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Chapter 21 Lipid Biosynthesis

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Chapter 21 Lipid Biosynthesis Powered By Docstoc
					   Chapter 21:
Lipid Biosynthesis
 Comparison of FAO and Synthesis (Mammals)*

                           Oxidation                Synthesis
Location                Mitochondria                  Cytosol
Acyl carrier                  CoA                          ACP
Carbon units                     2                          2
Acceptor/donor             CoA (C2)            Malonyl CoA (C3)
Redox                         NAD+                    NADPH
cofactors
Enzyme                      Multiple             Multifunctional
organization                                       complex
           From Biochemistry by Moran, Scrimgeour et al.
          Three Processes
• Transport of mitochondrial acetyl-CoA
  from matrix to cytosol through citrate
  transport system
• Carboxylation of acetyl-CoA to from
  malonyl-CoA, the substrate for elongation
  by acetyl-CoA carboxylase
• Assembly of FA by fatty acid synthase

            REGULATED STEP
Citrate Transport System
       (CTS)                                                                                           STEP 1
       2 results:
1.     Acetyl-CoAmatrix 
       Acetyl-CoAcytosol
                                                            NOTE: 50% of NADPH is generated by CTS;
2.     NADH  NADPH                                                 Remainder is from PPP.
                                             CO2 + NADPH       NADP+


      H+                       pyruvate
                                                       Malic enzyme




                                                                                 Malate
                                 CoASH + NAD+
              pyruvate
                                                                                           NAD+
                                 PDH
                                    CO2 + NADH                                       malate
                                                                                     dehydrogenase
HCO3- + ATP
                                                                                          NADH + H+
     pyruvate
     carboxylase
                                                                            oxaloacetate
   Pi + ADP                   Acetyl CoA
                                                                      Pi + ADP            Acetyl CoA

                                              CoASH                                  Citrate lyase

                                                                         ATP               CoASH
           oxaloacetate

                                             citrate                   citrate
                          Citrate Synthase
                                                                 dicarboxylate ion

                    MATRIX                                                       CYTOSOL
    Step 2: Acetyl-CoA Carboxylase

•   Biotin- and ATP-dependent carboxylation
    – (Not unlike other biotin-dependent reactions
      (e.g., pyruvate carboxylase)
•   2 step reaction
    1. Formation of carboxybiotin intermediate
    2. Tranfer of activated CO2 to acetyl-CoA to
       from malonyl-CoA
Step 3: FA Synthase

1.   Loading
2.   Condensation
3.   Reduction (of keto group)
4.   Dehydration
5.   Reduction (of enoyl)
                O                                                  -                   O
                                                                       O
      H2   H2                                                                   H2
H3C   C    C    C        S        ACP                                      C    C      C   S    ACP

           butyryl-ACP                                        O                        malonyl-ACP

                                        HS-ACP                                  O

                                                              H2           H2
                                                       H3C    C            C    C      S   KS




                             HS     KS                                               CO2




                                                                           O           O

                                                         H2   H2                H2
                                                 H3C     C    C            C    C      C   S    ACP




                                                                                       O

                                                         H2   H2           H2   H2
                                                 H3C     C    C            C    C      C   S    ACP
           Thermodynamics
• Often, anabolic reactions require
  carboxylation followed by decarboxylation:
  why?
  – Energy from ATP consumed (in carboxylase
    reaction) is conserved in thioester bonds.
  – To overcome unfavorable free energy change
    (reverse of beta oxidation), acetyl group is
    converted to malonyl group, which is
    chemically prepared for nucleophilic attack on
    carbonyl carbon of growing chain
                    Regulation

• In fed state, carbohydrates serve as fuel and FA
  precursors (storage)
• In fasted state, [glucose] decreases so its
  synthesis increase (gluconeogenesis) and FA
  are mobilized for fuel
• -oxidation is regulated by
   – CAT I activity in liver
   – FA release from adipocytes
• FA synthesis is regulated by activity of acetyl-
  CoA carboxylase
        Regulation of FA synthesis, continued
• Short term
   – Palmitate exerts feedback inhibition on acetyl-CoA carboxylase
   – Citrate activates pathway by
      • Allosterically up-regulating enzyme
      • Becoming available as substrate for enzyme
      • Inhibiting PFK-1 (in glycolysis)
   – Covalent modification (phosphorylation) inhibits enzyme
     (glucogon- and epinephrine- stimulated, cAMP-dependent)
• Long term: gene expression
   – Changes in levels of expressed acetyl-CoA carboxylase (and
     FA synthase)
      • Insulin stimulates and starvation inhibits expression of enzymes
      • High levels of polyunsaturated FA (from diet) lowers concentration
        of synthetic enzymes
      • Exercise and starvation alter blood [glucose], resulting in changes
        in hormone levels
          – Long-term increases in -oxidation enzymes and (decreases in FA
            synthetic enzymes) results
         Elongation and desaturation
• Elongases add 2-C units
  to palmitate
• Desaturases add double
  bonds
  – Mammals lack certain
    desaturases and must
    obtain those FA from DIET
    (“essential fatty acids”)
  Eicosonoids
• 20-carbon FA derived from
  arachidonic acid
• Potent messenger molecules;
  unlike hormones, these act locally
  and are short-lived
• Thromboxanes (cyclic pathway)
  are vaso-constrictors; cause
  platelet aggregation
• Alternate (linear) pathway
  generates leukotrienes, once
  called “slow-reacting substance of
  anaphylaxis”
   TAG and glycerophospholipid
           synthesis
• FA have 2 possible fates, depending on
  metabolic needs:
  – Energy storage
  – Membrane lipids
                                 H2C          OH


                                     C        O     DHAP


                                 H2C          OPO32-


                   NADH + H+
                                           glyceraldehyde-2-P
                                 +           dehydrogenase
                        NAD


                                 H2C           OH


                        HO           CH       glycerol-3-P

                                                                O
                                 H2C           OPO32-

                                               CoA         S    C    R1
               Acyltransferase

                                                CoASH
                                                    O


                            H2C           O         C    R1
                                                                            First and second
                                 CH monoacylphosphate
                     HO
                                    (lysophosphatidate)                 acyltransferases have
                    O       H2C           OPO32-                         different specificities:
CoA        S        C       R2
                                                                    1: Preference for saturated FA
                                              Acyltransferase       2: Preference for unsaturated
                     CoASH
                                                    O
                                                                    FA
               O            H2C           O         C   R1


      R2       C        O        CH      phosphatidate


                            H2C           OPO32-
Strorage   Membranes
TAG Cycle:
 75% of FA are esterified as TAG
                    Synthesis of glycerol:
                     Glyceroneogenesis
• Analogous to gluconeogenesis
  (pyruvate to DHAP)
• Regulated (in part) by PEP
  carboxykinase
• Reciprocally regulated by cortisol and
  other sterol hormones
   – Liver: PEP carboxykinase expression 
   – Adipose: PEP carboxykinase expression 
    Membrane Lipid Biosynthesis
•   Steps to glycero- and sphingolipid
    construction:
    1. Synthesis of backbone (glycerol or
       sphingosine)
    2. FA exterification (or amidation)
    3. Head group attachment (e.g., choline,
       serine, ethathanolamine, etc.)
    4. Head group modification
Acidic lipid



Neutral lipid
                                                                                                      O


                                                                O               H 2C         O        C        R1


                                                      R2        C       O         CH        phosphatidate


                                                                                H 2C         OPO32-




                                                                        H2O
                                                                                            phosphatidate phosphatase


                                                                            Pi
                                                                                                          O


                                                                    O            H 2C            O        C     R1
                O

                                                       R2           C       O          CH                                                            CDP-ethanolamine
CoA   S         C     R3
                                                                1,2-DAG                                                     phosphothanolamine
                               acyltransferase
                                                                                 H 2C            OH                         transferase
          CoA        SH                                                                                                                              CMP



                                                                                                                                                      O
                                  O

                                                                                                                               O         H2 C    O     C    R1
      O             H2C    O      C      R1                    CDP-choline
                                                                                           phosphocholine
                                                                                                                       R2      C     O     CH         O
                                                                                           transferase
R2    C     O        CH           O
                                                                        CMP                                                    PE
                                                                                                                                         H2 C    O     P    O    (CH2)2 NH3+
                    H2C    O      C      R3
      TAG
                                                                                                                                                      O-




                                                                                                 O


                                                           O            H 2C           O         C        R1


                                                 R2        C        O       CH                   O

                                                           PC
                                                                        H 2C           O         P        O         (CH2)2 N(CH3)3


                                                                                                 O-
               Head Group Activation
                                                     H2        H2
                                           HO        C         C         N+(CH3)3

Substitute ethanolamine                                         ATP
                                                           choline kinase
for choline                                                   ADP
                                          O

                                                          H2        H2
                                  -
                                  O       P     O         C         C       N+(CH3)3


                                          O-                        CTP
                                                           CTP-phosphocholine
                                                           transferase
                                                                    PPi
                          NH2




                      N                                   O

                                                                             H2     H2
                                                -
                                                O         P         O        C      C    N+(CH3)3

                  O       N
                                                          O


                                      H2C       O         P         O-
                                  O

                              H       H                   O

                          H                H        CDP-Choline
                              H       OH
Plasmologen (e.g., PAF):
An ether lipid
           CH2OH
                 serine
+
H3N        CH
                                               O
                                                                                             Sphingolipid synthesis
                                CoAS           C     (CH2)14CH3
           COO-
                                               palmitoyl-CoA



                                                                                                                                     CH2OH
                                  H+
    3-ketosphinganine                                                                                                  +
                                                                                                                       H3N           CH
             synthase
                                  CoASH
             CO2                                                                                                                    HC       C     C     (CH2)12CH3
                                                                                                                                             H     H

                                                                                                                                     OH
                                                                                                                                                 sphingosine
                                                                                                             O
                           CH2OH

                                                                                                   R1        C        SCoA
             +
                 H 3N      CH      3-ketosphinganine
                                                                                                                                          sphingosine acyl transferase
                           C       (CH2)14CH3
                                                                                                                  HSCoA
                           O
                                 NADPH + H+
3-ketosphinganine
        reductase                                                                                                 O                  CH2OH
                                  NADP+                                                                                         +
                                                                                                                           H2
                                                                                                        R1        C        N         CH
                           CH2OH
                                                                                                                  ceramide
             +                                                                                                                      HC      C     C     (CH2)12CH3
                 H 3N      CH          sphinganine                                                                                          H     H
                                                                                                                                                        UDP-galactose
                                                                                                             PC                     OH
                          HC       (CH2)14CH3                                                                                                                                            OH

                                                                                                  DAG                                                             UDP
                          OH

                                 e-carrier (OX)            H2O                               O
       sphinganine                                                                                                                                                                            OH
                               FAD                                                                                                                                                       O
    dehydgrogenase                                                                                      H2        H2                                                                               OH
                                 e-carrier (RED)          1/2 O2                                                                                                  O            H2C   O
                                                                        O          H2C   O   P    O     C         C        N+(CH3)3
                                                                                                                                                                         H2+                  OH
                           CH2OH                                             H2+
                                                                                                                                                         R1       C      N      CH
                                                                   R1   C    N      CH       O-

             +                                                          ceramide                                                                               cerebroside
                 H3N       CH                                                                                                                                                  HC    C   C    (CH2)12CH3
                                                                                   HC    C   C     (CH2)12CH3                                                                        H   H
                                                                                         H   H
                          HC       C       C       (CH2)12CH3                                                                                                                   OH
                                   H       H                                        OH

                           OH
                                        sphingosine
Cholesterol Synthesis
                Stage 1:
                Acetyl-CoA  mevalonate




First committed step
•Large integral membrane protein of ER
•Active site on cytosolic face
Stage 2: mevalonate  activated isoprenes
Stage 3:
Acivated isoprenes  squalene




 To squalene synthesis
Stage 4:
Squalene  cholesterol
      Regulation of Cholesterol
           Biosynthesis
• Regulation slide
  – Dolichol: Involved in
     • Addition of sugars to proteins
     • Addition of sugar to bacterial cell walls
     • The pathway for for N-linked glycoprotein
       synthesis
  – Ubiquinone
  – Farnesylated proteins (membrane “anchors”)
Cholesterol Transport
             Cholesterol (and lipid) Transport
• Dietary fat and cholesterol are assembled into chylomicrons
  (intestines)
     – Chylomicrons bind to sites in capillaries where lipoprotein lipase (LPL)
       hydrolyze TAGs, which are taken up by tissue
     – Smaller chylomicron remnants, now enriched in cholesterol, are
       released by capillaries and taken up by liver via receptor-mediated
       endocytosis
•    VLDL (synthesized in liver and containing free and esterified
    cholesterol) leave liver and are degraded by LPLs (much like
    chylomicrons) and are converted to IDL and LDL
     – Most cholesterol is esterified by lecithin-cholesterol acyl transferase
       (LCAT)
     – Lipoproteins are removed, except for apoB-100
         • ApoB-100 is one of the largest monomeric proteins known
         • ApoB-100 and B-48 are encoded by the same gene
• LDL (containing apoB-100 (and some apoE) is taken up by liver via
  receptor-mediated endocytosis
• Newly-formed vesicles fuse with lysosomes where
     – apoB-100 is degraded
     – Cholesteryl esters are hydrolyzed to free cholesterol, which is
       incorporated into membranes (as needed)
     – Excess cholesterol is re-esterified by ACAT for storage
       ApoE
       B-48
ApoE
C-II
B-48


              ApoE
              C-II
              B-100
             dietary fat + cholesterol


Intestine
                                  chylomicrons                                   chylomicron
                                                                                 remnants



                                                        free fatty acids




Bile acids                                 adipose
                                           tissue
                                                                           extrahepatic tissue    HDL
                                                                           (with LDL receptors)

                            FFA




                                                     FFA


  Liver                                   VLDL           IDL                     LDL




                                         HDL receptor


                                         chylomicron remnant receptor


                                         LDL receptor
   Cholesterol (and lipid) Transport, continued

• Overabundance of cholesteryl esters is
  prevented
  – High levels of cellular cholesterol suppress:
     • Expression of LDL receptors
     • Biosynthesis of cholesterol
• HDL: “Reverse cholesterol transport”
  – Scavenges cholesterol*
  – Assembled from components of other lipoproteins
  – Contains LCAT, which converts free cholesterol to
    cholesteryl esters
  – Cholesterol is delivered to liver*, where some is
    converted to bile acids/salts for excretion



                 *Mechanism is poorly understood
                          Acetyl CoA




                                          Ubiquinone
      Terpenes             Isopentyl
      (plants)           pyrophosphate
                                         Vitamins A, E, K


                                                                                 OH

    Cholesteryl
    esters               Cholesterol
                                                            OH


    Bile salts                                                   1,25-Dihydroxyvitamin D3

                                                       HO
                    OH


                                                OH




O
     testosterone                        -estradiol

                         HO

				
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