Zaidi (Lipid Metabolism I/II) Flashcards
Fatty Acid Synthesis and precursor/end product
- occurs primarily in liver; also in adipose tissue, brain, kidneys, lactating mammary glands
- requires coordination between cytosolic and mitochondrial reactions
Precursor: Acetyl-CoA (2 carbon molecule)
End product: Palmitic Acid (16 carbon molecule)
3 phases of FA Synthesis
Phase 1: Cytosolic entry of Acetyl-CoA
- made in mitochondria/needed in cytoplasm
Phase 2: Generation of Malonyl-CoA
- Acetyl-CoA carboxylated to Malonyl-CoA
Phase 3: Fatty Acid Chain formation
- fatty acid synthase catalyzes 7 reactions to form palmitate
What is the most important substrate of Fatty Acid Synthesis?
Malonyl-CoA –> RATE LIMITING STEP
Phase I: Cytosolic Entry of Acetyl-CoA
- oxaloacetate + Acetyl-CoA in mitochondria = citrate (citrate synthase)
- citrate shuttle moves citrate into cytoplasm
- ATP citrate lyase breaks citrate into Acetyl-CoA (FA synthesis in cytosol) and Oxaloacetate (+: glucose/insulin and -: PUFA/leptin)
- Oxaloacetate –> Malate (back into mitochondria) –> pyruvate (back into mitochondria) –> oxaloacetate
Phase I: Regulation of Oxaloacetate
1) malate into mitochondria via transporter and oxidized to OAA by malate dehydrogenase
2) cytosolic malate –> pyruvate (malic enzyme), transported into mitochondria via transporter and carboxylated to OAA by pyruvate carboxylase
Phase II: Synthesis of Malonyl-CoA
- ACoA –acetyl CoA carboxylase –> MCoA (3 carbon)
- acetyl CoA carboxylase = RATE LIMITING ENZYME of FA Biosynthesis pathway
- ACC adds CO2 to ACoA; uses ATP and BIOTIN (MUST HAVE)
+ regulation: insulin, citrate
- regulation: glucagon, epinephrine, palmitate, PUFA, high AMP
Malonyl CoA
- FA synthesis substrate
- REGULATOR = inhibits carnitine acyltransferase (RATE LIMITING STEP IN FA DEGRADATION)
- prevents FA synthesis and degradation from happening simultaneously
Phase III: Fatty Acid Chain Formation
- occurs on Fatty Acid Synthase Complex (FAS)
- two carbons from malonyl CoA are added to growing fatty acyl chain in 7 reactions = palmitate (16:0)
Fatty Acid Synthase
- large multi-enzyme complex (2 identical dimers)
- arranged in head to tail conformation
- 7 enzyme activities and an acyl carrier protein (ACP)
Stoichiometry of Palmitate Synthesis
IN: 1 ACoA + 7 MCoA + 14 NADPH + 14 H
OUT: Palmitate + 14 NADP + 8 CoA + 6 H2O
Reactions catalyzed by FAS (4)
- Condensation (Acetyl + Malony = B-ketoacyl group)
- Reduction (B-ketoacyl group –> B-hydroxyl group)
- Dehydration (B-hydroxyl group –> trans-enone group)
- Reduction (trans-enone group –> 4 C fatty acyl group)
Repeat 6 more times = Palmitate
Regulation of Fatty Acid Synthesis
- ATP citrate lyase (Phase 1)
- Acetyl-CoA Carboxylase (Phase 2 - RLS)
- Fatty Acid Synthase (Phase 3)
expression induced by low fat, high carb diet (25-100 fold more active in FED STATE)
Regulation of ATP Citrate Lyase
(+) regulation: phosphorylation, glucose/insulin
(-) regulation: PUFAs, leptin
Regulation of Acetly-CoA Carboxylase (Allosteric/Phosphorylation/Induction)
1) Allosteric
- (+) –> citrate
- (-) –> palmitate
2) Phosphorylation (-)/Dephosphorylation (+)
- (+) –> insulin
- (-) –> epinephrine, glucagon, AMP
3) Induction: high carb/low fat
Regulation of Fatty Acid Synthase
1) Allosteric –> phosphorylated sugar = inc. activity
2) Induction/Repression @ gene level
- (+) –> insulin, glucocorticoid hormones, Hi carb/Lo fat
- (-) –> high fat, starvation, high PUFA
Synthesis of Longer FA chain - ELONGATION
- palmitate converted to longer chain FA in SMOOTH ENDOPLASMIC RETICULUM/MITOCHONDRIA
- lengthened 2 carbons at a time, NADPH = reducing power
- SER: Malonyl CoA = carbon donor
- Mito: Acetyl CoA = carbon donor
BRAIN NEEDS LONGER FATTY ACID CHAINS (C18-C24)
Desaturation
- introduction of double bonds in FA, occurs in SER and uses NADH/NADPH w/oxygen
- Acyl CoA Desaturases (4, 5, 6, 9) –> where DB is at in chain
- FA w/DB beyond C9/C10 cannot be synthesized in humans
- ingest precursors: Essential Fatty Acids (Omega-3/Omega-6)
Essential Fatty Acids
- humans cannot synth omega-3/6 FA; need to ingest in diet or their precursors: linoleic acid (6) and linolenic acid (3)
- Linoleic Acid –> arachidonic acid (precursor for prostaglandins, leukotrienes, thromboxanes)
- Linolenic Acid –> eicosapentanoic acid (EPA) and docosahexanoic acid (DHA)
Human Desaturases cannot…
introduce unsaturation beyond C9 and the methyl end (omega end)
What are FA incorporated into?
- triacylglycerols (storage form of lipids)
- used to provide energy (adipose tissue)
What is the major source of carbon for fatty acid synthesis?
dietary carbohydrates
Eicosanoid Hormones
- short-lived local hormones
- stimulate inflammation, regulate blood flow to particular organs, control ion transport across membranes, modulate synaptic transmission, induce sleep
Aspirin: blocks enzyme that converts arachidonate into postaglandin
TAGs (Triacylglycerol)
- glycerol and 3 fatty acid chains
MAG (monoglycerol) - 1 FA
DAG (diacylglycerol) - 2 FA
TAG (triacylglycerol) - 3 FA
Sources of TAGs
dietary TAG: processed in intestinal cells
de novo TAG: in hepatocytes/adipocytes
Mobilization of FA from Adipocytes
- glucagon/epinephrine –> GPCR –> adenylate cyclase
- Adenylate cyclase activates cAMP
- cAMP activates Protein Kinase A
- PKA phosphorylates HS Lipase (DAG to MAG) and Perilipin (ATGL causes TAG to DAG)
- MAG Lipase (MAG to FA + glycerol)
Inhibition: Insulin Receptor (RTK): protein phosphatase (PP1) dephosphorylates/inactivates HS Lipase (hormone sensitive lipase)
Perilipin
- proteins that coat lipid droplets in adipocytes/muscle cells
- regulate lipolysis by controlling physical access to lipid breakdown enzymes
- overexpression of Perilipin 1 inhibits lypolysis, and knock-out has converse effect
- target of obesity treatment
Overview of Fatty Acid Breakdown
Phase I: FA activation (cytosol)
Phase II: Beta-oxidation (mitochondrial matrix)
Outer mitochondrial matrix: FA not permeable
Inner mitochondrial matrix: FACoA not permeable
Phase I: Fatty Acid Activation
- 1st rxn in FA metabolism (traps FA in cells), makes it metabolically active
- Acyl-CoA synthetase
2 steps: FA –> Acyl adenylate –> Acyl-CoA
- PP hydrolysis makes it IRREVERSIBLE
FA in cytosol, FA-CoA in Inner Membrane Space
Translocation to Mitochondrial Matrix
- Translocase moves Acyl Carnitine (carnitine + FACoA) across inner mitochondrial membrane (not permeable to FA-CoA)
- Carnitine acyltransferase I (RATE LIMITING ENZYME inhibited by Malonyl-CoA) adds carnitine and FACoA
- Carnitine acyltransferase II removes carnitine from FACoA once inside mitochondrial matrix
Phase II: Beta Oxidation Steps
Four steps
1) Oxidation - Acyl CoA Dehydrogenase (ACAD)
2) Hydration - Enoyl CoA Hydratase
3) Oxidation - 3-Hydroxyacyl CoA Dehydrogenase
4) Thiolysis - Acetyl CoA Acetyltransferase (B-keto thiolase)
Steps of Beta Oxidation (4)
- ACAD oxidizes B carbon = FADH2 and trans-enoyl CoA; FADH2 enters ETC and generates 2 ATP
- Enoyl CoA Hydratase saturates alkene with water to for B hydroxy acyl CoA
- B hydroxy acyl CoA dehydrogenase oxidizes carbon to form ketoacely CoA and NADH; NADH enters ETC and generates 3 ATP
- Acyl CoA acyl transferase attaches sulfur of CoA to ketone formed from cleavage of acetyl CoA from fatty acyl chain which is shortened by 2C
ATP released from Beta Oxidation of Palmitic Acid
FADH2 : 7 x 2 = 14 ATP
NADH : 7 x 3 = 21 ATP
ACoA : 8 x 12 = 96 ATP
Total = 131 ATP - (2 used to activate) –> NET = 129
Links between Lipid and Carbohydrate Oxidation
“fat burns in the flame of carbohydrates”
- ACoA enters TCA cycle –> combines w/Oxaloacetate to form citrate
- low OOA (low carbs) = ACoA NOT utilized –> condenses to form Ketone Bodies
- fasting/diabetes = OAA converted to glucose by gluconeogenesis
Ketone Bodies
- water-soluble and acidic compounds
- acetoacetate, B-hydroxybutyrate, acetone
- produced in LIVER ONLY
- provide energy for peripheral tissues during fasting, and brain during starvation
Formation of Ketone Bodies
- Acetoacetate becomes D-3-Hydroxybutyrate and Acetate
- Acetoacetate –CoA transferase–> Acetoacetyl CoA
- Acetoacetyl CoA –Thiolase–> 2 Acetyl CoA
Diabetic Ketoacidosis
Adipose Tissue: no glucose = FA acids released
Liver: no glucose = OAA lvl low, CAC slows, FA converted to ketone bodies –> blood pH drops –> coma and death
