In eukaryotes (non-photosynthetic), almost all the acetyl-CoA used in fatty acid synthesis is formed in mitochondria from pyruvate oxidation and from the catabolism of the carbon skeletons of amino acids. Acetyl-
CoA arising from the oxidation of fatty acids does not represent a significant source of acetyl –CoA for fatty acid biosynthesis in animals because the two pathways are regulated reciprocally.
The mitochondrial inner membrane is impermeable to acetyl –CoA, so an indirect shuttle transfers acetyl group equivalents across the inner membrane Fig. 11.
Fig. 11. Shuttle for transfer of acetyl groups from mitochondria to the cytosol. The outer mictochondrial membrane is freely permeable to all of these compounds. Pyruvate derived from amino acid catabolism in the mitochondrial matrix, or
from glucose by glycolysis in the cytosol, is converted to acetyl-CoA in the matrix. Acetyl groups pass out of the mitochondrion as citrate; in the cytosol they are delivered as acetyl-CoA for fatty acid synthesis. Oxaloacetate is reduced to
malate, which returns to the mitochondrial matrix and is converted to oxaloacetate. An alternative fate for cytosolic malate is oxidation by malic enzyme to generate cytosolic NADPH; the pyruvate produced returns to the mitochondrial matrix.
Intra-mitochondrial acetyl –CoA first reacts with oxaloacetate to form citrate, in the citric acid cycle reaction catalyzed by citrate synthase. Citrate then passes into the cytosol through the mitochondrial inner membrane at the citrate transporter. In the cytosol, citrate is cleaved by citrate lyase regenerates acetyl– CoA in an ATP dependent reaction. Oxaloacetate cannot return to the mitochondrial matrix directly as there is no oxaloacetate transporter. Instead, oxaloacetate is reduced by cytosolic malate dehydrogenase to malate, which returns to the mitochondrial matrix on the malate-α-ketoglutarate transporter in exchange for citrate. There it is reoxidized to oxaloacetate to complete the shuttle.
Alternatively, the malate produced in the cytosol is used to generate cytosolic NADPH through the activity of malic enzyme Fig. 12.
COO– CHOH CH3 COO– Malate
NADP+ NADP + H+ COO– C=O + CO2 CH3 Pyrvuate
Fig. 12 NADPH produced by Malic enzyme Palmitate
(16:0) Elongation
Stearate (18:0)
Desaturation Palmitoleate
(16:1) (∆9)
Oleate (18:1) (∆9)
Desaturation Vaccenate
(18:1) (∆11) Elongation Elongation
Longer, saturated fatty acids
Linoleate 18:2 (∆9,12) Desaturation (in plants only)
Desaturation Desaturation
(in plants only) α-linolenate 18:3 (∆9,12,15)
γ-linolenate 18:3 (∆6,9,12)
Elongation Other polyunsaturated
fatty acids Eicosatrienoate 20:3 (∆8,11,14)
Desaturation Eicosatetraenate (=Arachidonate) 20:4 (∆5,8,11,14)
Palmitate is the precursor of stearate and longer chain saturated fatty acids, as well as the monounsaturated fatty acids, palmitoleate and oleate. Mammals can not convert oleate into linoleate or a linolenate, hence required in the diet as essential fatty acid (EFAs). Conversion of linoleate into other polyunsaturated fatty acids and eicoandnoids is also outlined.
Biosynthesis of Long-chain Fatty Acids
Palmitate (a C16 fatty acid) is the major product of the fatty acid synthase system. This system is also called the de novo system in that the palmitate is constructed from acetyl –CoA (ACP) and malonyl – CoA (ACP). In plants and animals, the most important fatty acids are the C18 fatty acids, namely stearic, oleic, linoleic and linolenic. These C18 fatty acids, namely stearic, oleic, linoleic and linolenic are synthesized by elongation system which differ marked by from the de novo system. Palmitate acts as a precursor of other long chain fatty acids (Fig. 13). It may be lengthened to from stearate (18:0) or even larger saturate fatty acids by further addition of acetyl groups though the action of fatty acid elongation systems present in the endoplasmic reticulum (= microsomes) and mitochondria in the case of animals and in the soluble cytosol in the case of plants.
In animals
Endoplasmic reticulum membrane
Palmitoyl - CoA Malonyl - CoA
+ NADPH Stearyl - CoA Mitochondrial outer and inner membrane
Palmitoyl - CoA Acetyl - CoA
+ NADPH Stearyl - CoA In plants
Soluble cytosolic system
Palmitoyl -ACP Malonyl - ACP
+ NADPH Stearyl - ACP
Although different enzyme systems are involved and coenzyme A, rather than ACP, is the acyl carrier directly involved in animals fatty acid synthesis, the mechanism of elongation is otherwise identical with that employed in palmitate synthesis : donation of two carbons by malonyl ACP, followed by reduction (with NADH), dehydration and another reduction (with NADPH) to the saturated C18 product, stearyl-CoA. Fasting largely abolishes chain elongation. Elongation of stearyl–CoA in brain increase rapidly during myelination in order to provide C22 and C24 fatty acids that are present in sphingolipids.
BIOSYNTHESIS OF UNSATURATED FATTY ACIDS
Palmitate and stearate serve as precursors of the two most common monounsaturated fatty acids of animal tissues, palmitoleate and oleate. Both of them possess a single cis double bond between C-9 and C-10. The double bond is introduced into the fatty acid chain by an oxidative reaction catalyzed by fatty acyl – CoA desaturase (Fig. 14). The enzyme is an example of a mixed function oxidase. Two different substrates, a fattyacyl – CoA and NADPH, simultaneously undergo two electron oxidations.
CH3–(CH2)m–CH2–CH2–(CH2)n C O
S–CoA
O2 + 2H+
CH3–(CH2)m–CH=CH–(CH2)nC O S–CoA
+ 2H2O Fatty acyl-CoA desaturase
2 Cyt b5 (Fe2+) 2 Cyt b5
(Fe3+)
Cyt b5 reductase (FAD) Cyt b5 reductase
(FADH2)
NADPH +H+ NADP+
Fig. 14
+ Saturated fatty acyl-CoA
Monounsaturated fatty acyl-CoA
The pathway of electron transfer in the desaturation of fatty acids by a mixed function oxidase in vertebrates
[Two different substrates a fatty acyl-CoA and NADPH – undergoes oxidation by molecular oxygen. These reactions occur on the lumenal face of the ER. A similar pathway, but with different electron carries, exists in plants. The path of electron flow includes a cytochrome (cytochrome b5) and a flavoprotein (cytochrome b5 reductase), both of which like fatty acyl – CoA desaturase it self are present in the smooth ER.]
Mammalian hepatocytes can readily introduce double bond at ∆9 position of fatty acids but cannot introduce additional double bond in the fatty acid chain between C-10 and the methyl terminal end. Thus, linoleate, 18 : 2 (∆9,12) and α-linolenate, 18 : 3 (∆9,12,15) cannot be synthesized by mammals, but plants can synthesize both.
The plant desaturases that introduce double bonds at ∆12 and ∆15 positions are located in the endoplasmic reticulum.
These enzymes, in fact, act not on free fatty acids but on a phospholipid called phosphotidylcholine which contains at least one oleate linked to glycerol (fig.15). Because linoleate and linolenate are necessary precursors for the synthesis of other products, they are essential fatty acids (EFAs) for mammals and must be obtained from plants material in the diet.
Fig. 15. Oxidation of phosphatidylcholine-bound oleate by desaturases, producing polyunsatturated fatty acids
Once ingested, linolenate, eicosatrienoate and eicosatetraenoate (=arachidonate), can be made only from linoleate. Arachidonate, 20: 4 (∆5,8,11,14) is an essential precursor of regulatory lipids, the eicosanoids.