清华大学生物化学--脂肪分解代谢.ppt

Chapter 17 Fatty Acid Catabolism.1.The good and bad sides of using triacylglycerols as an energy storage Highly reduced,with an energy of complete oxidation more than twice that for the same weight of carbohydrates or proteins(38 kJ/g vs 18 kJ/g).Highly hydrophobic:does not raise osmolarity of cytosol,nor add extra weight;but must be emulsified before digestion and transported by special proteins in blood.Chemically inert:no undesired chemical reactions with other constituents;but must be activated(by attaching the carboxyl group to coenzyme A)to break the stable C-C bonds.Fatty acids is a central energy source in animals(especially the liver,heart,and resting skeletal muscle),many protists,and some bacteria;the only energy source for hibernating animals and migrating birds;not a major energy source in plants.Sources for cells to obtain fatty acids:diet;stored lipid droplets;synthesized from the excess carbohydrates and amino acids by some special organs(e.g.,liver).2.Dietary triacylglycerols are emulsified and absorbed by the intestineBile salts(e.g.,taurocholate牛黄胆酸牛黄胆酸 and glycocholate甘胆酸甘胆酸),synthesized from cholesterol in liver,emulsifies macroscopic fat particles into microscopic mixed micelles for better lipase action and absorption.Fatty acids generated from triacylglycerol(catalyzed by the intestinal lipase)diffuse into intestinal epithelial cells,be reconverted into triacylglycerol,and packed with cholesterol esters and specific apolipoproteins in chylomicrons(乳糜微滴乳糜微滴).Processing of dietary lipids in vertebrates.Processing of dietary lipids in vertebrates.Molecular structure of a chylomicron.Molecular structure of a chylomicron.Triacylglycerols are converted into fatty acids and glycerols in the capillaries by the action of lipoprotein lipases activated by apoC-II on chylomicrons,which in turn are absorbed mainly by adipocytes and myocytes for storage and energy consumption.The leftover of the chylomicrons(containing mainly cholesterol and apolipoproteins)will be taken up by the liver by endocytosis;triacylglycerols will be used as the energy source for the liver cells,converted to ketone bodies or transported to adipose tissues after being packed with apolipoproteins.3.Mobilization of stored triacylglycerols in adipocytes needs hormone to signal the demandThe hormones epinephrine and glucagon,signaling a lack of glucose in the blood,will bind to receptors on adipocyte surface and activate the hormone-sensitive lipase and perilipin molecules on the surface of the lipid droplets.Fatty acids released are carried to energy-demanding tissues(e.g.,skeletal muscle,heart,and renal cortex)via the 62 kD monomeric serum albumin(each binding as many as 10 fatty acids).Mobilization oftriacylglycerols stored in adipose tissue.Entry of glycerol into the glycolyticpathway.4.Early labeling experiments(1904):fatty acids are degraded by sequential removal of two-carbon unitsWhen dogs were fed with odd-numbered fatty acids attached to a phenyl group,benzoate was excreted;and when fed with even-numbered,phenylacetate was excreted.Hypothesis:the b b-carbon is oxidized,with two-carbon units released by each round of oxidation.These experiments are a landmark in biochemistry,in using synthetic label(the phenyl group here)to elucidate reaction mechanism,and was done long before radioisotopes was used in biochemistry!.Franz Knoops labelingexperiments(1904):fatty acids are degradedby oxidation at the b b carbon,i.e.,b b oxidation b bb ba aa a.5.Fatty acid oxidation was found to occur in mitochondria Enzymes of fatty acid oxidation in animal cells were localized in the mitochondria matrix.Revealed by Eugene Kennedy and Albert Lehninger in 1948.6.Fatty acids are activated on the outer membrane of mitochondriaFatty acids are converted to fatty acyl-CoA(a high energy compound)via a fatty acyl-adenylate intermediate(enzyme-bound,mixed anhydride)by the action of fatty acyl-CoA synthetases(also called fatty acid thiokinase).Pyrophosphate is hydrolyzed by inorganic pyrophosphatase,thus two anhydride bonds of ATP are consumed to form one high-energy thioester bond,thus pulling the reaction forward:a common phenomena in biosynthetic reactions.Acyl-adenylates are often formed when COOH groups are activated in biochemistry.Conversion of a fatty acid to a fatty acyl-CoA.7.Activated(long chain)fatty acids are carried into the matrix by carnitineThe fatty acyl group is attached to carnitine (肉碱肉碱)via transesterification by the action of carnitine acyltransferase I located on the outer membrane of mitochondria,forming fatty acyl-carnitine,leaving the CoA in the cytosol.The acyl carnitine/carnitine transporter moves acyl-carnitine across the inner membrane of mitochondria via facilitated diffusion.Medium-chain acyl-CoAs seem to enter the matrix by themselves,without being carried by carnitine.Fatty acid entry into mitochondria via the acyl-carnitine/carnitine transporter.The acyl group is then transferred back to CoA to form fatty acyl-CoA by the action of carnitine acyltransferase II located on the inner face of the inner membrane.This entering step seems to be rate-limiting for fatty acid oxidation in mitochondria and diseases have been found to be caused by a defect of this step(with aching muscle cramp,especially during fasting,exercise or when on a high-fat diet).8.Fatty acyl-CoA is oxidized to acetyl-CoA via multiple rounds of b b oxidation The b b oxidation consists of four reactions:dehydrogenation hydration dehydrogenation thiolytic cleavage.The b b-oxidation pathway.The 1st dehydrogenation is catalyzed by the membrane-bound acyl-CoA dehydrogenase,converting acyl-CoA to trans-2-enoyl-CoA with electrons collected by FAD and further transferred into the respiratory chain via the electron-transferring flavoprotein(ETF),also bound to the inner membrane of the mitochondria.Acyl-CoA dehydrogenase exists in three isozymes acting on the acyl-CoAs of long(12-18C),medium(4-14C)and short(4-8C)chains.The hydration step,stereospecifically catalyzed by enoyl-CoA hydratase,converts the trans-2-enoyl-CoA to L-b b-hydroxylacyl-CoA(the cis isomer can also be converted,but to the D isomer).The 2nd dehydrogenation is catalyzed by L-b b-hydroxylacyl-CoA dehydrogenase,converting L-b b-hydroxylacyl-CoA to b b-ketoacyl-CoA(not act on the D isomer),with electrons collected by NAD+.The acyl-CoA acetyltransferase(or commonly called thiolase)catalyzes the nucleophilic attack of CoA to the carbonyl carbon,cleaving b b-ketoacyl-CoA between the a a and b b carbon(thiolysis),generating two acyl-CoA molecules with one entering the citric acid cycle and the other reentering the b b oxidation pathway.The first three reactions of the b b oxidation used the same reaction strategy as the last three reactions in the citric acid cycle,all involving the oxidation of a highly reduced carbon(from-CH2-to-CHO-).A conserved reaction sequence to induce a carbonyl function on the carbon b b to a carboxyl group.Stages of fatty acid oxidation.Palmitol-CoA+7CoA+7FAD+7NAD+7H2O 8 acetyl-CoA+7FADH2+7NADH+7H+The complete oxidization of each 16-carbon palmitate(to H2O and CO2)yields 106 ATP(32 ATP per glucose,both having about 60%of actual energy recovery).Palmitoyl-CoA+23O2+108Pi+108ADP CoA+108ATP+16CO2+23H2O*.Hibernating grizzly bears use body fat as their sole fuel.9.Oxidation of unsaturated fatty acids requires one or two auxiliary enzymes,an isomerase and a reductase The isomerase converts a cis-3 double bond to a trans-2 double bond.The reductase(2,4-dienoyl-CoA reductase)converts a trans-2,cis-4 structure to a trans-3 structure,which will be further converted to a trans-2 structure by the isomerase.NADPH is needed for the reduction(from two double bonds to one).Oxidation of amonounsaturatedfatty acid:the enoyl-CoA isomerase helps toreposition the double bond.Both an isomerase anda reductase are neededfor oxidizing apolyunsaturated fatty acid.10.Propionyl-CoA generated from odd-number fatty acids(and three amino acids)is converted to succinyl-CoAOdd-number fatty acids,commonly found in the lipids of some plants and marine organisms,will also be oxidized via the b b oxidation pathway,but generating a propionyl-CoA at the last step of thiolysis.Propionyl-CoA is converted to succinyl-CoA via an unusual enzymatic pathway of three reactions.Propionyl-CoA can beconverted to succinyl-CoAvia a carboxylation,an epimerization andan intramolecular groupshifting.Propionyl-CoA is first carboxylated at the a carbon to form D-methylmalonyl-CoA,catalyzed by propionyl-CoA carboxylase.The D-methylmalonyl-CoA is then epimerized to the L-isomer by the action of an epimerase.The L-methylmalonyl-CoA is then converted to succinyl-CoA by an intramolecular rearrangement via free radical intermediates,with the catalysis of methylmalonyl-CoA mutase,using 5-deoxyadenosylcobalamin(or coenzyme B12)as a coenzyme.Coenzyme B12 enzymes act in intramolecular rearrangements,methylation,and reduction of ribonucleotides to deoxynucleotides.Pernicious anemia(恶性贫血恶性贫血)is caused by a failure to absorb vitamin B12,which may in turn be caused by a deficiency of the intrinsic factor,a 59 kD glycoprotein needed for vitamin B12 absorption.Vitamin B12 is synthesized by a few intestinal bacteria(not plants).3-D structure of coenzyme B12 and penicillin.11.The rate of b b-oxidation in mitochondria is limited by the entering rate of fatty acyl-CoAFatty acyl-CoA in the cytosol can enter mitochondria for oxidative degradation or be converted to triacylglycerol in the cytosol when excess glucose is present that can not be oxidized or stored as glycogen and is converted into fatty acids for storage.The key regulatory protein is carnitine acyltransferase I,which is inhibited by malonyl-CoA,the first intermediate for fatty acid synthesis from acetyl-CoA.Coordinated regulation of fatty acid synthesis and breakdown.12.Fatty acid oxidation also occurs in peroxisomes(glyoxysomes)Peroxisome is the principle organelle in which fatty acids are oxidized in plant cells.Also via the four-step b b oxidation pathway,but the electrons collected by FAD are directly passed to O2,producing the harmful H2O2,which is immediately converted to H2O and O2.Energy released is dissipated as heat,not conserved in ATP.Comparison of b b-oxidation in mitochondrion and inperoxisome/glyoxysome.The acetyl-CoA produced in animal peroxisomes is transported into cytosol,where it is used in the synthesis of cholesterol and other metabolites.The acetyl-CoA produced in plant peroxisomes/glyoxysomes(especially in germinating seeds)is converted to succinate via the glyoxylate cycle,and then to glucose via gluconeogenesis.Triacylglycerols as glucose source in seeds.The b b-oxidation enzymes in mitochondria and peroxisomesare organized differently:being separate enzymes in mitochondria(as in gram-positive bacteria)and one complex in peroxisomes(as in gram-negative bacteria).Bound to the inner mitochondrial membrane.13.Acetyl-CoA in liver can be converted to ketone bodies when carbohydrate supply is not optimalUnder fasting or diabetic conditions,oxaloacetate concentration in hepatocyte will be low:the rate of glycolysis is low(thus the supply of precursors for replenishing oxaloacetate is cut off)and oxaloacetate is siphoned off into gluconeogenesis(to maintain blood glucose level).The acetyl-CoA generated from active fatty acid oxidation can not be oxidized via the citric acid cycle and will be converted to acetoacetate,b b-hydroxylbutyrate,and acetone(i.e.,the ketone bodies,酮体酮体)in mitochondria for export to other tissues.Ketone body formation and export from the liver.Formation of ketone bodies from acetyl-CoA.14.Ketone bodies are converted back to acetyl-CoA in extrahepatic tissuesD-b b-hydroxylbutyrate is reoxidized to acetoacetate(catalyzed by the dehydrogenase).Acetoacetate is then converted to acetoacetyl-CoA using succinyl-CoA(catalyzed by b b-ketoacyl-CoA transferase).Acetoacetyl-CoA is cleaved to two acetyl-CoA(again catalyzed by thiolase).Heart,skeletal muscle and the renal cortex use acetoacetate in preference to glucose.The brain adapts to the utilization of acetoacetate during starvation and diabetes.b b-Hydroxybutyrate can be used as a fuel by extrahepatic tissuesNot present in liver.Overproduction of ketone bodies in uncontrolled diabetes or severely reduced calorie intake canlead to acidosis and ketosis.KeywordsActivation and transport of fatty acids b b-oxidation-reaction and regulationKetone bodies.Words of the weekketosisacidosisketoacidosisalkalosis.SummaryDiet triacylglycerols are emulsified by bile salts in the intestines before being absorbed and transported in blood as chylomicron particles.Stored triacylglycerols are mobilized in response to hormones and fatty acids in blood are carried by serum albumin.Fatty acids are activated to the acyl-CoA form and is then carried into mitochondria by carnitine with the help of two carnitine acyltranseferase isozymes(I and II)located on the outside and inside of the inner membrane.Acyl-CoA is converted to acetyl-CoA after going through multiple rounds of the four-step(dehydrogenation,hydration,dehydrogenation and thiolysis)b b-oxidation pathway.Oxidative degradation of unsaturated fatty acids need two extra enzymes:an isomerase and a reductase.The propionyl-CoA generated from odd-numbered fatty acids is converted to succinyl-CoA after being carboxylated,epimerized and intramolecularly rearranged(with help from a coenzyme B12).The rate of b b-oxidation pathway is controlled by the rate at which acyl-CoA is transported into mitochondria.The b b-oxidation pathway also occurs in peroxisomes using similar isozymes,but generates H2O2.Excess acetyl-CoA(under conditions when glucose metabolism is not optimal)can be converted to ketone bodies in the liver cells and reconverted into acetyl-CoA in extrahepatic cells.