Cytosol fatty acid synthesis

The acetyl-CoA derived from amino acid degradation is normally insufficient for fatty acid biosynthesis, and the acetyl-CoA produced by pyruvate dehydrogenase and by fatty acid oxidation cannot cross the mitochondrial membrane to participate directly in fatty acid synthesis. Instead, acetyl-CoA is linked with oxaloacetate to form citrate, which is transported from the mitochondrial matrix to the cytosol (Figure 25.1). Here it can be converted back into acetyl-CoA and oxaloacetate by ATP-citrate lyase. In this manner, mitochondrial acetyl-CoA becomes the substrate for cytosolic fatty acid synthesis. (Oxaloacetate returns to the mitochondria in the form of either pyruvate or malate, which is then reconverted to acetyl-CoA and oxaloacetate, respectively.)... 

This transport is accomplished by carnitine (L-jS-hydroxy-y-trimethylammonium butyrate), which is required in catalytic amounts for the oxidation of fatty acids (Figure 18-1). Carnitine also participates in the transport of acetyl-CoA for cytosolic fatty acid synthesis. Two carnitine acyl-transferases are involved in acyl-CoA transport carnitine palmitoyltransferase I (CPTI), located on the outer surface of the inner mitochondrial membrane, and carnitine palmitoyltransferase II (CPTII), located on the inner surface. 

The end-product of cytosolic fatty acid synthesis is palmitate (C16 0) longer-chain fatty acids (up to C24) and unsaturated fatty acids are synthesized from palmitate in the endoplasmic reticulum and mitochondria. 

FIGURE 20.23 Export of citrate from mitochondria and cytosolic breakdown produces oxaloacetate and acetyl-CoA. Oxaloacetate is recycled to malate or pyruvate, which re-enters the mitochondria. This cycle provides acetyl-CoA for fatty acid synthesis in the cytosol. 

COMPARTMENTALIZED PYRUVATE CARBOXYLASE DEPENDS ON METABOLITE CONVERSION AND TRANSPORT The second interesting feature of pyruvate carboxylase is that it is found only in the matrix of the mitochondria. By contrast, the next enzyme in the gluconeogenic pathway, PEP carboxykinase, may be localized in the cytosol or in the mitochondria or both. For example, rabbit liver PEP carboxykinase is predominantly mitochondrial, whereas the rat liver enzyme is strictly cytosolic. In human liver, PEP carboxykinase is found both in the cytosol and in the mitochondria. Pyruvate is transported into the mitochondrial matrix, where it can be converted to acetyl-CoA (for use in the TCA cycle) and then to citrate (for fatty acid synthesis see Figure 25.1). /Uternatively, it may be converted directly to 0/ A by pyruvate carboxylase and used in glu-... 

Intermediates in fatty acid synthesis are linked covalently to the suifhydryl groups of special proteins, the acyl carrier proteins. In contrast, fatty acid breakdown intermediates are bound to the —SH group of coenzyme A. Fatty acid synthesis occurs in the cytosol, whereas fatty acid degradation takes place in mitochondria. 

Providing Cytosolic Acetyl-CoA and Reducing Power for Fatty Acid Synthesis... 

Eukaryotic cells face a dilemma in providing suitable amounts of substrate for fatty acid synthesis. Sufficient quantities of acetyl-CoA, malonyl-CoA, and NADPH must be generated in the cytosol for fatty acid synthesis. Malonyl-CoA is made by carboxylation of acetyl-CoA, so the problem reduces to generating sufficient acetyl-CoA and NADPH. 

FIGURE 25.1 The citrate-malate-pyruvate shuttle provides cytosolic acetate units and reducing equivalents (electrons) for fatty acid synthesis. The shuttle collects carbon substrates, primarily from glycolysis but also from fatty acid oxidation and amino acid catabolism. Most of the reducing equivalents are glycolytic in origin. Pathways that provide carbon for fatty acid synthesis are shown in blue pathways that supply electrons for fatty acid synthesis are shown in red. 

Glycolysis, the pentose phosphate pathway, and fatty acid synthesis are all found in the cytosol. In gluconeo-genesis, substrates such as lactate and pyruvate, which are formed in the cytosol, enter the mitochondrion to yield oxaloacetate before formation of glucose. 

Citrate is isomerized to isocitrate by the enzyme aconitase (aconitate hydratase) the reaction occurs in two steps dehydration to r-aconitate, some of which remains bound to the enzyme and rehydration to isocitrate. Although citrate is a symmetric molecule, aconitase reacts with citrate asymmetrically, so that the two carbon atoms that are lost in subsequent reactions of the cycle are not those that were added from acetyl-CoA. This asymmetric behavior is due to channeling— transfer of the product of citrate synthase directly onto the active site of aconitase without entering free solution. This provides integration of citric acid cycle activity and the provision of citrate in the cytosol as a source of acetyl-CoA for fatty acid synthesis. The poison fluo-roacetate is toxic because fluoroacetyl-CoA condenses with oxaloacetate to form fluorocitrate, which inhibits aconitase, causing citrate to accumulate. 

Pymvate dehydrogenase is a mitochondrial enzyme, and fatty acid synthesis is a cytosohc pathway, but the mitochondrial membrane is impermeable to acetyl-CoA. Acetyl-CoA is made available in the cytosol from citrate synthesized in the mitochondrion, transported into the cytosol and cleaved in a reaction catalyzed by ATP-citrate lyase. 

Calculating energy costs for the synthesis of a CK, fatty acid from acetyl-CoA is not as simple as you might first think. The major complication is that acetyl-CoA is made in the mitochondria, but fatty acid synthesis occurs in the cytosol—acetyl-CoA can t cross the mitochondrial membrane. Acetyl-CoA gets out of the mitochondria disguised as citrate. The acetyl-CoA is condensed with oxaloacetate to give citrate, and the citrate leaves the mitochondria. In the cytosol, the citrate is cleaved by an ATP-dependent citrate lyase into acetyl-CoA and oxaloacetate ... 

If we count the NADPH (cytosol) as 3 ATP equivalents, which could have been oxidized by the TCA cycle if they hadn t been used for fatty acid synthesis, then the synthesis of Cx6 fat requires... 

In common with cholesterol synthesis described in the next section, fatty acids are derived from glucose-derived acetyl-CoA. In the fed state when glucose is plentiful and more than sufficient acetyl-CoA is available to supply the TCA cycle, carbon atoms are transported out of the mitochondrion as citrate (Figure 6.8). Once in the cytosol, citrate lyase forms acetyl-CoA and oxaloacetate (OAA) from the citrate. The OAA cannot re-enter the mitochondrion but is converted into malate by cytosolic malate dehydrogenase (cMDH) and then back into OAA by mitochondrial MDH (mMDH) Acetyl-CoA remains in the cytosol and is available for fatty acid synthesis. 

The fatty acid synthesis pathway can be seen to occur in two parts. An initial priming stage in which acetyl-CoA is converted to malonyl-CoA by a carboxylation reaction (Figure 6.9) is followed by a series of reactions which occur on a multi-enzyme complex (MEC), which achieves chain elongation forming C16 palmitoyl-CoA. The whole process occurs in the cytosol. 

Fatty acid synthesis Adipose, liver and muscle Cytosol 6.3.2.1... 

The basic building block for fatty acid synthesis is acetyl-CoA, produced from glucose, fructose or amino acids (Figure 11.1). Acetyl-CoA formation from these precursors occurs within the mitochondrion and so, because fatty acid synthesis occurs in the cytosol, acetyl-CoA must be transported across the mitochondrial membrane. Trans-... 

Figure 11.3 Mechanism of transfer of acetyl-CoA out of the mitochondrion. In the mitochondrion, acetyl-CoA reacts with oxaloacetate to form citrate, which is transported across the mitochondrial inner membrane. In the cytosol, citrate is split to re-form citrate and oxaloacetate, catalysed by citrate lyase. It has been shown that inhibition of citrate lyase inhibits fatty acid synthesis.

The physiological pathway starts with acetyl-CoA in the cytosol. It is converted to malonyl-CoA, which is the flux-generating step for fatty acid synthesis catalysed by... 

Figure 11.6 The physiological pathway for fatty acid synthesis acetyl-CoA to palmitoyl-CoA. The pathway starts with the conversion of acetyl-CoA to malonyl-CoA in the cytosol, which is the flux-generating step catalysed by acetyl-CoA carboxylase. The pathway can be considered to end with formation of palmitoyl-CoA rather than palmitate, since it has several fates formation of triacylglycerol and phospholipids or acylation of other compounds.

Fatty Acid Synthesis Occurs in the Cytosol of Many Organisms but in the Chloroplasts of Plants... 

In the photosynthetic cells of plants, fatty acid synthesis occurs not in the cytosol but in the chloroplast stroma (Fig. 21-8). This makes sense, given that NADPH is produced in chloroplasts by the light reactions of photosynthesis ... 

Net Equation of Fatty Acid Synthesis Write the net equation for the biosynthesis of palmitate in rat liver, starting from mitochondrial acetyl-CoA and cytosolic NADPH, ATP, and C02. 

The energy for the carbon-to-carbon condensations in fatty acid synthesis is supplied by the process of carboxylation and then decarboxylation of acetyl groups in the cytosol. The carboxylation of acetyl CcA to form malonyl CoA is catalyzed by acetyl CoA carboxylase (Figure 16.7), and requires HC03 )and ATP. The coenzyme is the vitamin, biotin, which is covalently bound to a lysyl residue of the carboxylase. 

The glycolytic pathway produces pyruvate, which is the primary source of the mitochondrial acetyl CoA to be used for fatty acid synthesis. It also produces cytosolic reducing equivalents of NADH. Pyruvate enters the mitochondria. 

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