FATTY ACID SYNTHESIS

 

·      Not an exact reversal of β-oxidation reactions.

·      Occurs in cytosol.

·     

Using an Acyl Carrier Protein (ACP or ACP-SH) instead of CoA for the activation of FAs.

·      CO2 (as HCO3-) is required but is not incorporated into the elongated FA product.

·      Sequential addition of 2-C units derived from acetyl-CoA.

·      The activated unit for the 2-C addition is a 3-C unit in the form of malonyl-ACP.

·      Citrate is required for maximal activity.

·      NADPH is required as a reductant.

·      Elongations stop at palmitate (C16).  Further elongation and/or insertion of double bonds require additional enzymes.

SOURCES OF ACETYL-CoA

·      Cytoplasmic acetyl-CoA synthetase

     CH3-COO- + ATP + CoA → CH3-CO-CoA + AMP + PPi

·      Mitochondrial pyruvate dehydrogenase complex

     Pyruvate + NAD+ + CoA → Acetyl-CoA + NADH + CO2 + H+

·      Mitochondrial b-oxidation of FAs

·      Mitochondrial degradation of some amino acids

 

TRANSPORT OF ACETYL-CoA (Fig. 19-24):    Tricarboxylate transport system

Note:  Malate can also be transported across inner mitochondrial membrane.  But this latter shuttle will not generate NADPH in cytosol (also see discussion below).

 

SOURCES OF NADPH

1.  Tricarboxylate transport system

·      Cytosolic malate dehydrogenase: 

     oxaloacetate + NADH + H+ → malate + NAD+

·       Cytosolic malic enzyme:  malate + NADP+ → pyruvate + CO2 + NADPH

·       pyruvate is  transported into mitochondria

·       Mitochondrial pyruvate carboxylase:

     pyruvate + CO2 + ATP + H2O → OAA + ADP + Pi + 2 H+

·       NADP+ + NADH + ATP + H2O → NADPH + NAD+ + ADP + Pi + 2 H+

·      For every cycle of the tricarboxylate transport system, one acetyl-CoA and one NADPH are gained in cytosol.   At the same time, one NADH plus one ATP are consumed in cytosol and one ATP plus one acetyl-CoA are consumed in mitochondrion.

2.  Pentose phosphate pathway.

 

Role of Citrate

1.  Activator for acetyl-CoA carboxylase (see below) - THE committed step and A rate-limiting Enz for FA synthesis.

2.  Citrate functions in the transport of acetyl-CoA from mitochondria to cytosol - the Citrate Shuttle (same as that shown in Tricarboxylate Transport System).

=============================================================

Acetyl-CoA carboxylase (in cytosol)

Acetyl-CoA + H*CO3- + ATP → -OO*C-C-CO-CoA + H+ + ADP + Pi

                                                            malonyl-CoA

·      Allosteric: Requires citrate or isocitrate for activation.  Inhibited by palmitoyl-CoA.

·      Requires CO2. (or bicarbonate HCO3-)

·      The committed step (= the first irreversible step in a pathway) and one of the rate-limiting steps.

 

Fatty Acid Synthase

·      In higher organisms, as a single polypeptide chain containing 7 activity domains.

·      In bacteria, the constituent enzymes of fatty acid synthase are dissociated upon isolation.

One Cycle of Elongation  (See Fig. 19-26)

1a. Malonyl-CoA/Acetyl-CoA-ACP transacylase

       Acetyl-CoA + ACP ⇌  Acetyl-ACP + CoA

1b. Malonyl-CoA/Acetyl-CoA-ACP transacylase

       Malonyl-CoA + ACP  ⇌  Malonyl-ACP + CoA


2a+2b. β-Ketoacyl-ACP synthase (Condensing enzyme)

3.   β-Ketoacyl-ACP reductase

·      NADPH-dependent, contrary to β-oxidation.

·      Produces D-β-hydroxyacyl-ACP (or 3-D-hydroxyacyl-ACP), contrary to L-β-hydroxyacyl group in β-oxidation.

4.  β-Hydroxyacyl-ACP dehydrase

·      Produces Δ2-trans-enoyl-ACP.

5.  Enoyl-ACP reductase (→ Butyryl-ACP  →  ´ 6 for Palmitoyl-ACP)

·      NADPH-dependent, contrary to β-oxidation.

Recyle 6 more times to obtain Palmitoyl-ACP

6.   Palmitoyl thioesterase

      Palmitoyl-ACP + H2O  à  R-COO- + ACP

See Fig. 19-23 for comparison of FA synthesis versus oxidation.

 

ENERGETIC CONSIDERATION

OXIDATION OF PALMITATE

130 ATP are formed from the complete oxidation of palmitate (see earlier notes).

SYNTHESIS OF PALMITATE

·      7 Acetyl-CoA + 7 CO2 + 7 ATP → 7 Malonyl-CoA + 7 ADP + 7 Pi + 7 H+

·      Acetyl-CoA + 7 Malonyl-CoA + 14 NADPH + 7 H+  →  Palmitate + 7 CO2 + 14 NADP+ + 8 CoA + 6 H2O

·      8 Acetyl-CoA + 7 ATP + 14 NADPH →

                     Palmitate + 14 NADP+ + 8 CoA + 6 H2O + 7 ADP + 7 Pi

8 Acetyl-CoA  →  into mitochondria → TCA  → 8 ´ 12 =                      96 ATP

7 ATP                                                                                                           7 ATP

14 NADPH → 14 NADH → Glycerol 3-P shuttle → 14 FADH2 →        28 ATP

or                                      → Malate-Aspartate shuttle → 14 NADH →  42 ATP

                                                                                               Total = 131 or 145 ATP

 

 

FURTHER ELONGATION OF PALMITATE

1.  Mitochondria

     See Fig. 19-29 - Last step requires NADPH instead of FADH2, otherwise identical to reversal of FA oxidation.

2.  Endoplastic Reticulum

     Acyl-CoA (Cn) + Malonyl-CoA → Acyl-CoA (Cn+2)  →  → Similar to normal FA synthesis except uses CoA instead of ACP.

UNSATURATION


Desaturases

·      X:  At least 5C, could contain double bonds.

·      Y:  Always saturated.

·      e.g. Stearoyl-CoA (C18) → Oleoyl-CoA (cis-Δ9)

·      Oleoyl-CoA can be further elongated and unsaturated to obtain poly-unsaturated FAs.

·      Mammals have Δ9-, Δ6-, Δ5-, and Δ4-fatty acyl-CoA desaturases but cannot generate double bonds beyond C-9. 

·       Therefore, FAs such as linoleate (18:2, cis-Δ9,Δ12) and linolenate (18:3, cis-Δ9,Δ12,Δ15) are ESSENTIAL FAs for mammals.