Lipid/Fat metabolism Chapter 4 Lipid metabolism allows polar bears to thrive in arctic climates and to endure months of hibernation.

Introduction Fatty acids have 4 major roles in the cell: Building blocks of phospholipids and glycolipids Added onto proteins to create lipoproteins, which targets them to membrane locations Fuel molecules - source of ATP Fatty acid derivatives serve as hormones and intracellular messengers

The oxidation of f.acids – source of energy in the catabolism of lipids Both triacylglycerols and phosphoacylglycerols have f.acids as part of their covalently bonded structures The bond between the f.acids and the rest of the molecule can be hydrolyzed (as shown in the fig.) FIGURE 21.1 The release of fatty acids for future use. The source of fatty acids can be a triacylglycerol (left) or a phospholipid such as phosphatidylcholine (right). Fig. 21-1, p.569

FIGURE 21. 2 Several phospolipases hydrolyze phosphoacylglycerols FIGURE 21.2 Several phospolipases hydrolyze phosphoacylglycerols. They are designated A1, A2, C, and D. Their sites of action are shown. The site of action of phospholipase A2 is the B site, and the name phospholipase A2 is the result of historical accident (see text). Fig. 21-2, p.569

p.569

FIGURE 21.3 Liberation of fatty acids from triacylglycerols in adipose tissue is hormone dependent. Fig. 21-3, p.570

Fatty acids oxidation begins with activation of the molecule. A thioester bond is formed between carboxyl group of f.acid and the thiol group of coenzyme A (CoA-SH) (esterification reaction – in cytosol) FIGURE 21.4 The formation of an acyl-CoA.

FIGURE 21.5 The role of carnitine in the transfer of acyl groups to the mitochondrial matrix. Fig. 21-5, p.571

FIGURE 21.6 The -oxidation of saturated fatty acids involves a cycle of four enzyme-catalyzed reactions. Each cycle produces one FADH2 and one NADH, and it liberates acetyl-CoA, resulting in a fatty acid that is two carbons shorter. The  symbol represents a double bond, and the number associated with it is the location of the double bond (based on counting the carbonyl group as carbon one). Fig. 21-6, p.572

β-oxidation takes place in mitochondria. When a f.acid with an even number of C atoms undergoes successive rounds of β-oxidation cycle, the product is acetyl-CoA. No. of molecules of acetyl-CoA produced = ½ the no. of C atoms in the original f.acid. (as shown in fig above) The acetyl-CoA enters the TCA cycle (the rest of oxidation to CO2 and H2O taking place via TCA cycle and ETC) β-oxidation takes place in mitochondria. FIGURE 21.7 Stearic acid (18 carbons) gives rise to nine 2-carbon units after eight cycles of -oxidation. The ninth 2-carbon unit remains esterified to CoA after eight cycles of -oxidation have removed eight successive two-carbon units, starting at the carboxyl end on the right. Thus, it takes only eight rounds of -oxidation to completely process an 18-carbon fatty acid to acetyl-CoA.

There are two sources of ATP: The energy released by the oxidation of acetyl-CoA formed by β-oxidation of f.acids can be used to produce ATP. There are two sources of ATP: Reoxidation of the NADH and FADH2 produced by β-oxidation ATP production from processing acetyl-CoA via TCA cycle and oxidative phosphorylation NADH and FADH2 produced by β-oxidation and TCA cycle enter ETC and ATP produced through oxidative phosphorylation Table 21-1, p.575

Comparison Carbohydrate Lipids 32 moles of ATP produced from complete oxidation of CHO (but, glucose is 6C atoms, so 6 x 3 = 18 C atoms. Therefore, 32 x 3 = 96 ATP. e.g stearic acid: 18 C atoms = produced 120 moles of ATP Reason? F.acid is all hydrocarbon except carboxyl group – exists in highly reduced state H2O is produced in oxidation of f.acids – can be a source of water for organisms that live in desert

Camel Kangaroo rats p.575a

The catabolism of odd-carbon f.acids FIGURE 21.8 The oxidation of a fatty acid containing an odd number of carbon atoms. Fig. 21-8, p.576

The catabolism of unsaturated f.acids The -oxidation of unsaturated f.acids does not generate as many ATPs as it would for a saturated f.acids (same C atoms) – the presence of double bond the acyl-deH2ase step skipped – fewer FADH2 will be produced FIGURE 21.9 -oxidation of unsaturated fatty acids. In the case of oleoyl-CoA, three -oxidation cycles produce three molecules of acetyl-CoA and leave cis-3-dodecenoyl-CoA. Rearrangement of enoyl-CoA isomerase gives the trans-2 species, which then proceeds normally through the -oxidation pathway.

FIGURE 21. 9 -oxidation of unsaturated fatty acids FIGURE 21.9 -oxidation of unsaturated fatty acids. In the case of oleoyl-CoA, three -oxidation cycles produce three molecules of acetyl-CoA and leave cis-3-dodecenoyl-CoA. Rearrangement of enoyl-CoA isomerase gives the trans-2 species, which then proceeds normally through the -oxidation pathway. Fig. 21-9b, p.577

FIGURE 21.10 The oxidation pathway for polyunsaturated fatty acids, illustrated for linoleic acid. Three cycles of -oxidation on linoleoyl-CoA yield the cis-3, cis-6 intermediate, which is converted to a trans-2, cis-6 intermediate. An additional round of -oxidation gives cis-4 enoyl-CoA, which is oxidized to the trans-2, cis-4 species by acyl-CoA dehydrogenase. The subsequent action of 2,4- dienoyl-CoA reductase yields the trans-3 product, which is converted by enoyl-CoA isomerase to the trans-2 form. Normal -oxidation then produces five molecules of acetyl-CoA. Fig. 21-10a, p.578

FIGURE 21.10 The oxidation pathway for polyunsaturated fatty acids, illustrated for linoleic acid. Three cycles of -oxidation on linoleoyl-CoA yield the cis-3, cis-6 intermediate, which is converted to a trans-2, cis-6 intermediate. An additional round of -oxidation gives cis-4 enoyl-CoA, which is oxidized to the trans-2, cis-4 species by acyl-CoA dehydrogenase. The subsequent action of 2,4- dienoyl-CoA reductase yields the trans-3 product, which is converted by enoyl-CoA isomerase to the trans-2 form. Normal -oxidation then produces five molecules of acetyl-CoA. Fig. 21-10b, p.578

Ketone bodies Substances related to acetone (“ketone bodies”) are produced when an excess of acetyl-CoA arises from β-oxidation Occurs because when there are not enough OAA to react with acetyl-CoA in TCA cycle When organisms has a high intake of lipids and low intake of CHO or starvation and diabetes The reactions that result in ketone bodies start with the condensation of two molecules of acetyl- CoA to produce acetoacetyl-CoA

synthesis of ketone bodies in liver mitochondria the odor of acetone can be detected on the breath of diabetics whose not controlled by suitable treatment Acetoacetate and β-hydroxybutyrate are acidic, their presence at high [ ] overwhelms the buffering capacity of the blood to lowered the blood pH is dealt by excreting H+ into the urine, accompanied by excretion of Na +, K + and water → results in severe dehydration and diabetic coma synthesis of ketone bodies in liver mitochondria transport ketone bodies in the bloodstream; water soluble other organs such as heart muscle and renal cortex can use ketone bodies (acetoacetate) as the preferred source of energy even in brain, starvation conditions lead to the use of acetoacetate for energy FIGURE 21.11 The formation of ketone bodies, synthesized primarily in the liver.

FATTY ACID SYNTHESIS The anabolic reaction takes place in cytosol Important features of pathway: Intermediates are bound to sulfhydral groups of acyl carrier protein (ACP); intermediates of β-oxidation are bonded to CoA Growing fatty acid chain is elongated by sequential addition of two-carbon units derived from acetyl CoA Reducing power comes from NADPH; oxidants in β- oxidation are NAD+ and FAD Elongation of fatty acid stops when palmitate (C16) is formed; further elongation and insertion of double bonds carried out later by other enzymes

Step 1 FIGURE 21.12 The transport of acetyl groups from the mitochondrion to the cytosol. Fig. 21-12, p.581

Step 2 FIGURE 21.13 The formation of malonyl-CoA, catalyzed by acetyl-CoA carboxylase. Fig. 21-13, p.581

Malonyl-CoA inhibits carnitine acyltransferase I FIGURE 21.14 (b) A mechanism for the acetyl-CoA carboxylase reaction. Bicarbonate is activated for carboxylation reactions by formation of N-carboxybiotin. ATP drives the reaction forward, with transient formation of a carbonyl-phosphate intermediate (Step 1). In a typical biotin-dependent reaction, nucleophilic attack by the acetyl-CoA carbanion on the carboxyl carbon of N-carboxybiotin—a transcarboxylation—yields the carboxylated product (Step 2). Malonyl-CoA inhibits carnitine acyltransferase I Fig. 21-14b, p.582

Pathway of palmitate synthesis from acetyl-CoA and malonyl-CoA The biosynthesis of f.acids involves the successive addition of two-carbon units to the growing chain. - Two of the three C atoms of the malonyl group of malonyl-CoA are added to the growing fatty-acid chain with each cycle of the biosynthetic reaction Fig. 21-15, p.583

Fig. 21-15a, p.583

Step 3 Fig. 21-15b, p.583

This reaction require multienzyme complex : fatty acid synthase Step 4 Fig. 21-15c, p.583

FIGURE 21.16 Structural similarities between coenzyme A and the phosphopantetheine group of ACP. Fig. 21-16, p.584

There are several additional reactions required for the elongation of f.acid chain and the introduction of double bonds. When mammals produce f.acids with longer chains than that of palmitate, the reaction does not involve cytosolic f-acid synthase. There are two sites for chain lengthening reactions: ER (endoplasmic reticulum) and mitochondrion. Table 21-2, p.586

FIGURE 21.17 A portion of an animal cell, showing the sites of various aspects of fatty-acid metabolism. The cytosol is the site of fatty-acid anabolism. It is also the site of formation of acyl- CoA, which is transported to the mitochondrion for catabolism by the -oxidation process. Some chainlengthening reactions (beyond C16) take place in the mitochondria. Other chain-lengthening reactions take place in the endoplasmic reticulum (ER), as do reactions that introduce double bonds. Fig. 21-17, p.586

Table 21-3, p.599

Lipoproteins classified according to their densities: Lipids are transported throughout the body as lipoproteins Both transported in form of lipoprotein particles, which solubilize hydrophobic lipids and contain cell-targeting signals. Lipoproteins classified according to their densities: chylomicrons - contain dietary triacylglycerols chylomicron remnants - contain dietary cholesterol esters very low density lipoproteins (VLDLs) - transport endogenous triacylglycerols, which are hydrolyzed by lipoprotein lipase at capillary surface intermediate-density lipoproteins (IDL) - contain endogenous cholesterol esters, which are taken up by liver cells via receptor- mediated endocytosis and converted to LDLs low-density lipoproteins (LDL) - contain endogenous cholesterol esters, which are taken up by liver cells via receptor-mediated endocytosis; major carrier of cholesterol in blood; regulates de novo cholesterol synthesis at level of target cell high-density lipoproteins - contain endogenous cholesterol esters released from dying cells and membranes undergoing turnover