Biochemistry II - Synthesis of Lipids

Lipids Biological Functions

  • Energy storage.
  • Membrane Constituents.
  • Anchors for membrane proteins.
  • Enzyme cofactors.
  • Signaling molecules.
  • Pigments, detergents, transporters, and antioxidants.

Fatty Acid Synthesis and Breakdown

  • Separate processes: different pathways, enzymes, and cell compartments.
  • Malonyl-CoA involved in synthesis but not breakdown.

Catabolism vs Anabolism of Fatty Acids

  • Catabolism:
    • Produces acetyl-CoA, NADH, and FADH2.
    • Occurs mainly in mitochondria (animals).
  • Anabolism:
    • Requires acetyl-CoA, malonyl-CoA, and NADPH.
    • Takes place in the cytosol (animals) or chloroplasts (plants).

Fatty Acid Synthesis Overview

  • Built in passes, processing one acetate unit at a time (from malonyl-CoA).
  • Each pass reduces a carbonyl carbon to a methylene carbon.

Location of Fatty Acid Synthesis

  • Cytosol (animals, yeast) or chloroplasts (plants).

Synthesis of Fatty Acids: Malonyl-CoA Formation

  • Acetyl-CoA carboxylase catalyzes the first step, using biotin as a prosthetic group.
  • Reaction involves two steps: carboxylation of biotin and transfer of CO2 to acetyl-CoA to yield malonyl-CoA.

Biotin (Vitamin B7)

  • Prosthetic group in carboxylation reactions.
  • Deficiency is rare but can be caused by excessive raw egg-white consumption (avidin binds biotin).

Acetyl-CoA Carboxylase Reaction

  • Two-step reaction involving CO2 binding to biotin and transfer to acetyl-CoA.

Fatty Acids Synthases I and II

  • FAS I (vertebrates): single multifunctional polypeptide, homodimer, main product is palmitic acid (16:0).
  • FAS II (bacteria, plants): separate proteins, more variable products.

Fatty Acid Synthase Complex

  • Acyl carrier protein (ACP) carries acyl groups in thioester linkage
  • Malonyl/Acetyl-CoA-ACP transacetylase (MAT) Transfers acetyl group from CoA to Cys residue of KS
  • b-Ketoacyl-ACP synthase (KS) Condenses acyl and malonyl groups
  • b-Ketoacyl-ACP reductase (KR) Reduces b-Ketoacyl group to b-hydroxyl group
  • b-Ketoacyl-ACP dehydrotase (HD) Removes H2O from b-hydroxyl-ACP creating double bond
  • Enoyl-ACP reductase (ER) Reduces double bond, forming saturated acyl-ACP

Acyl Carrier Protein (ACP)

  • Contains 4'-phosphopantetheine prosthetic group (from pantothenic acid/vitamin B5).
  • Fatty acid linked to -SH group via thioester.
  • Flexible arm carrying intermediates between enzyme active sites.

Fatty Acid Synthesis Process

  • Acetate unit from malonyl-CoA added to a growing chain and reduced.
  • Involves four steps: condensation, reduction, dehydration, reduction.

Four-Step Reaction in Fatty Acid Synthase I

  • Preparation: Malonyl CoA and acetyl CoA bind to FAS I, activating acyl groups.
  • Requires two thiols on FAS I: one from 4-phosphopantethine in ACP, one from Cys.

Four-Steps in Fatty Acid Synthase I Reaction in Mammals

  • Step 1: Condensation, Step 2: First Reduction, Step 3: Dehydration, Step 4: Second Reduction

Enzymes in Fatty Acid Synthase

  • Chain transfer/charging: malonyl/acetyl-CoA ACP transferase
  • Condensation w/ acetate: b-ketoacyl-ACP synthase (KS)
  • Reduction of carbonyl: b-ketoacyl-ACP reductase (KR)
  • Dehydration of alcohol: b-hydroxyacyl-ACP dehydratase (DH)
  • Reduction of alkene: enoyl-ACP reductase (ER)

Stoichiometry of Palmitate Synthesis

  • 7<br/>ewlineacetyl−CoA+7<br/>ewlineCO2+7<br/>ewlineATP→7<br/>ewlinemalonyl−CoA+7<br/>ewlineADP+7<br/>ewlinePi7 <br /> ewline acetyl-CoA + 7 <br /> ewline CO_2 + 7 <br /> ewline ATP \rightarrow 7 <br /> ewline malonyl-CoA + 7 <br /> ewline ADP + 7 <br /> ewline Pi
  • acetyl−CoA+7<br/>ewlinemalonyl−CoA+14<br/>ewlineNADPH+14<br/>ewlineH+→palmitate+7<br/>ewlineCO<em>2+8ewlineCoA+14ewlineNADP++6ewlineH</em>2Oacetyl-CoA + 7 <br /> ewline malonyl-CoA + 14 <br /> ewline NADPH + 14 <br /> ewline H^+ \rightarrow palmitate + 7 <br /> ewline CO<em>2 + 8 ewline CoA + 14 ewline NADP^+ + 6 ewline H</em>2O

Acetyl-CoA Transport

  • Transported into the cytosol with a cost of 2 ATPs.
  • Total cost: 3 ATPs per 2-C unit.

Sources of NADPH

  • Pentose phosphate pathway and malic enzyme.

Regulation of Fatty Acid Biosynthesis

  • Regulation via acetyl-CoA carboxylase (ACC) and carnitine acyltransferase I.
  • Insulin activates ACC; glucagon inactivates ACC.
  • Allosteric regulation: palmitoyl-CoA inhibits, citrate activates.

Palmitate Elongation and Desaturation

  • Elongation systems in ER and mitochondria add 2-C units (stearate is common).
  • Desaturation catalyzed by fatty acyl-CoA desaturase (requires NADPH).

Plant Desaturases

  • Act on fatty acids bound to glycerol in phosphatidylcholine.

Eicosanoids

  • Derived from arachidonate.
  • Hormones including prostaglandins, leukotrienes, thromboxanes.

Conversion of Arachidonate to Eicosanoids

  • PGH2 synthase (COX) converts arachidonate to PGG2 and then to PGH2.
  • NSAIDs inhibit COX activity (aspirin, ibuprofen).

Fat (Triacylglycerol) and Phospholipids

  • Animals and plants store fat as triacylglycerols.
  • Animals, plants, and bacteria make phospholipids for cell membranes.
  • TAGs and phospholipids have glycerol backbone and fatty acids.

Synthesis of TAGs and Phospholipids

  • Glycerol 3-phosphate from DHAP (glycolysis) or glycerol (glycerol kinase).
  • Phosphatidic acid is precursor to TAGs and phospholipids.

Regulation of Triacylglycerol Synthesis

  • Insulin stimulates synthesis; lack of insulin increases lipolysis, fatty acid oxidation.

Triacylglycerol Cycle

  • Free fatty acids released by lipolysis are reesterified to form TAGs.
  • Glyceroneogenesis makes DHAP for glycerol 3-phosphate generation.

Biosynthesis of Membrane Phospholipids

  • Begins with phosphatidic acid or diacylglycerol; attach head group to C-3 OH group.
  • Requires activation by CDP.

Phospholipid Synthesis in E. coli and Eukaryotes

  • E. coli: phosphatidylserine and phosphatidylglycerol synthesis.
  • Eukaryotes: similar strategies but different enzymes.

Synthesis of Sphingolipids

  • Similar pathways and head-group attachment to ceramide

Cholesterol Derivation

  • Cholesterol is synthesized from isoprene

Cholesterol Roles

  • Component in animal membrane bilayers.
  • Precursor of steroid hormones and bile salts.
  • Cholesterol biosynthesis stages include:
    • Condensation of acetate units to mevalonate.
    • Conversion of mevalonate into activated isoprene.
    • Polymerization of isoprene units to form squalene.
    • Cyclization of squalene to steroid nucleus.

HMG-CoA Reductase

  • Key regulatory enzyme in cholesterol synthesis.

Cholesterol Synthesis (Condensation of 3 acetate units to mevalonate)

  • Key enzyme that's regulated is hydroxymethylglutaryl -CoA reductase or HMG -CoA reductase (HMGR)

Cholesterol Esterification

-Increases lipophilicity.

ATP and NADPH Requirements for Cholesterol Synthesis

  • Synthesis is energy intensive (36 ATPs and 26 NADPHs).

Lipoproteins and Lipid Transport

  • Lipids transported in blood as lipoproteins (chylomicrons, VLDL, LDL, HDL).

Regulation of Cholesterol Metabolism

  • Regulated by dietary intake, HMGR activity, ACAT activity, and LDL receptor uptake.

Regulation of HMG CoA Reductase

  • Phosphorylation decreases activity; dephosphorylation increases activity.

Cholesterol Metabolism Regulation via AMPK

  • AMP-Dependent Protein Kinase (AMPK) responds to an increase in AMP by catalyzing phosphorylation of several key proteins thereby regulation their activities.

Summary of Cholesterol Metabolism Regulation

  • Protein degradation, proteolytic Regulation of HMG-CoA Reductase

Cholesterol and Bile Acids and Salts

  • Bile acids perform four physiologically significant functions:

Cardiovascular Disease (CVD)

  • Very high LDL-cholesterol and low HDL-cholesterol levels are associated with atherosclerosis.

Familial Hypercholesterolemia

  • Due to genetic mutation in LDL receptor

Reverse Cholesterol Transport by HDL

-HDL picks up cholesterol from nonliver tissues, including foam cells at growing plaques.

Cholesterol-Derived Steroids

  • Includes adrenal gland-synthesized steroids (mineralcorticoids, glucocorticoids) and gonad-synthesized steroids (progesterone, androgens, estrogens).