Outline

Introduction (Figure 18.1)

Utilization and Transport of Fat and Cholesterol

> > 5-25% of mammalian body weight is lipid

> > 90% of this lipid is triacylglycerols (fat)

> > Fat stored in adipocytes

> Fats as Energy Reserves

> > Glycogen binds water - fats are anhydrous

> > Intracellular fat contain 6 times the potential metabolic energy of glycogen

> > 400,000 kJ of fuel in body fat, 100,000 kJ in protein, 2500 kJ in glycogen, 170 kJ in glucose

> > 40% of Western diet calories come from fat

> Fat Digestion and Absorption (Figure 18.3 )

> > Triacylglycerols come from

> > > 1. Diet
> > > 2. de novo biosynthesis (primarily liver)
> > > 3. Adipocyte storage

> > Bile salts (Figure 18.4)

> Transport of Fat to Tissues: Lipoproteins

> > Classification and Functions of Lipoproteins (Figure 18.5)

> > > Lipoproteins (Table 18.1)

> > > > Chylomicrons, VLDL, IDL, LDL, HDL

> > > Apolipoproteins (Table 18.2)

> > > Lipoproteins solubilize 500 mg of total lipid per 100 ml blood

> > Transport and Utilization of Lipoproteins (Figure 18.7)

> > > Chylomicrons carry dietary fat from intestines (Figure 18.3)

> > > VLDL carries triacylglycerols synthesized in liver (Figure 18.3)

> > > > VLDLs are degraded to IDLs

> > > Lipoprotein lipase (Figure 18.6)

> Cholesterol Transport and Utilization in Animals (Diagram)

> > Phosphatidylcholine + Cholesterol <=> Lysolecithin + Cholesterol Ester (catalyzed by Lecithin:Cholesterol Acyl Transferase)

> > LDL is lipoprotein class that is richest in cholesterol

> > > Single molecule of B-100 apolipoprotein is LDL's primary protein component

> > > Cholesterol primarily synthesized in liver - LDL important in transport

> > The LDL Receptor and Cholesterol Homeostasis (Figure 18.10)

> > > Atherosclerosis

> > > Familial hypercholesterolemia

> > > LDL receptor transports cholesterol into cell by receptor mediated endocytosis

> > > > Receptors clustered into coated pits which contain clathrin

> > > > Imported cholesterol moves to endoplasmic reticulum and has 3 effects

> > > > > 1. Suppresses endogenous cholesterol synthesis by inhibiting HMG-CoA Reductase

> > > > > 2. Activates acyl-CoA:cholesterol acyltransferase (ACAT) to make acyl-cholesterol esters

> > > > > 3.Regulates synthesis of LDL receptor

> Cholesterol, LDL, and Atherosclerosis

> > Unknowns

> > > Why do diets rich in saturated fat tend to elevate serum cholesterol levels?

> > > Why to the polyunsaturated fats called -3 fatty acids tend to depress levels of serum cholesterol?

> > Plaque formation (Atherosclerosis)

> > > Ready oxidation of LDLs

> > > > Include peroxidation of unsaturated fatty acids, hydroxylation of cholesterol, and oxidation of amino acids in apoprotein.

> > > LDL taken into a class of white blood cells (via scavenger receptor) at site of injury/oxidation

> > > > Unlimited uptake of LDL by white blood cells converts them to foam cells. This attracts more white cells and they form chief chemical constituents of a plaque at the site.

> > "Good/Bad" Cholesterol

> > > LDL called bad cholesterol because of link to atherosclerosis

> > > HDL called good cholesterol because high HDL levels counter atherosclerosis by transporting cholesterol back to the liver from peripheral tissues

> Mobilization of Stored Fat

> > Begins with hydrolysis of fat to glycerol and free fatty acids (is hormonally regulated - comparable to glycogen metabolism - Figure 18.11)

> > Hormonal activation involves triacylglycerol lipase ( = hormone-sensitive lipase)

> > > Catalyzes release of fatty acid from position 1 or 3 of fat

> > Freed fatty acids bound in blood plasm to serum albumin (10 fatty acids each)

> > 95% of energy of fat from fatty acids, 5% from glycerol

> > Fatty acid catabolized to acetyl-CoA

Fatty Acid Oxidation

> Early Experiments (Figure 18.13)

> > Knoop's evidence suggested fatty acids broken into two carbon units in breakdown (Figure 18.12)

> > Lelois and Lehninger demonstrated fatty acid oxidation in cell-free liver homogenates

> > Kennedy and Lehninger showed process occurs in mitochondria

> > Lynen showed ATP-dependent activation of fatty acid involves linkage of carboxyl group to thiol group of coenzyme A

> Fatty Acid Activation and Transport into Mitochondria (Diagram)

> > Fatty acyl-CoA ligases (specific for short, medium, or long chain fatty acids) catalyze formation of fatty acyl thioester conjugate with coenzyme A (Diagram)

> > > Short and medium chain ligases found in mitochondria. Long chain ligase found in endoplasmic reticulum and outer mitochondrial membrane

> > ATP energy drives endergonic thioester formation (Figure 18.14)

> > Carnitine acyltransferase I catalyzes exchange of CoA for carnitine to carry acyl group into mitochondrion. Carnitine acyltransferase II on inside of mitochondrion catalyzes reverse reaction to yield acyl-CoA and free carnitine (Figure 18.15)

> The -Oxidation Pathway (Figure 18.16)

> > Fatty acids broken down sequentially into acetyl-CoA units (called -oxidation)

> > Process similar to succinate oxidation in the Citric Acid Cycle (dehydrogenation, hydration, dehydrogenation, attack of CoA to carbon to release acetyl-CoA and fatty acyl-CoA two carbons shorter than the original. (Note - unsaturated fatty acids are oxidized slightly differently).

> > Reaction 1: The Initial Dehydrogenation (Diagram, Figure 18.17)

> > > Catalyzed by fatty acyl-CoA dehydrogenase

> > > Electrons passed to enzyme-bound FAD and then to Coenzyme Q (like succinate dehydrogenase)

> > Reactions 2 and 3: Hydration and Dehydrogenation (Diagram)

> > > Catalyzed by enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase, respectively.

> > > Electrons passed to NAD+

> > Reaction 4: Thiolytic Cleavage (Diagram)

> > > Catalyzed by -ketothiolase (thiolase)

> Energy Yield from Fatty Acid Oxidation (Diagram)

> > ATP yield (including ATP from oxidative phosphorylation) = 129 ATP

> Oxidation of Unsaturated Fatty Acids (Table 10.1, Figure 18.18)

> > Enoyl-CoA isomerase

> > 2,4-dienoyl-CoA reductase

> Oxidation of Fatty Acids with Odd-numbered Carbon Chains (Diagram, Figure 18.19)

> > Yields propionyl-CoA

> > > Propionyl-CoA carboxylase

> > > Methylmalonyl-CoA epimerase

> > > Methylmalonyl-CoA mutase (B 12 enzyme)

> > Methylmalonic acidemia

> Control of Fatty Acid Oxidation

> > By fatty acid substrate availability by hormonal control of fat mobilization

> > Triacylglycerol lipase regulated by hormonally controlled cAMP regulatory cascades

> > > Glucagon and epinephrine cause fat breakdown - leads to fatty acid accumulation

> > In liver, malonyl-CoA controls uptake of fatty acyl-CoAs from cytosol into mitochondria by inhibiting carnitine acyltransferase I

> Peroxisomal -Oxidation of Fatty Acids (Diagram)

> > FAD-linked acyl-CoA dehydrogenase transfers electrons directly to oxygen (not electron transport chain) to form hydrogen peroxide.

> > In animals, oxidation stops at C4 and C6 acyl-CoAs.

> > May have function in oxidizing very long chain fatty acids.

> -Oxidation of Fatty Acids (Figure 18.20)

> > Deficient in Refsum's disease - patients accumulate phytanic acid (derived from phytol)

> Ketogenesis (Figure 18.21, Diagram)

> > Ketone body formation - occurs when acetyl-CoA levels are high

> > Thiolase

> > HMG-CoA synthase

> > HMG-CoA lyase

> > Important in starvation

Fatty Acid Biosynthesis

> Relationship of Fatty Acid Synthesis to Carbohydrate Metabolism (Figure 18.22)

> > Acetyl-CoA from fatty acid and carbohydrate catabolism can be made into fatty acids, but not into glucose in animals.

> Early Studies of Fatty Acid Synthesis

> Biosynthesis of Palmitate from Acetyl-CoA (Figure 18.23)

> > Synthesis of Malonyl-CoA (Diagram, Figure 18.24, Diagram)

> > > Acetyl-CoA carboxylase (biotin-containing - is primary regulatory enzyme of pathway)

> > Acyl Carrier Protein (Diagram)

> > > Same thioester linkage to fatty acid as Coenzyme A and same phosphopantetheine moiety too, but is linked to a protein instead of adenosine-3' phosphate-5' bisphosphate (Figure 18.26)

> > > Malonyl-CoA-ACP transacylase & Acetyl-CoA-ACP transacylase

> > From Malonyl-ACP to Palmitate (Figure 18.27, Figure 18.23, Diagram 1, #2, #3)

> > > Fatty acid synthase

> > > 8 acetyl-CoA + 7 ATP + 14 NADPH + 13H<=> Palmitate + 14 NADP+ +8 CoASH + 6 H 2O + 7 ADP + 7Pi + 7 H+

> > Multifunctional Proteins in Fatty Acid Synthesis (Figure 18.29, Figure 18.30)

> > > Intermediates between acetyl-CoA and palmitate do not accumulate in cells that are synthesizing fatty acids

> > Transport of Acetyl Units and Reducing Equivalents into the Cytosol (Figure 18.31, Diagram 1, #2, #3, #4)

> > > Citrate shuttle

> Elongation of Fatty Acid Chains

> > Elongation beyond palmitate occurs in mitochondria and endoplasmic reticulum

> > Uses acyl-CoAs, Malonyl-CoAs, and NADPH (Diagram 1, #2)

> Fatty Acid Desaturation (Table 10.1, Diagram, Figure 18.33)

> > Fatty acyl-CoA desaturase (Figure 18.32)

> > Mammals cannot introduce double bonds beyond position #9 in the fatty acid chain.

> > Thus, linolenic acid is an essential fatty acid

> Control of Fatty Acid Synthesis

> > Largely hormonal control (Figure 18.34)

> > Insulin stimulates fatty acid synthesis by stimulating glucose entry into cells and activating pyruvate dehydrogenase complex.

> > Acetyl-CoA carboxylase inhibited by long chain fatty acyl-CoAs

> > NADPH may be limiting

Biosynthesis of Triacylglycerols (Diagram 1, #2, #3)

> > Occurs through glycerol-3-phosphate and diacyglycerol-3-phosphate - intermediates in fat and phospholipid biosynthesis.

> Variant Fatty Acid Synthesis Pathways that Lead to Antibiotics (Figure 18.35)

> > Polyketides

> > > Erythromycin (from Saccharopolyspora erythraea)

> > > Oxytetracycline (from Streptomyces rimosus)

> > Polyketide synthesis pathway

> > > Starts with propionyl-CoA and Malonyl-CoA

> > > Complex lacks a dehydratase domain and two modules lack the ketoacyl reductase domain

Biochemical Insights Into Obesity

> Ob gene product = leptin

> > Leptin is a hormone that binds receptor in brain

> > When leptin binds receptor, feeding behavior is controlled

> Serotonin - controls satiety

> > Fenfluramine acts by increasing serotonin levels