Lipids 3 2024.pdf

LEARNING OBJECTIVES 1. Understand the physiological importance of the lipid metabolic pathways: a) de novo synthesis of fatty acids b) beta-oxidation c) ketolysis 2. Describe the De novo synthesis of fatty acids 3. Understand the pathway for mobilization of fats (from adipose tissue) 4. Describe the...

LEARNING OBJECTIVES 1. Understand the physiological importance of the lipid metabolic pathways: a) de novo synthesis of fatty acids b) beta-oxidation c) ketolysis 2. Describe the De novo synthesis of fatty acids 3. Understand the pathway for mobilization of fats (from adipose tissue) 4. Describe the beta-oxidation process (including the role of the carnitine shuttle) 5. Understand what are ketone bodies and its relevance 6. Briefly describe ketolysis LIPIDS – fatty acid metabolism Fatty acids are synthesized from and oxidized to a common compound → ACETYL-COA Fatty acids are oxidized in the mitochondria and synthesized in the cytoplasm Physiological conditions that promote FA synthesis largely inhibits oxidation (and viceversa) → Preventing futile cycling! Image from Quora.com LIPIDS – De novo synthesis of Fatty Acids Fatty acid synthesis (de novo) occurs mainly in the cytosol of: Liver cells (hepatocyte) Mammary glands (lactating) Adipose tissue cells (adipocyte)  Other tissues can synthesize FA in small quantities → kidneys, brain and lungs  Animals can synthesize all FAs they need except for the essential FAs, which must be supplied through the diet  Short FAs are produced in lactating mammary glands (butiric acid, caproic acid → milk fat) LIPIDS – De novo synthesis of Fatty Acids Substrates are: Excess carbohydrates and proteins from the diet that exceed the body‘s needs for these nutrients during feeding period  Acetyl Coenzyme A (ACoA) from mitochondria is key  The process requires ATP and NADPH Primary product:  Palmitate or palmitic acid (16 C) is the primary end product of De novo FA Synthesis  Can be further elongated in smooth endoplasmic reticulum (sER)  Brain cells can produce very long FA required for synthesis of brain phospholipids  Certain enzymes present in sER can cause desaturation of LCFA by adding cis double bonds  A variety of polyunsaturated FA (PUFA) can be produced by desaturation + elongation LIPIDS - De Novo Synthesis of FA 1. Cytosolic Acetyl CoA Production Move acetate units from mitochondrial acetyl CoA to the cytosol (from the mitochondrial matrix) Mitochondrial ACoA is produced mainly by oxidation of pyruvate Since CoA portion of ACoA cannot cross inner mitochondrial membrane, acetyl group must be incorporated into citrate for membrane transport Citrate is produced by condensation of ACoA with oxaloacetate (OAA) Citrate in the cytosol is then cleaved to OAA and ACoA by ATPcitrate lyase This process is stimulated when mitochondrial [citrate] is high → this happens when [ATP] is high → high energy signal From: Harvey and Ferrier. Biochemistry LIPIDS - De Novo Synthesis of FA 2. ACoA carboxylation to Malonyl CoA Carboxylation of ACoA to malonyl CoA is catalyzed by acetyl CoA carboxylase (ACC) Biotin (Vit H or B7) and ATP are required in the carboxylation process This is the rate-limiting step and the regulated step in FA synthesis  ACC is allosterically activated by citrate, and inactivated by palmitoyl CoA (pathway end product-negative feedback)  ACC synthesis is also stimulated by high-calory and highcarbohydrate diets (nutrient availability) and hormonally (insulin) From: Harvey and Ferrier. Biochemistry LIPIDS - De Novo Synthesis of FA 3. Synthesis of Palmitate 16:0 All other reactions of fatty acid synthesis (in eukaryotes) are driven by the enzyme fatty acid synthase (FAS) The result is the production of palmitate (a fully saturated fatty acid, 16:0) This involves the addition of two carbons from malonyl CoA to the carboxyl end of a series of acyl acceptors (amino acid such as cysteine). Pentose phosphate pathway provides reductant NADPH Carbons provided directly by ACoA (via malonyl CoA) are red, carbons released as CO2 are blue From: Harvey and Ferrier. Biochemistry LIPIDS - Storage LIPIDS ARE STORED AS TRIACYLGLYCEROL The three FA in TAG are generally not the same type: C-1 often saturated, C-2 unsaturated, C-3 either Presence of unsaturated FA decreases the Tm of the lipid Newly synthesized fatty acids can be stored as mono- (one), di- (two) or triacylglycerols (three molecules of fatty acids) Lipids caloric value per unit mass is over twice as great as carbs and proteins (i.e.,9 kCal/gm for TG compared to about 4 kCal/gm for carbohydrate and From: Harvey and Ferrier. Biochemistry protein) LIPIDS - Storage Stored as lipid droplets in adipocytes (fat depot)  a small part is stored in the liver and released into the blood as VLDL Mobilization of fat (from adipose tissue) requires release from their TAG form → lipolysis  With help of Hormone-Sensitive-Lipase (HSL) (stimulated by epinephrin and glucagon) FATTY ACID β-OXIDATION Major pathway for FA catabolism → Occurs in the mitochondria  Long chain fatty acids must form an active intermediate (fatty acyl CoA) before being oxidized inside the mitochondria  The carnitine shuttle is required to transport fatty-acyl-CoA into the mitochondria (rate limiting transport) From: Harvey and Ferrier. Biochemistry FATTY ACID β-OXIDATION Carnitine: Carnitine is a compound synthesized from amino acids lysine and methionine in liver and kidneys (abundant in mitochondrial membranes of muscle tissue) Carnitine can be taken up from the diet  mostly from animal products (red meat, poultry, dairy) Carnitine deficiencies cause decreased ability of tissues to use LCFA as fuel  Can be caused by cellular defects, genetic or medical conditions, or due to liver or kidney pathology Carnitine shuttle (CPT1) can be inhibited by malonyl CoA, so newly synthesized FA cannot be transferred into mitochondria to be degraded L- Carnitine FATTY ACID β-OXIDATION  It is a cyclic process, each cycle is catalyzed by enzymes with chainlength specificity  Each cycle produces: 1 acetyl-CoA + 1 NADH + 1 FADH2 First cycle of β-oxidation : A sequence of four reactions that involve the β-carbon and cause the shortening of the FA by two carbons at the carboxyl end an oxidation that produces FADH2 a hydration a second oxidation that produces NADH a CoA-dependent thiolytic cleavage that frees a molecule of acetyl CoA From: Harvey and Ferrier. Biochemistry SUMMARY OF THE ENERGY YIELD FROM THE OXIDATION OF 1 PALMITOYL CoA (16 CARBONS) From: Harvey and Ferrier. Biochemistry Oxidation of 1 palmitoyl CoA:  8 ACoA  7 NADH  7 FADH2 Final products after full oxidation (beta oxidation, TCA cycle and OxPhos)  ATP  CO2  H2O *2 ATP: activation of palmitate to palmitoyl CoA (fatty acyl CoA) LIPIDS - KETONES KETONE BODIES: ALTERNATIVE FUEL FOR CELLS  The adult liver mitochondria can convert ACoA from fatty acid oxidation (beta oxidation) into ketone bodies → acetoacetate, ß- hydroxybutyrate and acetone  Acetoacetate and ß-hydroxybutyrate are free soluble lipids  Transported in the blood plasma to peripheral tissues (muscle, brain, kidney, mamary gland, small intestine, fetal liver)  In peripheral tissue cells, it can be converted back into acetyl CoA, which enters the What happens to acetone? Acetone is metabolically inert, can cause fruity smell on the breath and urine of ketotic patients TCA cycle for ATP production  Can be used in the biosynthesis of glycerophospholipids, sphingolipids and sterols  Negative feedback on hormone sensitive lipase (HSL) activity in adipocytes Cerebral metabolic adaptation and ketone metabolism after brain injury FYI https://pubmed.ncbi.nlm.nih.gov/17684514/ LIPIDS - KETONES Important energy supply for peripheral tissues: Are water soluble, can be transported without albumin or lipoproteins Can cross blood brain barrier and placental barrier Are used proportionally to their concentration in the blood by extrahepatic tissue If concentrations are high enough cardiac and skeletal muscle, intestinal mucosa cells, renal cortex, brain, fetus can use ketone bodies LIPIDS - KETONES Lipolysis of triglycerides in adipocytes stimulates production of ketone bodies  Increased Acetyl CoA (due to beta oxidation of FFAs) → exceeds the oxidative capacity of the liver → stimulating the production of ketone bodies As a result, ketone bodies save glucose (important during fasting and prolonged exercise) During fasting, fatty acids mobilized from adipose tissue move to the liver Fatty acid oxidation produces high amounts of NADH (exceeding oxidative capacity of TCA/OxPhos in the liver)  ACoA goes into ketogenesis LIPIDS- KETOLYSIS KETOLYSIS: KETONE BODIES USED BY THE PERIPHERAL TISSUES In normal conditions, the liver constantly produces low levels of ketone bodies  However, it increases during fasting (or pathologic conditions such as diabetes mellitus) when ketone bodies are required as source of energy to peripheral tissues KETOLYSIS in peripheral tissue 3-hydroxybutyrate is oxidized to acetoacetate Acetoacetate + CoA molecule → Acetoacetyl CoA Acetoacetyl CoA → 2 Acetyl-CoA  Ketone bodies (KB) synthesis occur in the liver → KB used in peripheral tissues From: Harvey and Ferrier. Biochemistry  KB are hydrophilic → quickly transported via plasma  Mammalian RBC and liver cannot use KB as source of energy RBC lack mitochondria hepatocyte lack thiophorase