Lecture 23 Lipogenesis Biochemistry & Genetics PDF

PBBS508 Biochemistry & Genetics L23: Lipogenesis Asst. Prof. Witchuda Saengsawang, Ph.D.  Learning Objectives VB3 Describe when and where lipogenesis occurs. VB4 Describe in detail the process of converting mitochon...

PBBS508 Biochemistry & Genetics L23: Lipogenesis Asst. Prof. Witchuda Saengsawang, Ph.D.  Learning Objectives VB3 Describe when and where lipogenesis occurs. VB4 Describe in detail the process of converting mitochondrial acetyl CoA into cytoplasmic malonyl-Co A including the enzymology and regulation. VB5 Describe the reactions carried out by FA synthase. VB6 Describe where the carbons in the growing chain originate. VB7 Explain the significance of NADPH as substrate and palmitate and CO2 as products in this pathway. VB8 Explain how fatty acids are elongated and desaturated. VB9 Describe essential fatty acids. VB10 Describe triacylglycerol synthesis and contrast the substrates used in liver versus adipose tissue for this process.  storage > - form in Adipose Tissue Lipogenesis Acid When?: excess carbohydrate (more than need for glycolysis and glycogenesis) Fatty - synthesis A diet rich in carbohydrates leads to stimulation of both the glycolytic and lipogenic pathways. 7 De novo lipogenesis : excess glucose is converted to fatty acids in liver (Excess amino acid as well) TAG synthesis: TAG is then formed from Fatty acids and glycerol TAGs is stored in adipose tissues Glucose 16 Carbon fatth Acids - Fatty acids (palmitate) Use Elongate/Desaturate TAGs 3 FatY AcidS ↳ ↓ 1 Glycero De Novo lipogenesis or Fatty acid synthesis Where? Fatty acid synthesis occurs primarily in the cytoplasm of: Liver (major) Lactating mammary gland Central nervous system Adipose tissue (very little) Glucose -> Pyruvate -> Acetyl-CoA -> 16C Fatty Acid (palmitate) FA synthesis is the process of combining 8 two-carbon fragments (acetyl groups from acetyl CoA) to form a 16-carbon saturated fatty acid called palmitate (palmitic acid, 16:0). Acetyl-CoA Palmitate can then be modified to give rise to the other fatty acids (not all). (eg. chain elongation -> longer fatty acids, desaturation -> unsaturated fatty acids or shortening). Overview of Fatty Acid synthesis Glucose -> Pyruvate -> Acetyl-CoA -> Fatty Acid (Palmitate) 8 Acetyl CoA + 7 ATP + 14 (NADPH + H+) t Need Substrate: Acetyl CoA (need ATP and NADPH) Muse First product -> Malonyl CoA à Final product: Palmitate Palmitate(16:0) + 8 CoA + 7 CO2+ 7 (ADP + Pi) + 14 NADP+ + 6 H2O Two enzymes: Acetyl CoA Carboxylase and Fatty acid synthase Where does Acetyl CoA come from? Glucose is first catabolized to pyruvate by glycolysis in the cytoplasm Pyruvate is transported into the mitochondria. Pyruvate dehydrogenase (PDH) decarboxylates pyruvate, forming Acetyl CoA (in the mitochondria) FomTC S From GlYCOMSis Produces H How to get Acetyl CoA in the cytoplasm? Where does NADPH come from? Fatty acid synthesis occurs in the cytoplasm Acetyl CoA is combined with oxaloacetate (OAA) to form Citrate (by citrate synthase) in mitochondria Citrate shuttle transports Citrate from mitochondria to cytoplasm In cytoplasm then Citrate Lyase splits Citrate back into Acetyl CoA*** and oxaloacetate (OAA) Majority of NADPH comes from HMP shunt* (Malic enzyme convert Malate to Pyruvate and also generate some NADPH) Citrate synthase * Citrate Lyase What is rate limiting step? Conversion of Acetyl CoA to Malonyl CoA by enzyme Acetyl CoA carboxylase is rate limiting step***. Acetyl CoA carboxylase*** is Rate-limiting enzyme for fatty acid synthesis (De novo lipogenesis). This enzyme is an important target for regulations. Enzyme Rate limiting Lipogenesis St *** for (2C) * (3C) ↳ Adds the Carbon How is regulated? Allosteric Regulators Hormonal regulation: Citrate activates ACC Insulin activates ACC and Fatty acid synthase State Fasting Palmitate inhibits ACC Glucagon inhibits ACC and Fatty acid synthase - Fed Glucagon/Epinephrine Insulin state - + - + *** + - Synthesis of palmitate by Fatty Acid Synthase Fatty Acid Synthase is a muti-functional dimeric enzyme that synthesize palmitate (FA) Substrates and cofactor: Acetyl CoA + Malonyl CoA + NADPH * Fatty Acid Synthase Detail 8 Acetyl CoA (1 Acetyl CoA+ 7 Malonyl CoA ) + 7 ATP + 14 (NADPH + H+) Palmitate(16:0) + 8 CoA + 7 CO2+ 7 (ADP + Pi) + 14 NADP+ + 6 H2O The series of reactions is catalyzed by the muti-functional dimeric Condensation enzyme Fatty Acid Synthase. Dehydration Reduction Each monomer of the enzyme has 7 catalytic activities Reduction The synthesis consists of 4 reactions which repeat for total 7 rounds 1. Condensation: CO2 is released 2. Reduction: Reduction (use NADPH) 2 molecules of NADPH are End of 1st round 3. Dehydration: H O is released required per round. 2 4. Reduction: Reduction (use NADPH) The process is terminated when palmitoyl thioesterase (TE) recognizes the palmitoyl-S-ACP. Final goal: Palmitate(16:0) Relationship of Glucose Metabolism and FA Synthesis Linked to Glucose Metabolism Lippincott Fig. 16.11  Palmitate is the Precursor of Other Fatty Acids The FA product released is palmitate which is a 16C fatty acid and no double bond (saturate). Elongation and unsaturation of fatty acids occurs in both the mitochondria and endoplasmic reticulum. Elongation Malonyl CoA is the source of carbons added to the # 1C of palmitate (+NADPH) Brain cells make very long fatty acid chains. Desaturation (adding cis double bonds) Involves 4 Fatty Acyl-CoA Desaturases (4, 5, 6 and 9 desaturase) Requires O2, NADPH and Cytochrome b5. These enzymes introduce double bonds at C4, C5, C6 or C9 CANNOT introduce double bonds beyond C9*** ***That’s why linoleic acid (18:2 9, 12) or omega-6 and linolenic acid (18:3 9,12,15) or omega-3 are essential fatty acids. Linoleic acid is also required for the synthesis of arachidonic acid (prostaglandin synthesis). Lippincott Fig. 16.4 Storage of fatty acids as Triacylglycerols (TAGs) or Triglycerides (TG) Triacylglycerols consist of 3 fatty acids esterified to a molecule of glycerol. TAGs can be obtained from diet (review Lecture 22 on how it’s digested and absorbed) TAGs can also be de novo synthesized from fatty acids and glycerol 3-phosphate in liver and adipose tissues TAGs are stored as liquid droplets in the cytoplasm of adipose tissue The TAG synthesized in the liver are mostly released from liver into the blood in the VLDL particle Recall: Chylomicron deliver dietary-derived TAGs from……………..to…….......................then……..… The three fatty acids in a TAG are usually not of the same type. C1 is typically saturated fatty acid (most commonly palmitate) C2 is typically unsaturated C3 can be either. Lippincott Fig. 16.12 Triacylglycerols Synthesis Glycerol 3-Phosphate + 3 Fatty acyl CoA (activated Fatty acids) TAGs synthesis from Glycerol 3-phosphate and Fatty acyl CoA Occurs primarily in liver and adipose tissues Sources of Glycerol 3-phosphate* 1. Reduction of DHAP from glycolysis both in adipose tissue and liver (by Glycerol 3-P dehydrogenase) 2. Phosphorylation of free glycerol by glycerol kinase, found only in liver Adipocytes can uptake glucose only in the (DHAP) presence of insulin (using (DHAP) GLUT……) When glucose and insulin levels are low adipocyte synthesis of TAGs is very low. Fatty Acyl CoA synthesis Fatty acid is attached to CoA to form Fatty Acyl CoA by Fatty acyl CoA synthetase. (same reaction that happens in the enterocyte before TAGs is reassembled)  TAG Synthesis Detail 1. Fatty Acyl-CoA (Acyl-CoA) is esterified to glycerol-3-phosphate to yield 1,monoacylglycerol phosphate 1 (lysophosphatidic acid) (Acyltransferase) 2. A second molecule of acyl-CoA is then esterified to yield 1,2-diacylglycerol phosphate (phosphatidic acid) a key intermediate in synthesis of phospholipids (Acyltransferase) 3. The phosphate is then removed, by phosphatidic acid phosphatase, to yield 1,2-diacylglycerol (Phosphatase) 2 4. A third Fatty Acyl CoA is then added (Acyltransferase). 3 4 Lippincott Fig. 16.14 Summary 1. When and how is fatty acid (palmitate) synthesized? (which tissues? location? substrates? products? Shuttle?) 2. What is Rate limiting enzyme/step for fatty acid synthesis? How is it regulated? 3. How are TAG synthesized? (which tissues? location? substrates?, sources of substrates?) 4. What are essential fatty acid? What is the synthesis limitation that makes them essential?