Lipid Metabolism Part I: Cholesterol and Bile Acid Metabolism PDF

Lipid metabolism Part I: metabolism of cholesterol and bile acid Zhang, Dong PhD, Professor Biomedical Sciences Department [email protected] Harvey and Ferrier: Lippincott Biochemistry 8th Edition: Chapter 18 Marks’ Basic Medical Biochemistry, 6e, Chapter 32, 34 Office of Academic Affairs ...

Lipid metabolism Part I: metabolism of cholesterol and bile acid Zhang, Dong PhD, Professor Biomedical Sciences Department [email protected] Harvey and Ferrier: Lippincott Biochemistry 8th Edition: Chapter 18 Marks’ Basic Medical Biochemistry, 6e, Chapter 32, 34 Office of Academic Affairs  Learning Objectives 1. Compare and contrast the structure and function of cholesterol and cholesterol esters. 2. Distinguish the mechanisms by which cholesterol biosynthesis is regulated by energy availability, hormones, food intake and pharmacological manipulation. 3. Interpret the effect of up-regulating or down-regulating plasma cholesterol levels on the intracellular synthesis of cholesterol and the transcriptional regulation of genes that are involved in cholesterol homeostasis. 4. Explain the bile acid synthesis, metabolism, and their clinical relevance Structure and function of cholesterol  Overview of cholesterol Major functions of cholesterol: 1. A structural component of all cell membranes – modulating their fluidity* 2. Precursors of: o Vitamin D o Sterol hormones o Bile acids Liver plays a central role in cholesterol homeostasis Cholesterol contains a steroid nucleus.  Steriods Steroids are derivatives of a fused, four ring system (A to D, Steroid Nucleus). There are two methyl group substitutions (C18 and C19) and 17 ring carbons for a total of 19 carbons in the steroid skeleton. The carbons are numbered as shown. You DO NOT need to memorize the structure!!!  Sterols Sterols are a subclass of steroids that always contain a hydroxyl group at carbon 3 and an aliphatic chain at least eight carbons long attached to carbon 17 The hydroxyl group at carbon 3 can be esterified to produce sterol esters. Cholesterol has a double bond at carbon 5  Nomenclature of Steriods and Sterols: α, β and ∆ Substituents that are above the plane of the ring system are said to be in β orientation. β substituents are shown connected to the ring system by a solid line. The methyl groups in steroids, carbon 18 and 19 are always in β orientation. Substituents oriented below the plane of the ring system are said to be in the α orientation. Groups in α-orientation are shown connected to the ring system by dashed lines. Double bonds are indicated by a ∆ symbol followed by a superscript that denotes the number of the carbon atom where the double bond is located.  Roles of cholesterol-I: Membrane Component Cholesterol, which is interspersed between membrane phospholipids, maintains membrane fluidity: is an important component of mammalian plasma membranes and myelin (about 25% of total lipid); ( present at much lower concentrations in organelle membranes); buffers the fluidity of cell membranes  Roles of cholesterol-II: Vitamin D synthesis A B C D Vitamin D3 and calcium metabolism: Vitamin D3 (vit. D3) is converted to a hormone that enhances intestinal Ca2+ absorption and demineralization of the bone Vit. D3 is formed endogenously from cholesterol in skin when exposed to UV 1,25-Dihydroxycholecalciferol is the active form of vit. D3 PTH: parathyroid hormone  Roles of cholesterol-III: Steroid hormone synthesis Synthesis: Shortening the hydrocarbon chain Oxidation of the steroid nucleus Steroid hormones: (sex steroids, glucocorticoids, mineralocorticoids) Progesterone Testosterone Estradiol Cortisol others Genetic diseases related to steroid hormone synthesis  Roles of cholesterol-IV: Synthesis of Bile acids/salts Quantitatively, the most important product of cholesterol are bile acids. 7-hydroxylase catalyzes the rate-limiting step + Cholesterol of bile acid synthesis. 3 7 3 7 Primary bile acids:  Plant Sterols Plants do not have significant amounts of cholesterol. But they do have other similar sterols such as stigmasterol, sitosterol, sitostanol etc. Plant sterols are not absorbed efficiently by the human digestive system as seen in the next slide) Plant sterols in the diet seem to inhibit the absorption of cholesterol in diet. Structural comparison between cholesterol and plant sterols Absorption rate: 33% Absorption rate: 4.2% Absorption rate: 4.8% Absorption rate: 0-3%  Cholesterol Balance Western diets contain roughly 400-600 mg /day of cholesterol; about half is absorbed. About 800 mg / day of cholesterol is synthesized by the body. This production is balanced mostly by the excretion of bile acids, about 1000-1100 mg/ day. So, if we want to decrease cholesterol levels in the body, we could: 1. Decrease dietary cholesterol uptake 2. Decrease cholesterol synthesis in the body 3. Increase the excretion of bile acids Biosynthesis of cholesterol The structure of my lectures related to key metabolic pathways 1) What is the purpose or importance of the pathway? 2) What is (are) the committed, or rate-limiting, or irreversible reaction(s)? The enzyme(s)? 3) What are the featured reactions of the pathway and enzymes catalyzing the reactions? 4) How is the pathway regulated? 5) Clinical correlation.  Biosynthesis of cholesterol Most tissues can synthesis cholesterol: liver (the main source), adrenals, gonads, intestine, placenta. Biosynthesis is metabolically regulated.  Biosynthesis of cholesterol: Stage 1 Stage 1: Synthesis of mevalonate from acetyl CoA Cholesterol is synthesized in the cytoplasm Citrate transport system moves excess acetyl CoA from mitochondrial matrix to the cytoplasm glycolysis Pyruvate Pyruvate carboxylase dehydrogenase Citrate synthase  Biosynthesis of cholesterol: Stage 1 (continued) Stage 1: Synthesis of mevalonate (6C) from acetyl CoA (2C) 1. Synthesis of acetoacetyl CoA by thiolase 2. Cytosolic HMG CoA synthase condenses acetyl CoA and acetoacetyl CoA 3. HMG CoA reductase is the key regulatory point in cholesterol biosynthesis – Reduces HMG CoA to form mevalonate – Requires 2X NADPH – Located in the ER membrane – Rate-limiting – Irreversible 3X Acetyl CoA  Biosynthesis of cholesterol: Stage 2-4 6C 5C Synthesis of isopentenyl PP (IPP, isoprene unit) – another key intermediate of cholesterol synthesis IPP is the activated 5-carbon unit You DO NOT need to memorize this!!!  Fates of cholesterol The cholesterol may then be esterified : Transfer of acyl groups from fatty acyl CoA by acyl CoA- cholesterol acyltransferase (ACAT) and stored in the cell as cholesterol esters Secreted out as a part of lipoproteins (e.g., VLDL) Converted to cholesterol esters by the transfer of an acyl group from carbon 2 of lecithin (also called Phosphatidylcholine) by Lecithin - cholesterol acyltransferase (LCAT) in HDL in the plasma. Structure of Lecithin (Phosphatidylcholine) Lecithin is a phospholipid Regulation of cholesterol biosynthesis  Regulation of cholesterol biosynthesis-I: transcription Transcriptional control of cholesterol biosynthesis: Sterol Regulatory Element Binding Protein (SREBP) controls the rate of synthesis of HMG CoA reductase SREBP DNA-binding domain binds to the Steroid Response Element (SRE) of the HMG CoA reductase gene and turns on its transcription when cholesterol levels are low SREBP is an ER membrane protein, associated with SCAP (SREBP Cleavage Activating Protein) – when [Cholesterol] is high, cholesterol binds to SCAP and inactivates it. – when [Cholesterol] is low, SCAP-SREBP is transported to Golgi In Golgi, SCAP activates proteases, which then cleave the SREBP and release the DNA binding domain, which then translocates to the nucleus and binds the SRE. SREBP then activates the transcription of genes involved in cholesterol biosynthesis, including HMG CoA reductase.  Regulation of cholesterol biosynthesis-II: proteolysis Cholesterol Regulation of cholesterol biosynthesis at the HMG CoA reductase-catalyzed step: Post-transcriptional control – Proteolysis of HMG CoA reductase enzyme – mRNA of HMG CoA reductase is degraded when cholesterol is abundant  Regulation of cholesterol biosynthesis-III: phosphorylation FAST Regulation of cholesterol biosynthesis through phosphorylation and dephosphorylation of HMG CoA reductase: AMP↑ (low energy signal) → activation of AMP-activated protein kinase (AMPK) → phosphorylation Glucagon and high level of sterols activate AMPK, then result in phosphorylation→cholesterol ↓ Cholesterol Synthesis ↑ Insulin activates a protein phosphatase, then results in dephosphorylation→cholesterol ↑ FED  Hormonal regulation of metabolism Mobilizing hormone Storage hormone ↓Cholesterol synthesis ↑Cholesterol synthesis Insulin activates a PHOSPHATASE Glucagon activates a KINASE (PKA, AMPK)  Summary of the regulation of cholesterol biosynthesis - HMG CoA reductase Catalyzes the rate limiting step in cholesterol biosynthesis Cholesterol functions as a feedback inhibitor of HMG CoA reductase synthesis through a complex transcriptional regulatory mechanism: a protein complex SREBP:SCAP is retained in the ER membranes when high levels of cholesterol are present. When cholesterol is low, SREBP:SCAP is exported to the Golgi, SREBP is cleaved and a cleavage fragment of SREBP enters the nucleus to serve as a transcription factor and stimulates the expression of HMG CoA reductase HMG CoA reductase degradation is accelerated by high levels of cholesterol. AMP level controls HMG CoA reductase activity through reversible phosphorylation (by AMPK); the phosphorylated enzyme (present under high AMP conditions) is less active than the unphosphorylated enzyme. Insulin and thyroid hormone stimulate HMG CoA reductase activity. Glucagon, cholesterol, chylomicron remnants and Low density lipoproteins (LDL) inhibit HMG CoA reductase activity HMG CoA reductase is the target of cholesterol lowering drugs - statins  Synthesis of Bile Acids The terms, bile acids and bile salts, are often used interchangeably!!!  The role of bile salts in the digestion of lipid Dietary lipids (fat) have low solubility Bile salts act as the detergent to emulsify dietary lipids Bile is a mixture of water, phosphatidylcholine (lecithin), bile salts and cholesterol Cholesterol is the precursor of bile salts Bile salts are synthesized in the liver and stored in the gallbladder Bile salts are reabsorbed in the GI tract for re- circulation back to the liver (enterohepatic circulation) A significant amount of bile salts are excreted in the feces everyday. Bile salts excretion is the major way that Cholesterol is eliminated from the body  Synthesis of bile salts-I Bile acids (24C) are detergents derived from cholesterol (27C) by: increasing hydroxylation of the steroid nucleus removing three carbons (-3C) from the aliphatic chain adding a carboxyl group at carbon 24 saturating the double bond in cholesterol Hydroxylation and carboxylation increase the solubility of bile salts. Primary bile acids:  Synthesis of bile salts-II Bile acids made in the liver directly from cholesterol; cholic acid and chenodeoxycholic acid are also known as the 'primary bile acids’ Hydroxylation at carbon 7 by 7α-hydroxylase is the rate limiting step in bile acid synthesis Deoxycholic acid and lithocholic acid are formed in the intestine from the primary bile acids by intestinal bacterial enzymes and are thus called the 'secondary bile acids’ Secondary bile salts are less soluble (because they lost a OH) and more likely to be excreted  Synthesis of bile salts-III Conjugated bile salts: the bile acids can also be conjugated to glycine or taurine via peptide bonds through the carbon 24 carboxylate. Conjugated bile salts have lower pKa A higher percentage of the conjugated bile salts is present in the ionized form at the pH of intestine (~6) Better detergents Summary of bile salts metabolism The bile acids secreted into the intestine are efficiently absorbed back into the blood stream in the ileum > 95% Venous blood from the ileum goes into the portal vein and directly to the liver. The liver efficiently extracts the bile acids and re-uses them to produce bile. Less than a gram a day of bile salts are excreted in the feces. The liver → bile duct → duodenum → ileum → portal vein → liver cycle of bile acids is called the enterohepatic circulation. The efficiency of the liver in extracting bile acids may be compromised under conditions of liver disease, resulting in bile acids in the blood.  Bile salt deficiency: cholelithiasis Cholelithiasis (cholesterol gallstone disease):  the imbalance of cholesterol, phospholipids, and bile salts leads to the formation of stones in the gallbladder.  Summary Key concepts: steriod, sterol, steroid nucleus, plant sterols, bile acids/bile salts, primary bile salts, secondary bile salts, conjugated bile salts, enterohepatic circulation Function of cholesterol The rate-limiting step and committed step of cholesterol biosynthesis and the key enzymes involved Key regulatory mechanisms of cholesterol biosynthesis Function and synthesis of bile acids Clinical correlations: disease symptoms, biochemical basis, and treatment (Cholelithiasis) Where does acetyl CoA come from? Carbohydrates Fatty acids Amino Acids  Lecture Feedback Form: https://comresearchdata.nyit.edu/redcap/surveys/?s=HRCY448 FWYXREL4R