospholipids, sphingophospholipids, glycolipids) Phospholipids = glycerol + 2 FA + phosphate group + hydrophilic compound Phosphatidylethanolamine (cephalin) Phophatidylinositol Phosphatidylcholine (lecithin) choline Sphingophospholipids = sphingosine + FA + phosphate residue + amino alcohol or sugar alcohol Ceramide = sphingosine + fatty acid Sphingomyelin acyl residue Glycolipids = sphingosine + FA + sugar or oligosaccharide residue The phosphate group is absent. Galactocerebroside Degradation of phospholipids Phospholipases are divided according to the type of the bond which is cleaved. Figure is found on http://web.indstate.edu/thcme/mwking/lipid-synthesis.html#phospholipids Synthesis of cholesterol • substrate: acetyl-CoA • product: cholesterol • function: de novo synthesis of endogenous cholesterol • subcelullar location: cytosol and endoplasmic reticulum • organ location: liver, intestine, adrenal cortex, ovaries, testes and placenta make the largest contributions to the body´s cholesterol pool Cholesterol is a constituent of cellular membranes and it is present in all animal tissues. Biosynthesis of isoprenoids and steroids Figure is found on http://web.indstate.edu/thcme/mwking/cholesterol.html Regulation of cholesterol synthesis • reduction of HMG-CoA to mevalonic acid is catalyzed by HMG-CoA reductase – rate-limiting and key regulatory step • expression of HMG-CoA reductase gene is inhibited by cholesterol ● activity of HMG-CoA reductase is increased by insulin and thyroxine but glucagone has the opposite effect Figure was adopted from textbook T. M. Devlin et al.: Textbook of Biochemistry With Clinical Correlations, 4th ed. Lipoproteins ● Chylomicrons carry TAG (fat) from the intestine to the liver and adipose tissue ● VLDL carry (newly synthesized) TAG from the liver to peripheral tissues ● IDL are intermediate between VLDL and LDL ● LDL carry cholesterol from the liver to cells of the body ● HDL collects cholesterol from body´s tissues and brings it back to the liver Figure was assumed from http://http://www.britannica.com/EBchecked/topic- art/720793/92254/Cutaway-view-of-a-low-density-lipoprotein-complex-The-LDL Metabolism of lipoproteins Figure was assumed from www.media-2.web.britannica.com/eb-media/42/96842-... Enzyme lipoprotein lipase (LPL) is present on the surface of the vascular endothelia. This enzyme hydrolyses TAG in chylomicrons → chylomicrons remnants → liver LPL also catalyzes cleavage of TAG in VLDL → IDL. LPL synthesis is stimulated by insulin. Enzyme lecithin cholesterol acyltransferase (LCAT) catalyzes esterification of cholesterol with acyl. It is included in HDL formation. Figure was adopted from textbook T. M. Devlin et al.: Textbook of Biochemistry With Clinical Correlations, 4th ed.
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