Fat Metabolism Flashcards

Fat Metabolism

Introduction

• Fat metabolism involves storing a significant amount of energy.
• Sian Patterson, Ph.D., is an Associate Professor teaching this topic.

Overview of Glycogen and Fat Metabolism

• Glycogen Catabolism/Synthesis:
  • Interconversion of Glucose-1-Phosphate (Glc.1-P) and Glucose.
• Glycolysis:
  • Glucose converted to Pyruvate, which can then become Lactate.
• Gluconeogenesis (GNG):
  • Glycerol can be used to synthesize Glucose.
• Krebs Cycle/Citric Acid Cycle (CAC):
  • Oxaloacetate + Acetyl CoA enter the cycle.
• Electron Transport Chain (ETC) & Oxidative Phosphorylation:
  • ATP production.
• Fat Synthesis:
  • Glycerol + Acyl CoA form Triacylglycerols (TAGs) and Cholesterol.
• β Oxidation:
  • TAGs are broken down into Acetyl CoA.

Tissue-Specific Metabolism

• Brain:
  • Utilizes carbohydrates or ketone bodies for energy.
• Muscle:
  • Can use both carbohydrates and fats.
• Liver:
  • Plays a central role in both carbohydrate and fat metabolism.
• Epinephrine:
  • Hormone that promotes fat breakdown.
• Insulin:
  • Hormone that promotes fat synthesis and glucose uptake.

Learning Objectives

• Describe why, when, and where fats are made and broken down.
• Review how much ATP is made from reduced coenzymes in β oxidation via oxidative phosphorylation.
• Describe the role of ACC (Acetyl-CoA Carboxylase) and FAS (Fatty Acid Synthase) for fat synthesis.
• Compare lipid synthesis to lipolysis (β oxidation).
• Contrast the reciprocal regulation of fat metabolism.
• Describe the structure and function of lipoproteins.
• Summarize how lipoproteins are made and how they travel through the body.

Triacylglycerol (Triglyceride, TG)

• Extra carbons are stored as triacylglycerol molecules, primarily in fat cells but also in muscle and other tissues.
• Acyl chains are connected to a glycerol backbone via ester bonds.
• Fatty acyl chains are often saturated, allowing for close packing.
• Chains must be hydrolyzed off for use.

Fat Mobilization

• Process:
  • Triacylglycerol is broken down into Diacylglycerol, then Monoacylglycerol, and finally into Fatty Acids + Glycerol.
  • Fatty Acids are transported via albumin.
• Hormone Sensitive Lipase (HSL):
  • Epinephrine and glucagon signal via GPCRs to activate lipases.
  • Lipases hydrolyze ester bonds to produce free fatty acids (FFAs) and glycerol.
  • FFAs are transported in the blood via albumin for cellular use.

Fat Mobilization via GPCR Signaling

• Epinephrine and glucagon bind to G-protein coupled receptors (GPCRs) on adipose cells (and other tissues).
• The signaling response involves the release of Gα-GTP, activation of adenylyl cyclase, and cAMP production, leading to Protein Kinase A (PKA) activation.
• PKA phosphorylates and activates triacylglycerol lipase, also known as hormone-sensitive lipase (HSL).
• TAGs are hydrolyzed to 3 fatty acids (+ glycerol) that are released in the blood and transported via albumin.

Fat Metabolism Overview

• Fat Cell: Stores Triacylglycerol, which is broken down into Glycerol and Fatty acids.
• Liver Cell: Glycerol can be used for Gluconeogenesis to produce Glucose.
• Liver, Muscle, and Other Tissues:
  • Fatty acids are transported via albumin.
  • Fatty acid oxidation produces Acetyl CoA.
  • Acetyl CoA enters the Citric Acid Cycle (CAC).
  • NADH and FADH2 are produced, leading to ATP generation through oxidative phosphorylation.
  • $CO2$ and $H2O$ are byproducts.

Acyl CoA vs. Acetyl CoA

• Acyl CoA has more carbons than Acetyl CoA.
• Acetyl CoA has 2 carbons.
• Acyl CoA varies, e.g., 16 carbons.

Fatty Acid Activation in the Cytoplasm

• Acyl chains are trapped in the cell by the covalent addition of coenzyme A.
• Acyl CoA Synthetase catalyzes a reversible reaction that uses ATP (→ AMP) to form the fatty acyl CoA molecule.
• Pyrophosphate (PPi) hydrolysis to 2 Pi is favorable and drives this reaction in the forward direction.

ATP → AMP Energetics

• 95% of the ATP is made in oxidative phosphorylation using ADP as a substrate.
• To make ATP from AMP, AMP must be converted to ADP by sacrificing a 2nd ATP:
  • AMP + ATP \rightarrow 2 ADP \rightarrow 2 ATP
• Fatty acid activation by CoA requires 2 ATP (1 for activation and 1 for the production of ADP) and 1 $H_2O$ to hydrolyze PPi.

Mitochondrial Structure and Acyl CoA

• Acyl CoA is made in the cytoplasm, but β-oxidation occurs inside the mitochondrial matrix.
• Acyl CoA can pass through the outer membrane into the intermembrane space via porins but cannot cross the inner membrane because there is no transport mechanism.
• CoA is too large and water-soluble, making it unable to cross membranes.

Carnitine Shuttle

• The carnitine shuttle regulates what comes into the mitochondria.
• Carnitine Acyltransferase I (CAT I) moves the acyl chain onto carnitine from CoA.
• Acyl carnitine is translocated across the inner membrane in exchange for carnitine.
• Carnitine Acyltransferase II (CAT II) moves the acyl chain back onto CoA.
• Acyl CoA can then be catabolized in β-oxidation.

Regulation

• TAGs must be broken down into fatty acyl chains.
• Process is carnitine-dependent and hormone-signaling dependent.
• Fatty acyl chains + carnitine.
• Increased carnitine concentration saturates the system.

Beta Oxidation

• Repeated process involving Acyl CoA molecules with decreasing carbon numbers (e.g., 16:0 CoA, 14:0 CoA, 12:0 CoA, 10:0 CoA).
• Each round produces Acetyl CoA (2 carbons), $NADH$, and $FADH2$ while consuming $H2O$ and CoA.
• The last round starts with 4:0 CoA and produces 2 Acetyl CoAs.
• Acetyl CoA enters the Citric Acid Cycle (CAC), followed by Oxidative Phosphorylation.

Rounds of Beta Oxidation

• To breakdown a 16:0 CoA, the number of rounds (n) needed:
  • n = (\frac{\text{# of Cs}}{2}) - 1
  • For 16:0 CoA: \frac{16}{2} - 1 = 7 rounds.

Steps of β-oxidation

• Fatty acid oxidation occurs at the β-carbon (number 3 carbon in the fatty acid chain).
  1. Acyl CoA dehydrogenase: produces $FADH_2$ and creates a double bond in the fatty acid chain.
  2. Hydratase: adds water across the double bond (β-OH).
  3. Dehydrogenase: produces $NADH$ and creates a keto group on the β carbon in the chain.
  4. Thiolase: uses CoA to release acetyl CoA, producing a fatty acyl CoA chain with 2 fewer carbons.

Overall β-oxidation of 16:0 CoA

• Overall reaction:
  • 16:0 CoA + 7 $NAD^+$ + 7 FAD + 7 $H2O$ + 7 CoA → 8 acetyl CoA + 7 $NADH$ + 7 $FADH2$ + 7 $H^+$

ATP Calculation from Beta Oxidation

• Calculate the number of ATPs and $H_2O$ made by the complete oxidation of 16:0 CoA.
  • 7 $FADH_2$ yields 10.5 ATP ($7 \times 1.5$)
  • 7 $NADH$ yields 17.5 ATP ($7 \times 2.5$)
  • 8 Acetyl CoA enter Citric Acid Cycle (CAC) to produce GTP, NADH, and FADH2.

Grand Total for Complete Oxidation of 16:0 CoA

• Overall reaction:
  • 16:0 CoA + ADP + Pi + $O2$ → $CO2$ + ATP + $H_2O$ + CoA
  • Requires 2 ATP to start.
• Total ATP Yield:
  • 108 ATP from CAC
  • 16 H2O

Summary of Fatty Acid Breakdown

• Epinephrine and/or glucagon are required for TAG breakdown.
• Fatty acids travel in the bloodstream bound to serum albumin and enter the tissues.
• Fatty acids are activated in the cytoplasm by acyl CoA synthetase, requiring (2) ATP.
• The carnitine shuttle controls fatty acid entry into the matrix (CAT I, translocase, CAT II) in exchange for free carnitine.
• Acyl chains are broken down in β oxidation, producing Acetyl CoA, NADH, and $FADH_2$.
• β oxidation can produce a huge amount of ATP using the Citric Acid Cycle and oxidative phosphorylation.

Using Fats as Fuel

• Acetyl CoA from fat degradation needs to be processed via TCA to generate ATP in the mitochondria.
• TCA is dependent on the amount of oxaloacetate present.
• Cells need oxaloacetate to efficiently make ATP from fat.
• Oxaloacetate can be made from amino acids or pyruvate by pyruvate carboxylase.
• A blend of fuels, including glucose, amino acids, and fats, is needed for efficient energy production.

Localization of Fat Metabolism

• Fat metabolism processes occur in different cellular locations:
  • Cytoplasm: Fatty acid synthesis, glycolysis.
  • Mitochondria: β-oxidation, Citric Acid Cycle, oxidative phosphorylation.

Metabolic Pathways and Interconnections

• Glycolysis: Converts Glucose to Pyruvate.
• Beta Oxidation: Breaks down Fatty Acids to Acetyl-CoA.
• Citric Acid Cycle: Oxidizes Acetyl-CoA to produce $CO2$, $NADH$, $FADH2$, and ATP.
• Oxidative Phosphorylation: Uses $NADH$ and $FADH2$ to produce ATP and $H2O$.
• Gluconeogenesis (GNG): Synthesis of glucose from precursors such as glycerol, pyruvate, and amino acids.
• Urea Cycle: Processes $NH_3$

Low Carb Diets

• Biochemical basis and potential side effects of low carb diets.

Hormonal Response and Fat Synthesis

• As glucose levels rise, insulin signals for glucose uptake and catabolism in glycolysis. Fats and amino acids will also be imported for use in the cell.
• As ATP levels rise, glycolysis slows down, and excess glucose is stored as glycogen in muscle and liver cells.
• As ATP and NADH levels rise in the mitochondria, the citric acid cycle slows down, allowing for the synthesis of fat from Acetyl CoA.
• Excess macromolecules (carbs/fats/proteins) will be first broken down to smaller precursors and energy that can be used to make fats, but Acetyl CoA is made in the mitochondria…

Steps in Fatty Acid Synthesis

1. Export of mitochondrial Acetyl CoA to the cytoplasm for fat synthesis.
2. Carboxylation of acetyl CoA to malonyl CoA by Acetyl CoA Carboxylase (ACC).
3. Use of malonyl CoA to form 16:0 fatty acid chains by Fatty Acid Synthase (FAS).

Acetyl CoA Transport

• Step 1: Import & Export.
  • Citrate is transported out of the mitochondria.
  • ATP, amino acids, pyruvate.

Acetyl CoA Export as Citrate

• Coenzyme A cannot cross the inner mitochondrial membrane.
• Citrate synthase makes citrate in the Citric Acid Cycle, which can cross the inner membrane.
• Citrate is broken down by different enzymes to recreate acetyl CoA, regenerating a pyruvate molecule.
• The pyruvate can then return to the mitochondria, while Acetyl CoA is used for the synthesis of fatty acyl chains in the cytoplasm.

Fatty Acid Synthesis as an Anabolic Process

• Acyl chain synthesis occurs in the cytoplasm.
• Two enzymes are needed: Acetyl CoA Carboxylase and Fatty Acid Synthase.
• Acetyl CoA Carboxylase converts Acetyl CoA to malonyl CoA, an activated 2-carbon carrying precursor for fatty acid synthesis.
• Acetyl CoA Carboxylase is the committed and regulated step for fatty acid synthesis.
• Acetyl CoA Carboxylase uses ATP, while Fatty Acid Synthase uses NADPH as reducing power.

Acetyl CoA Carboxylase (ACC)

• Acetyl CoA is carboxylated using ATP to produce Malonyl CoA.
• The $CO_2$ group on malonyl CoA drives fatty acid synthesis by fatty acid synthase (step 3).

Reciprocal Regulation for Fats

• When fat synthesis is ON, fat degradation is OFF.
• This prevents the futile cycle of synthesizing chains and then breaking them down.
• The product of Acetyl CoA Carboxylase (ACC), malonyl CoA, inhibits CATI and shuts down the carnitine shuttle for the import of chains.
• ACC is also regulated for fat metabolism.

ACC Regulation via Phosphorylation

• Active carboxylase (ACC) can be inactivated by phosphorylation.

Regulation of Fat Metabolism

• Acetyl CoA Carboxylase is the committed step for FA synthesis.
• ACC is inhibited by phosphorylation with rising glucagon/epinephrine and AMP.
• Insulin and protein phosphatase activate ACC via dephosphorylation.
• Citrate can also stimulate ACC, while palmitoyl CoA (16:0 CoA) inhibits active ACC.
• The product, malonyl CoA, also shuts down fat breakdown by inhibiting Carnitine Acyltransferase 1.

Fatty Acid Synthase (FAS)

• Use of malonyl CoA to form fatty acid chains by Fatty Acid Synthase (FAS).
• The Acyl Carrier Protein (ACP) moves the intermediates between the different reaction sites.

Fatty Acid Synthase (FAS) Steps

• Step 1: A condensation reaction (KS) occurs with the release of $CO_2$, producing a chain that is 2 carbons longer.
• Step 2: Redox reaction requiring NADPH (KR).
• Step 3: Dehydration and $H_2O$ is released (DH).
• Step 4: 2nd redox reaction using NADPH producing a saturated chain (ER).
• Repeat: 2 carbons from another malonyl CoA can then be used for the next round of reactions.

Fat Synthesis Requirements

• The synthesis of fatty acids requires the actions of both Acetyl CoA carboxylase (ACC) and Fatty Acid Synthase (FAS).
• Seven cycles of FAS are required to generate 16:0. This involves 1 acetyl CoA and 7 malonyl CoA molecules.
• Lots of energy is required for this anabolic pathway to proceed (ACC needs 7 ATPs to make 7 malonyl CoAs, and FAS needs 14 NADPHs).
• Elongation of the 16:0 chain (18, 20, 22, 24) occurs in the ER, also using malonyl CoA and NADPH.
• Desaturases can introduce cis bonds, also requiring NADPH and oxygen.

Fatty Acid Synthase vs. Beta Oxidation

Triglyceride (TAG) Formation

• Elevated insulin promotes the activation of all the enzymes involved: ACC, FAS, and acyltransferases.
• TAGs produced in the liver can be released into the bloodstream as lipoproteins for storage or use.
• Triglycerides are made starting with phosphatidate.
• The phosphate is first removed by a phosphatase, producing diacylglycerol.
• Diacylglycerol acyltransferase adds the 3rd fatty acyl tail to make tri-acyl-glycerol.

Lipoproteins

• Lipoproteins are the transport molecules for hydrophobic TAGs and cholesterol esters throughout the body.
• They have a thin monolayer of phospholipids and cholesterol.
• The polar portions of the molecules face the aqueous exterior, while the hydrophobic ones face the hydrophobic interior.
• Apoproteins are on the surface of the lipoproteins and are oriented with their hydrophilic side chains outward and hydrophobic side chains facing inwards.

Cholesterol Metabolism

• Cholesterol is crucial for membranes and is metabolized to produce hormones, bile salts, and vitamin D3.
• Cholesterol is made from Acetyl CoA in the cytoplasm using carbons from fat/protein/carbohydrate catabolism.
• Cholesterol can be modified to a hydrophobic, acyl-carrying molecule called a cholesterol ester (CE) by acyltransferases.
• The production of cholesterol requires NADPH and ATP and is regulated by inhibiting HMG-CoA reductase (phosphorylation or statins).

Lipid Transport via Lipoproteins

• Lipoproteins are made in the liver or intestine.
• As they travel through the body, fatty acids and cholesterol are transferred to the tissues to produce energy, build membranes, or to produce other molecules like hormones.
• Whatever doesn’t get used returns to the liver as lipoprotein remnants, where it is recycled, and new lipoproteins are made with additional cholesterol and/or TAGs.
• HDL is known as good cholesterol and is the lipoprotein that picks up cholesterol to return it to the liver for processing.
• Problems may occur if there is an excess synthesis of fats, defects in the receptors that recycle lipoproteins, or problems with the expression and/or overactivity of HMG-CoA Reductase.

Biochemical Logic of Exercise and Fat Synthesis

• Exercise inhibits fat synthesis via kinases.
  • lipases → TAG → 3 fatty acyl chains
  • ↓ β oxidation

Key Messages

• Excess carbons from fats, carbohydrates, and proteins are stored as triacylglycerol molecules.
• GPCR signaling is required for TAG breakdown, with fatty acids traveling in the bloodstream bound to serum albumin.
• Fatty acids are activated by acyl CoA synthetase and then imported into the mitochondria by the carnitine shuttle for breakdown in β oxidation.
• A blend of fuels and $O_2$ are required to produce the maximum ATP output from fatty acyl chains.
• Fatty acid synthesis begins after Acetyl CoA is exported from mitochondria as citrate by Acetyl CoA Carboxylase (ACC), the regulated step in fat synthesis.
• Fatty Acid Synthase (FAS) and beta oxidation have similar reverse mechanisms but differ in many ways.
• Regulation is important for lipid metabolism and preventing CVD.