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).
FIGURE 21.3 Liberation of fatty acids from triacylglycerols in adipose tissue is hormone dependent.
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.
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.
FIGURE 21.11 The formation of ketone bodies, synthesized primarily in the liver.
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.
Metabolism of the lipidsFatty 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
Omega-3 f.acids shown toslow the development ofcardiovascular diseases
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.) Fig. 21-1, p.569
• 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)
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.
The oxidation ofunsaturatedf.acids does notgenerate as manyATPs as it wouldfor a saturatedf.acids (same Catoms) – thepresence ofdouble bond• the acyl-deH2asestep skipped –fewer FADH2 willbe produced
Ketone bodiesSubstances related to acetone (“ketone bodies”) are produced when an excess of acetyl-CoA arises from β- oxidationOccurs because when there are not enough OAA to react with acetyl-CoA in TCA cycleWhen organisms has a high intake of lipids and low intake of CHO or starvation and diabetesThe reactions that result in ketone bodies start with the condensation of two molecules of acetyl-CoA to produce acetoacetyl-CoA
• the odor of acetone can be detected on thebreath of diabetics whose not controlled bysuitable treatment• Acetoacetate and β-hydroxybutyrate are acidic,their presence at high [ ] overwhelms thebuffering capacity of the blood• to lowered the blood pH is dealt by excretingH+ into the urine, accompanied by excretion ofNa +, K + and water → results in severedehydration 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 renalcortex can use ketone bodies (acetoacetate) as thepreferred source of energy• even in brain, starvation conditions lead to theuse of acetoacetate for energy
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 (C 16) is formed; further elongation and insertion of double bonds carried out later by other enzymes
Pathway of palmitate synthesis from acetyl-CoA and malonyl-CoAThe biosynthesis off.acids involves thesuccessive addition oftwo-carbon units tothe growing chain.- Two of the three Catoms of the malonylgroup of malonyl-CoA are added to thegrowing fatty-acidchain with each cycleof the biosyntheticreaction
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