Mitochondria fatty acid synthesis

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. 

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. 

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 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.

One enzyme regulated by AMPK is acetyl-CoA carboxylase, which produces malonyl-CoA, the first intermediate committed to fatty acid synthesis. Malonyl-CoA is a powerful inhibitor of the enzyme carnitine acyl-transferase I, which starts the process of ]3 oxidation by transporting fatty acids into the mitochondrion (see Fig. 17-6). By phosphorylating and inactivating acetyl-CoA carboxylase, AMPK inhibits fatty acid synthesis while relieving the inhibition (by malonyl-CoA) of )3 oxidation (Fig. 23-37). 

Similarly, factors that stimulate acetyl-CoA carboxylase, the first enzyme in the pathway for fatty acid synthesis, also discourage fatty acid catabolism. This dual effect occurs because the first enzyme in the pathway leads to the formation of malonyl-CoA, which is a potent inhibitor of carnitine acyltransferase I. This inhibition prevents the transport of fatty acids into the mitochondrion, thereby, preventing fatty acid breakdown. 

Answer Malonyl-CoA would no longer inhibit fatty acid entry into the mitochondrion and jS oxidation, so there might be a futile cycle of simultaneous fatty acid synthesis in the cytosol and fatty acid breakdown in mitochondria. (See Fig. 17-12.)... 

OAA by pyruvate carboxylase (EC 6.4.1.1), thereby completing the net transport of the C2 unit (acetate) from the mitochondrion to the cytosol with the added advantage of having converted a reducing equivalent as NADH + H+ to NADPH + H+. This mechanism of C2 transport provides up to 50% of the NADPH + H+ for fatty acid synthesis in nonruminants. 

For the conversion of pyruvate to oxaloacetate and the formation of citrate in the mitochondrion, see Chap. 12. Acetyl-CoA for fatty acid synthesis is converted to malonyl-CoA this reaction is catalyzed by acetyl-CoA carboxylase. Seven molecules of acetyl-CoA are converted to malonyl-CoA for the synthesis of one molecule of palmitic acid. 

In animals and fungi there is a similar dichotomy. NADPH can be generated by cytosolic malic enzyme which catalyses the reaction malate + NADP+ — pyruvate + COg + NADPH. Cytosolic malate derives from the following successive reactions the pyruvate/ citrate shuttle on the mitochondrial inner membrane takes pyruvate to the mitochondrion in exchange for citrate cytosolic ATP citrate lyase catalyses ATP + citrate + CoA-SH —> acetylCoA (CH3CO-S-C0A) + oxaloacetate and cytosolic malate dehydrogenase, which catalyses NADH + oxaloacetate NAD+ + malate. This scheme provides both acetylCoA and NADPH for subsequent long chain fatty acid synthesis (see section on Fatty acid synthesis ). 

Transsport of fatty adds into the mitochondrion is regulated by a mechanism that plays a major role in controlling the overall rate of oxidation of fatty acids. This mechanism is in "communication" with the pathw ay for fatty acid synthesis. The fatty acid transport system is sensitive to the concentration of one fatty acid synthesis intermediate, malonyl-CoA. 

The study described in Figure 4.54 illustrates the role of cytosolic malonyl-CoA in controlling the entry of fatty acids into the mitochondrion. The activity of carnitine acyltransferasc w as measured after adding various concentrations of fatty acyl-CoA (0-250 jiM) to a suspension of mitochondria both w ith and without malonyl-CoA. Transferase activity increased with increasing concentrations of fatty acyl-CoA until a plateau was reached. At this point, the transferase was saturated and unable to operate at a faster rate when presented w ith higher concentrations of its substrate. The activity of the transferase w as impaired in the presence of malonyl-CoA. This indicates that an increase in the rate of fatty acid synthesis, with the resultant increase in concentration of malonyl oA, will impair the oxidation of fatty acids. Therefore, the pathways of fatty acid oxidation and synthesis are coordinated. Figure 4-54 also indicates that malonyl-CoA inhibition can be overcome by higher levels of fatty acy)-CoA. Other studies have revealed... 

The malic enzyme/citrate lyase pathway is shown in Figure 5.10. The 2-carbon units acetyl groups) for fatty acid synthesis are supplied by the activity of citrate lyase, which may be considered an enzyme of fatty acid biosynthesis. The reduced NADP is Supplied at the point of malic enzyme. Figure 5.10 reveals no net production or utilization of NAD in the cytoplasm. The NADPH + H generated in the cytoplasm is used for fatly acid synthesis, which regenerates NADP. One molecule of CO is produced in the cytoplasm. The diagram reveals no net production or utilization of CO in the mitochondrion. One molecule of NAD is... 

Although fatty acid synthesis occurs within the cytoplasm of most animal cells, liver is the major site for this process. (Recall, for example, that liver produces VLDL. See p. 349.) Fatty acids are synthesized when the diet is low in fat and/or high in carbohydrate or protein. Most fatty acids are synthesized from dietary glucose. As discussed, glucose is converted to pyruvate in the cytoplasm. After entering the mitochondrion, pyruvate is converted to acetyl-CoA, which condenses with oxaloacetate, a citric acid cycle intermediate, to form citrate. When mitochondrial citrate levels are sufficiently high (i.e., cellular energy requirements are low), citrate enters the cytoplasm, where it is cleaved to form acetyl-CoA and oxaloacetate. The net reaction for the synthesis of palmitic acid from acetyl-CoA is as follows ... 

In muscle, most of the fatty acids undergoing beta oxidation are completely oxidized to C02 and water. In liver, however, there is another major fate for fatty acids this is the formation of ketone bodies, namely acetoacetate and b-hydroxybutyrate. The fatty acids must be transported into the mitochondrion for normal beta oxidation. This may be a limiting factor for beta oxidation in many tissues and ketone-body formation in the liver. The extramitochondrial fatty-acyl portion of fatty-acyl CoA can be transferred across the outer mitochondrial membrane to carnitine by carnitine palmitoyltransferase I (CPTI). This enzyme is located on the inner side of the outer mitochondrial membrane. The acylcarnitine is now located in mitochondrial intermembrane space. The fatty-acid portion of acylcarnitine is then transported across the inner mitochondrial membrane to coenzyme A to form fatty-acyl CoA in the mitochondrial matrix. This translocation is catalyzed by carnitine palmitoyltransferase II (CPTII Fig. 14.1), located on the inner side of the inner membrane. This later translocation is also facilitated by camitine-acylcamitine translocase, located in the inner mitochondrial membrane. The CPTI is inhibited by malonyl CoA, an intermediate of fatty-acid synthesis (see Chapter 15). This inhibition occurs in all tissues that oxidize fatty acids. The level of malonyl CoA varies among tissues and with various nutritional and hormonal conditions. The sensitivity of CPTI to malonyl CoA also varies among tissues and with nutritional and hormonal conditions, even within a given tissue. Thus, fatty-acid oxidation may be controlled by the activity and relative inhibition of CPTI. 

The acetyl units needed for fatty-acid synthesis are generated in the mitochondrion, but utilized in the cy-... 

Transport of acetyl CoA from mitochondria to cytosol. The transport of the acetyl unit of acetyl CoA is moved out of the mitochondrion as citrate and reformed to acetyl CoA in the cytosol. The resulting oxaloacetate can participate in effective conversion of NADH to NADPH, the latter participating in cholesterol and fatty-acid synthesis. The acetyl carbons of... 

Lipogenesis is controlled by a number of mechanisms, including allosteric effectors, covalent modification, and availability of substrate. Pyruvate is an excellent potential precursor for fatty acids, particularly in the liver. One of the difficulties encountered is that pyruvate can proceed to acetyl CoA in the mitochondrion, however, acetyl CoA in the mitochondrion will not directly produce fatty-acid synthesis because this process occurs in the cytosol. 

The control of fatty-acid oxidation is related to the availability of circulating fatty acids and the activity of palmitoyl carnitine transferase 1. When circulating fatty acids are elevated, considerable fatty-acyl CoA is formed in a number of tissues, including the liver, which is sufficient to inhibit both acetyl CoA carboxylase in the cytosol and, indirectly, pyruvate dehydrogenase in the mitochondrion. Under this condition, neither malonyl CoA nor citrate would accumulate thus, there would be a diminution of fatty-acid synthesis. When large amounts of fatty... 

The acetyl units used for fatty-acid synthesis are transported from the mitochondrion as... 

Both malate enzyme and citrate lyase are part of the shuttle system that transports two-carbon units from the mitochondrion to the cytosol. Malate enzyme also generates reducing power in the form of NADPH, which is used for fatty acid synthesis however, the pentose phosphate pathway (see the text. Section 20.3) also serves as a source of NADPH, so that fatty acid synthesis can continue even if malate enzyme is deficient. Recall from page 515 of the text that malate can cross the mitochondrial membrane. Citrate lyase is more critical to fatty acid synthesis because it is required to generate acetyl CoA from citrate in the cytosol. Without cytosolic acetyl CoA, fatty acid synthesis cannot take place, and the cells cannot grow and divide. 

As shown in Figure 5.28, the first reaction in the synthesis of fatty acids is carboxylation of acetyl CoA to malonyl CoA. This is a biotin-dependent reaction (section 11.12.2) and, as discussed above (section 5.5.1), the activity of acetyl CoA carboxylase is regulated in response to insulin and glucagon. Malonyl CoA is not only the substrate for fatty acid synthesis, but also a potent inhibitor of carnitine palmitoyl transferase, so inhibiting the uptake of fatty acids into the mitochondrion for P-oxidation. 

Pyruvate is converted to phosphoenolpyruvate for glucose synthesis by a two-step reaction, with the intermediate formation of oxaloacetate. As shown in Figure 5.31, pyruvate is carboxylated to oxaloacetate in an ATP-dependent reaction in which the vitamin biotin (section 11.12) is the coenzyme. This reaction can also be used to replenish oxaloacetate in the citric acid cycle when intermediates have been withdrawn for use in other pathways, and is involved in the return of oxaloacetate from the cytosol to the mitochondrion in fatty acid synthesis — see Figure 5.26. Oxaloacetate then undergoes a phosphorylation reaction, in which it also loses carbon dioxide, to form phosphoenolpyruvate. The phosphate donor for this reaction is GTP as discussed in section 5.4.4, this provides regulation over the use of oxaloacetate for gluconeogenesis if citric acid cycle activity would be impaired. 

FIGURE 15.5 Transportation of acetyl-CoA from the mitochondrion for fatty acid synthesis... 

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