Finals Review Notes for CHE 534

Finals Review

Units 5A and 5B

Course Code: CHE 534

Fatty Acid Metabolism

Overview

• Catabolism and Anabolism of fatty acids proceed via different pathways.
  • Catabolism of fatty acids:
  • Also known as fatty acid degradation.
  • Key process: β-oxidation.
    • Produces acetyl-CoA.
    • Produces reducing power (NADH and FADH2) and ATP.
    • Takes place in the mitochondria.
    • Involves the carnitine shuttle for transportation into the mitochondria.
  • Anabolism of fatty acids:
  • Also known as fatty acid synthesis.
  • Requirement: Acetyl-CoA and malonyl-CoA.
  • Requires reducing power from NADPH.
  • Generates palmitate (C16 fatty acid).
  • Takes place in the cytosol (in animals).
  • Involves a citrate shuttle to transport acetyl-CoA from the mitochondria to the cytosol.

Location of Lipid Metabolism

Key Locations and Functions:

• Cytosol:
  • NADPH production via the Pentose Phosphate Pathway (PPP) and malic enzyme.
  • Fatty acid synthesis using fatty acid synthase (FAS).
  • Phospholipid synthesis.
• Mitochondria:
  • Fatty acid oxidation (β-oxidation) to produce acetyl-CoA and ATP.
  • Acetyl-CoA production via Pyruvate Dehydrogenase (PDH) complex.
• Endoplasmic Reticulum:
  • Cholesterol synthesis (both early and late stages).
  • Isoprenoid synthesis (early stages).
  • Synthesis of ketone bodies from acetyl-CoA.
  • Fatty acid elongation and desaturation processes.

NADPH in Fatty Acid Synthesis

Role and Sources

• Importance: NADPH is crucial for reductive biosynthesis of fatty acids.
• Location: Fatty acid synthesis occurs in cell compartments with high NADPH levels, notably in the cytosol of animals and yeast.
• Sources of NADPH:
  • In adipocytes:
  • Generated through the Pentose phosphate pathway.
  • Also produced via malic enzyme as malate converts to pyruvate + CO2.
  • In hepatocytes and mammary glands:
  • Generated via the pentose phosphate pathway generating NADPH when glucose-6-phosphate converts to ribulose 6-phosphate.

Pathways for NADPH Production

• Two primary pathways for NADPH production:
  1. Malic enzyme: Predominantly in adipocytes.
  2. Pentose phosphate pathway: Occurs in adipocytes, hepatocytes, and mammary glands.

Steps in Fatty Acid Biosynthesis

1. Shuttle of Acetyl-CoA: Citrate shuttle transports acetyl-CoA from mitochondria to cytosol.
2. Carboxylation of Acetyl-CoA: Converts Acetyl-CoA to Malonyl-CoA via Acetyl-CoA carboxylase (ACC).
   • Rate limiting step in fatty acid synthesis.
3. Reactions of Fatty Acid Synthesis (all catalyzed by fatty acid synthase - FAS):
   • Loading of Acetyl-CoA and Malonyl-CoA.
   • Condensation of the chains.
   • Reduction of keto group.
   • Dehydration to form a double bond.
   • Reduction to finish the chain extension.
   • Repeat loading with Malonyl-CoA to extend chains by two carbon atoms until reaching C16 (palmitate).

Formation of Malonyl CoA from Acetyl CoA

• Enzymatic Process:
  • Carboxylation reaction to form Malonyl CoA from Acetyl CoA.
  • Enzyme involved: Acetyl-CoA carboxylase (ACC).
  • It is:
  • Rate limiting enzyme: Controls the overall rate of fatty acid synthesis.
  • Irreversible reaction utilizing biotin and CO2 as substrate.
  • Requires ATP.

Fatty Acid Synthase (FAS)

• Function: Catalyzes the synthesis of fatty acids through a repeating process that extends fatty acyl chains by two carbons at each cycle.
  • Enzymatic Steps within the four-step sequence:
  1. Condensation - Enzyme: β-ketoacyl synthase, removing CO2.
  2. Reduction - Enzyme: β-ketoacyl ACP reductase, requiring NADPH as the electron donor.
  3. Dehydration - Enzyme: β-hydroxyacyl ACP dehydratase, produces water.
  4. Reduction - Enzyme: Enoyl ACP reductase, again using NADPH as electron donor.
  • Final outcome: Each cycle extends the fatty acid chain by two carbon atoms until it reaches 16 carbons (palmitate).
  • Thioesterase: Cleaves palmitate from fatty acid synthase.

Regulation of Fatty Acid Synthesis

• Key Regulatory Enzyme: Acetyl CoA Carboxylase (ACC).
  • Activation: Stimulated by citrate and insulin, which promotes fatty acid and triglyceride synthesis.
  • Inhibition: By glucagon and epinephrine; these hormones inhibit synthesis through phosphorylation of ACC (inactivating it).
  • Additional Note: Citrate also serves as a phosphofructokinase 1 (PFK1) inhibitor.

Elongation & Desaturation of Fatty Acids

Elongation Process

• Mechanism: Involves elongases located in the ER and mitochondria where:
  • E.g., C18:2 + Malonyl-CoA → C20:2 + CO2 through steps including condensation, reduction, dehydration, and hydrolysis.

Desaturation Process

• Mechanism: Involves desaturases located in the ER. These enzymes convert single C-C bonds to double C=C bonds by removing two hydrogens:
  • E.g., C20:2 → C20:3 + 2H⁺; and C20:3 → C20:4 + 2H⁺.

Breakdown of Fatty Acids (Catabolism)

Overview

• Fatty acids must be digested and transported before undergoing β-oxidation.
  • Digestion: Involves digestive enzymes known as lipases.
  • Transportation: Facilitated through lipoproteins.
  • β-Oxidation: Process involving breakdown of fatty acids into acetyl-CoA.

Lipase Enzymes and their Reactions

Steps in Fatty Acid Catabolism

1. TAG mobilization: Regulated by glucagon and epinephrine hormones.
2. Fatty Acid transport: Involves binding of fatty acids to serum albumin for transport within the bloodstream.
3. Fatty acid activation: Conversion of fatty acids to fatty acyl-CoA via fatty acyl-CoA synthetase in the cytosol of cells with mitochondria (except for brain cells).
4. Fatty Acid Transport into Mitochondria: Accomplished through the carnitine shuttle.
5. β-Oxidation Reactions: Occurs in the mitochondrial matrix leading to the generation of acetyl-CoA.

Steps Explained in Detail

Step 1: Mobilization of Stored TAGs

• Glucagon and Epinephrine bind to specific receptors on adipocytes:
  • This activation of Adenylyl cyclase generates cAMP, which triggers the breakdown of fatty acids.
  • Hormone-sensitive lipase (HSL) is activated when phosphorylated and works alongside perilipin, which recruits more lipases, catalyzing the breakdown of triglycerides into glycerol and free fatty acids.

Step 2: Fatty Acid Activation

• Transport Mechanism:
  • Serum albumin binds to fatty acids aiding their transport to tissues such as myocytes and heart tissues.
• Activation:
  • Fatty acids are activated in the cytosol to form acyl-CoA via Fatty acyl-CoA synthase:
  • Reaction type: Esterification occurs at the outer mitochondrial membrane.

Step 3: Carnitine Transport Mechanism

• Transport of Acyl-Carnitine:
  • The carnitine acyltransferases (CAT I and II) facilitate the transport of acyl-carnitine across mitochondrial membranes:
  • Carnitine is an important molecule for the translocation of fatty acyl-CoA into the mitochondrial matrix for β-oxidation.

Step 4: β-Oxidation Continuum

• Oxidation within Cellular Respiration:
  • β-oxidation is a systematic breakdown of fatty acids generating acetyl-CoA through these steps:
  1. First Oxidation: Catalyzed by Acyl CoA Dehydrogenase - generates FADH2.
  2. Hydration: Enzyme: Enoyl CoA Hydratase.
  3. Second Oxidation: Enzyme: L-3-Hydroxyacyl CoA Dehydrogenase - generates NADH.
  4. Thiolysis: Enzyme: β-Ketothiolase (or Thiolase), where coenzyme A shortens the acyl-CoA product by 2 carbon atoms.

Enzymes in Fatty Acid Catabolism

Vitamins and Fatty Acid Oxidation

• Activators: Pantothenate, which serves as part of Coenzyme A in fatty acid activation and synthesis.
• Shuttles in β-oxidation: Requires certain vitamins for complete oxidation:
  • Riboflavin, Niacin, and Pantothenate needed.
  • For odd-carbon chain fatty acids, Biotin and vitamin B-12 are also necessary.

Regulatory Differences in Fatty Acid Synthesis vs. β-Oxidation

• Key Regulatory Enzymes:
  1. Acetyl CoA Carboxylase (ACC): Regulatory enzyme for fatty acid synthesis.
  2. Carnitine Acyl Transferase I (CAT I): Regulatory enzyme for fatty acid β-oxidation.
• ACC Regulation:
  • Insulin promotes ACC activation through dephosphorylation, while glucagon and epinephrine inhibit it via phosphorylation.
• CAT I Regulation:
  • Inhibited by Malonyl CoA, which is produced from ACC activity.
  • High glucagon and epinephrine levels lead to phosphorylation of perilipin and hormone-sensitive lipase, enhancing fatty acid breakdown.

Ketone Bodies

Overview

• Role: Ketone bodies produced by the liver in mitochondria serve as an energy source for peripheral tissues (heart, skeletal muscle, renal cortex, and brain) during low glucose conditions (starvation).
• Structure: Composed of a carbon backbone with a double-bonded oxygen. Key ketone bodies include Acetoacetate, Acetone, and 3-Hydroxybutyrate. Acetone cannot be utilized for energy.
• Precursor: Acetyl CoA is the precursor for ketone body synthesis.

Biosynthesis of Glycerophospholipids

• Activated Intermediates: Involved in the formation of glycerophospholipids are CDP-alcohol or CDP-1,2-DAG.
• Reactions:
  • Reactants differ based on which reactant is activated:
  • CDP-Diacylglycerol reacts with an alcohol to yield phospholipids.
  • CDP-alcohol reacts with 1,2-DAG also producing phospholipids.

Degradation of Glycerophospholipids (GPL)

Enzyme Breakdown:

Phospholipase A2

• Found in mammalian tissues, pancreatic juice, and also in snake and bee venom.
• Activated by trypsin and requires bile salts for activity.
• Specific Reaction: Acts on phosphatidylinositol to release arachidonic acid, a precursor to prostaglandins.
• Inhibited by glucocorticoids such as cortisol.

Phospholipase A1

• Present in various mammalian tissues.

Phospholipase D

• Involved in signal transduction, generating phosphatidic acid (PA) from phosphatidylcholine and diacylglycerol from PA.

Phospholipase C

• Present in liver lysosomes and activates by the PIP2 system, playing a role in producing second messengers.

Eicosanoids

Overview

• Eicosanoids: Short-range hormones composed of 20 carbons, includes prostaglandins, leukotrienes, thromboxanes.
• Synthesis Steps: Arachidonic acid (20:4) is released from membrane phospholipids through the action of phospholipase A2 in response to stimuli (hormonal, etc.) to form eicosanoids.

Conversion of Arachidonate to Prostaglandins and Other Eicosanoids

Sequential Steps

1. Cyclooxygenase Activity: PGH2 synthase adds 2 molecules of O2 to form PGG2.
   • Aspirin and ibuprofen inhibit this reaction.
2. Peroxidase Activity of PGH2 Synthase: Converts peroxide to alcohol, creating PGH2, which serves as a precursor for cyclic eicosanoids (e.g., prostaglandins and thromboxanes).

Isoforms of PGH2 Synthase

• COX Enzymes:
  1. COX-1: Catalyzes prostaglandin synthesis regulating gastric mucin secretion.
  2. COX-2: Catalyzes prostaglandin synthesis in the response to pain, inflammation, and fever.
• NSAIDs:
  • Aspirin (irreversible inhibitor by acetylating a serine residue) and ibuprofen (competitive inhibitor resembling substrate).
  • Selective COX-2 inhibitors (e.g., Celebrex, Vioxx) target inflammation without affecting gastric mucus production.

Synthesis of Leukotrienes

• Produced via lipoxygenase activity, involves converting arachidonic acid into various leukotrienes through multi-step processes.

Cholesterol Synthesis

Steps Involved:

1. Condensation of Acetyl-CoA to form mevalonate.
2. Decarboxylation of mevalonate to generate a 5C isoprene unit.
3. Polymerization of these isoprene units to create a 30-carbon structure known as squalene.
   • 5C → 10C → 15C, then 15C + 15C → squalene.
4. Cyclization: Conversion of squalene into a cyclic molecule generating four steroid rings.

Stepwise Formation of Mevalonate from Acetyl-CoA

1. Acetoacetyl-CoA Formation:
   • Reaction: 2 Acetyl CoA → Acetoacetyl CoA
   • Enzyme: Thiolase.
2. Formation of HMG-CoA:
   • Reaction: Acetyl CoA + Acetoacetyl CoA → HMG-CoA,
   • Enzyme: HMG-CoA synthase (cytoplasmic).
3. Mevalonate Formation:
   • Reaction: HMG-CoA → Mevalonate,
   • Enzyme: HMG-CoA reductase (Key step, uses NADPH, targeted by statins).

Regulation of Cholesterol Metabolism

• Insulin stimulates HMG-CoA reductase activity, while glucagon down-regulates it, facilitating cholesterol synthesis or degradation through various intermediates and feedback mechanisms.

Bile Salts Synthesis in Liver

1. Synthesis: Cholesterol is converted to cholate.
2. Activation: Cholate + ATP + CoA → Cholyl-CoA + AMP + PPi.
3. Attachment of glycine or taurine results in glycocholate or taurocholate.

Cholesterol as Precursor for Steroid Hormones

• Cholesterol is synthesized in the mitochondria, where it undergoes hydroxylation by mixed-function oxidases, generating pregnenolone as the first step toward steroid hormone production.
  • Rate limiting reaction: Cleavage of side chains utilizing NADPH and O2.

Vitamin D Synthesis

• Site of Synthesis: Skin.
• Site of Action: Bones, intestine, kidney.
• Hydroxylation: First in liver (cholecalciferol → 25-hydroxyvitamin D) and then in kidneys (25-hydroxyvitamin D → 1α,25-Dihydroxyvitamin D3).

Vitamin D Function and Regulation

• Function: Regulates calcium homeostasis and mineral metabolism; influences gene expression related to calcium deposition and reabsorption.
• Regulation: Parathyroid hormone (PTH) stimulates vitamin D synthesis by activating the 1α-hydroxylase enzyme in response to low calcium and phosphate levels.

Major Lipoproteins in Blood

VLDL to LDL Conversion

• Process: TAG removal from VLDL creates LDL.
• Function of LDL: Delivers cholesterol to muscles and adipose tissues via the ApoB-100 apolipoprotein, facilitating receptor-mediated endocytosis.

Cholesterol Uptake & HDL Function

• Reversal Transport: HDL (good cholesterol) assists in carrying cholesterol back to the liver. Its primary proteins, Apo A-I and Apo A-II, participate in the formation of cholesteryl esters via lecithin-cholesterol acyl transferase (LCAT).
• Mechanism: ABC transporters export cholesterol from cells to HDL particles.

Conclusion

These notes aim to encapsulate the extensive topics covered within fatty acid metabolism, synthesis, degradation, and its regulatory mechanisms alongside lipid-related components in human biochemistry. Streamlined understanding of these pathways is imperative for a successful grasp of biochemistry concepts.