Fat and Cholesterol Metabolism Notes

NOTES NOTES FAT & CHOLESTEROL METABOLISM CHOLESTEROL METABOLISM osms.it/cholesterol-metabolism ▪ Cholesterol insoluble in water → moves through blood stream with lipoproteins ▪ Cholesterol used in cell membrane for flexibility, durability ▫ At ↓ temperature, cholesterol squeezed between phospholipid molecules, keeps membrane fluid ▫ At ↑ temperature, cholesterol pulls phospholipid molecules together ▪ Cholesterol used by adrenal glands, gonads; makes steroid hormones ▫ Adrenal glands form corticosteroids (e.g. cortisol, aldosterone); testes (testosterone); ovaries (estradiol, progesterone) CHOLESTEROL SYNTHESIS ▪ Mevalonate pathway; occurs in smooth endoplasmic reticulum Pathway ▪ Two acetyl-CoA molecules joined by acetyl-CoA acyltransferase → acetoacetylCoA, CoA ▪ HMG-CoA synthase combines acetoacetylCoA, acetyl-CoA → 3-hydroxy-3methylglutaryl-CoA (HMG-CoA), CoA ▪ HMG-CoA reductase reduces HMG-CoA into mevalonate, removes CoA-SH, water ▫ Rate limiting cholesterol synthesis step ▪ Mevalonate-5-kinase uses ATP to phosphorylate mevalonate → mevalonate5-phosphate ▪ Phosphomevalonate kinase uses ATP to phosphorylate mevalonate-5-phosphate → mevalonate pyrophosphate ▪ Mevalonate pyrophosphate decarboxylase removes carboxyl group → isopentenyl pyrophosphate ▪ Geranyl transferase condenses three isopentenyl pyrophosphate molecules → farnesyl pyrophosphate ▪ Squalene synthase condenses two farnesyl pyrophosphate molecules → squalene ▪ Oxidosqualene cyclase converts squalene into lanosterol (cyclization) ▪ Lanosterol converted into 7-dehydrocholesterol, eventually cholesterol Cholesterol synthesis regulation ▪ SREBP, INSIG1, SCAP (collection of proteins) ▫ ↓ cholesterol → INSIG1 falls off of SCAP-SREBP → SREBP cleaving → binds sterol regulatory element → ↑ HMG-CoA reductase gene expression CHOLESTEROL USE & STORAGE ▪ Majority of cholesterol used by liver, ends up as bile acids ▪ Include cholic acids, chenodeoxycholic acids ▫ Conjugation with taurine forms taurocholic acid, taurochenodeoxycholic acid respectively ▫ Conjugation with glycine forms glycocholic acid, glycochenodeoxycholic acid respectively ▪ Stored in gallbladder ▪ Released into intestines after meals, aids fat digestion ▪ Most reabsorbed by intestine; some eliminated through feces ▫ Enterohepatic circulation: reabsorbed bile acids enter portal bloodstream, return to liver cells OSMOSIS.ORG 13

Figure 2.1 Cholesterol synthesis via the mevalonate pathway. 14 OSMOSIS.ORG

Chapter 2 Biochemistry: Fat & Cholesterol Metabolism Figure 2.2 Cholesterol synthesis regulation. FATTY ACID SYNTHESIS osms.it/fatty-acid-synthesis ▪ Fatty acids: simplest lipid form ▫ Long carbon, hydrogen chain atoms ▫ Classification: short, medium, long, very long chain fatty acids ▪ Short, medium chain fatty acids ▫ Primarily obtained from diet ▪ Long, very long chain fatty acids ▫ Synthesized by liver, fat cells ▪ Synthesis: combine acetyl-CoA molecules into palmitoyl-CoA ▫ 16 carbon chain fatty acid; precursor to longer chain fatty acids BEFORE FATTY ACID SYNTHESIS ▪ Acetyl-CoA must be obtained ▪ In response to insulin, cells take in glucose ▫ Consumed as carbohydrates ▪ In cell, glycolysis breaks glucose down into pyruvate molecules ▪ Mitochondria convert pyruvate into acetylCoA using pyruvate dehydrogenase ▪ Typically, acetyl-CoA combines with oxaloacetate, enters citric acid cycle → forms citrate → forms electron carriers ( join electron transport chain in oxidative phosphorylation) → creates adenosine triphosphate (ATP) FATTY ACID SYNTHESIS ▪ ATP inhibits enzymes needed for citric acid cycle ▫ Allows additional acetyl-CoA to be funneled toward pathways involving fatty acid synthesis Stages ▪ Acetyl-CoA combines with oxaloacetate → forms citrate → crosses mitochondrial membrane into cytoplasm ▪ In cytoplasm, citrate lyase cleaves citrate into acetyl-CoA, oxaloacetate ▫ Malic enzyme converts oxaloacetate into pyruvate (NADP+ → NADPH in process), which can cross back into membrane ▫ Then converted back into oxaloacetate by pyruvate carboxylase ▪ Acetyl-CoA carboxylase adds carboxyl group to acetyl-CoA → forms malonyl-CoA ▫ Rate limiting fatty acid synthesis step ▫ Requires ATP, biotin, carbon dioxide (A- OSMOSIS.ORG 15

B-C) as cofactors ▫ Acetyl-CoA carboxylase: tightly regulated (hormonal, allosteric regulation); hormonal regulation uses insulin, glucagon to remove/ add phosphate group on acetyl-CoA carboxylase; insulin ↑ activity/vice versa; allosteric regulation uses citrate, fatty acids to ↑/↓ acetyl-CoA carboxylase activity by allosteric binding ▪ Multiple enzymes form fatty acid synthase complex (acyl carrier protein (ACP) on one end, cysteine amino acid on other) ▪ Acetyl-CoA ACP transacylase removes CoA group from acetyl-CoA, attaching resulting acetate to ACP → moves to cysteine residue ▪ Malonyl-CoA ACP transacylase removes CoA group from malonyl-CoA, attaching resulting malonate to ACP ▪ 3-ketoacyl-ACP synthase cuts off carbon (was added to malonate earlier), released as CO2 (leaving behind acetate) → condenses it with acetate on cysteine residue → forms four carbon chain (using one NADPH molecule for each process) ▪ Malonyl-CoA added across seven cycles forming 16 carbon chain fatty acid polymer ▫ Each cycle uses one acetyl-CoA (converted into malonyl-CoA), two NADPH molecules ▪ In total, eight acetyl-CoA molecules (including initial molecule) used along with 14 NADPH molecules Figure 2.3 Acetyl-CoA is produced by mitochondria using pyruvate molecules (made during glycolysis). ATP inhibits citric acid cycle enzymes so that acetyl-CoA can be used in fatty acid synthesis pathways. 16 OSMOSIS.ORG

Chapter 2 Biochemistry: Fat & Cholesterol Metabolism Figure 2.4 The citrate shuttle transports acetyl-CoA out of the mitochondria by combining it with oxaloacetate to form citrate. Once citrate is in the cytoplasm, it is converted back to oxaloacetate and acetyl-CoA, allowing acetyl-CoA to be used in fatty acid synthesis. Figure 2.5 Fatty acid synthesis. Malonyl-CoA added across seven cycles → 16 carbon chain fatty acid polymer called palmitoyl-CoA. OSMOSIS.ORG 17

FATTY ACID OXIDATION osms.it/fatty-acid-oxidation ▪ AKA β-oxidation ▪ Fatty acids broken down to produce energy ▪ Takes place in mitochondria of heart, skeletal muscles, liver cells OXIDATION PREPARATION ▪ Triglycerides (three fatty acids attached to glycerol) in adipocytes → broken down by hormone sensitive lipase ▫ ↓ blood glucose → ↑ glucagon → ↑ hormone sensitive lipase → ↑ fatty acid breakdown ▪ Fatty acids leave fat cells → enter bloodstream ▪ Albumin in blood binds to fatty acids → carries them to target cells ▪ Fatty acid dissociates from albumin → diffuses into cell ▪ Fatty acyl-CoA synthetase adds CoA to end of fatty acid (→ fatty acyl-CoA), using up two ATP molecules ▪ Fatty acyl-CoA cannot cross cell membrane, carnitine shuttle used ▫ Carnitine acyltransferase 1 (outer membrane) replaces CoA on fatty acid with carnitine (→ fatty acyl-carnitine) ▫ Fatty acyl-carnitine, CoA cross inner mitochondrial membrane ▫ Carnitine acyltransferase 2 (inner membrane) replaces carnitine on fatty acid with CoA (→ fatty acyl-CoA) OXIDATION PROCESS ▪ Occurs on ⍺, β carbon atoms of fatty acylCoA ▫ Acyl-CoA dehydrogenase moves one hydrogen from each carbon to nearby flavin adenine dinucleotide molecule (FAD) → FADH2, enoyl-CoA ▫ Enoyl-CoA hydratase transfers hydroxyl group to β carbon → β-hydroxyacylCoA ▫ β-hydroxyacyl-CoA dehydrogenase removes two hydrogens from β carbon 18 OSMOSIS.ORG transferring one to nicotinamide adenine dinucleotide (NAD) → NADH, β-ketoacyl-CoA ▫ β-ketothiolase cleaves off two carbon atoms → acetyl-CoA, fatty acyl-CoA molecule (two carbons shorter—which can be further oxidized) OXIDATION CYCLE ▪ One oxidation cycle: 1 NADH, 1 FADH2, 1 acetyl-CoA ▪ Fatty acids with even number of carbon atoms ▫ Oxidation repeats until just acetyl-CoA remains ▪ Fatty acids with odd number of carbon atoms ▫ Oxidation repeats until three carbon propionyl-CoA is left; propionyl-CoA is broken down differently ▪ Propionyl-CoA carboxylase ▫ Adds carboxyl group to propionyl-CoA → methylmalonyl-CoA ▫ Cofactors required: ATP, biotin, carbon dioxide (A-B-C) ▪ Methylmalonyl-CoA mutase ▫ Rearranges carbon atoms on methylmalonyl-CoA → succinyl-CoA ▫ Cofactor required: Vitamin B12 ▪ Succinyl-CoA ▫ Can enter citric acid cycle/used for heme synthesis ▪ Very long fatty acids (22 carbons atom/ longer) ▫ Peroxisomes may be needed ▫ Peroxisomal oxidation uses different enzymes until fatty acid is smaller than 22 carbon atoms ▪ NADH, FADH2 ▫ Can enter electron transport chain ▫ Creates ATP → approximately three + two ATP molecules

Chapter 2 Biochemistry: Fat & Cholesterol Metabolism ▪ Acetyl-CoA ▫ Can enter citric acid cycle ▫ Creates more NADH, FADH2 → approximate total of 12 ATP molecules Figure 2.6 Oxidation preparation requires the use of two ATP molecules and results in fatty acyl-CoA being present in the mitochondrial matrix. OSMOSIS.ORG 19

Figure 2.7 Oxidation preparation requires the use of two ATP molecules and results in fatty acylCoA being present in the mitochondrial matrix. Figure 2.8 Fatty acid oxidation when the fatty acid has an odd number of carbon atoms. 20 OSMOSIS.ORG

Chapter 2 Biochemistry: Fat & Cholesterol Metabolism KETONE BODY METABOLISM osms.it/ketone-body-metabolism KETONE BODIES ▪ Acetoacetate, β-hydroxybutyrate, acetone (all contain ketone C=O group) ▪ Produced by liver mitochondria using acetyl-CoA ▫ During physiological states such as fasting, carbohydrate-restrictive diets (e.g Atkins, ketogenic diet), intense exercise, pathological states such as Type 1 diabetes mellitus, alcoholism (lack of glucose to power cells) ▪ Released into bloodstream → picked up by majority of cells → re-converted into acetyl-CoA → enter mitochondria, produce ATP KETONE BODY BREAKDOWN ▪ β-hydroxybutyrate, acetoacetate in blood diffuses into peripheral tissue mitochondria ▫ β-hydroxybutyrate dehydrogenase converts β-hydroxybutyrate back into acetoacetate ▪ Thiophorase can add CoA from succinylCoA to acetoacetate → form acetoacetylCoA, succinate ▪ β-ketothiolase cleaves acetoacetylCoA with CoA → forms two acetyl-CoA molecules ▫ Can enter citric acid cycle to make ATP KETONE BODY SYNTHESIS ▪ Two acetyl-CoA molecules are joined (acetyl-CoA acyltransferase) → acetoacetyl-CoA + CoA ▪ HMG-CoA synthase ▫ Acetoacetyl-CoA + acetyl-CoA → 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) + CoA (rate-limiting ketone body synthesis step) ▪ HMG-CoA lyase removes acetyl-CoA from HMG-CoA → acetoacetate ▪ Remaining ketone bodies formed ▫ β-hydroxybutyrate dehydrogenase adds hydrogen from NADPH to acetoacetate → β-hydroxybutyrate ▫ Acetoacetate in blood spontaneously loses a carbon → acetone (exhaled through lungs) OSMOSIS.ORG 21

Figure 2.9 Ketone body synthesis. Figure 2.10 Ketone body breakdown. 22 OSMOSIS.ORG