Fatty acid synthesis NADPH source

Cholesterol synthesis. The early steps of cholesterol synthesis are presented, including the rate-limiting reaction HMGCoA reductase (in purple), with a general outline of the latter steps. Note, as with fatty-acid synthesis, NADPH is an important source of... 

F. 33.9. Sources of NADPH for fatty acid synthesis. NADPH is produced by the pentose phosphate pathway and by malic enzyme. OAA = oxaloacetate. 

The ketoacyl-ACP is then reduced to yield a hydroxyl group. In turn, this is dehydrated to yield a carbon-carbon double bond, which is reduced to yield a saturated fatty acid chain. Thus, the sequence of chemical reactions is the reverse of that in P-oxidation (section 5.5.2). For both reduction reactions in fatty acid synthesis, NADPH is the hydrogen donor. One source of this NADPH is the pentose phosphate pathway (section 5.4.2) and the other is the oxidation of malate (arising from oxaloacetate) to pyruvate, catalysed by the malic enzyme (see Figure 5.27). 

D. Major sources of the NADPH required for fatty acid synthesis... 

The rest of the steps in fatty acid synthesis are catalyzed by the fatty acid synthase com plex, which produces palmitoyl CoA from acetyl CoA and malonyl CoA, with NADPH as the source of reducing equivalents. 

A similar situation exists in the case of fatty acid synthesis, which proceeds from acetyl-CoA and reverses fatty acid breakdown. However, both carbon dioxide and ATP, a source of energy, are needed in the synthetic pathway. Furthermore, while oxidation of fatty acids requires NAD+ as one of the oxidants, and generates NADH, the biosynthetic process often requires the related NADPH. These patterns seen in biosynthesis of sugars and fatty acids are typical. Synthetic reactions resemble the catabolic sequences in reverse, but distinct differences are evident. These can usually be related to the requirement for energy and often also to control mechanisms. 

The reactions of fatty acid synthesis all take place in the cytosol, but acetyl-CoA is made in the mitochondria and can t cross the inner membrane. The Pyruvate-Malate Cycle (Citrate-Pyruvate Cycle) is used to take acetyl- groups to the cytosol while simultaneously providing a source of NADPH from NADH, and thus, coupling fatty acid synthesis to Glycolysis (Fig. 10.7). Note that the acetyl-CoA is first joined to oxaloacetate to make citrate which is readily transported out of the mitochondria using a co-transporter. The citrate is then cleaved to acetyl-CoA and oxaloacetate, a process requiring ATP to make it favourable (recall the condensation was spontaneous). Acetyl-CoA for fatty acid synthesis is now available in the cytosol, but oxaloacetate must be regenerated for the mitosol. 

Several Sources Supply NADPH for Fatty Acid Synthesis... 

Acetyl CoA is converted to malonyl CoA and into fatty acids as described previously. The enzyme that carries out the first committed step for fatty-acid synthesis, acetyl CoA carboxylase, is finely controlled both allosterically and covalently. This enzyme can occur in a monomeric inactive form or a polymeric active form. One factor that affects this is citrate, which stimulates the polymeric or active form of acetyl CoA carboxylase. Thus, citrate plays an important role in lipogenesis as (1) a source of cytosolic acetyl CoA, (2) an allosteric positive effector of acetyl CoA carboxylase, and (3) a provider of oxaloacetate in the cytosol, which can allow transhydrogenation from NADH to NADPH. An allosteric inhibitor of acetyl CoA carboxylase that causes dissociation to the monomeric form is fatty-acyl CoA. Thus, if exogenous fatty acids are available, there is little reason to synthesize more fatty acids. Fatty-acyl CoA in the cytosol decreases malonyl CoA formation by inhibiting acetyl CoA carboxylase. 

This last point is important because many of the enzymes of fatty acid synthesis require NADPH. The pentose phosphate pathway (Section 18.4) is the principal source of NADPH in most organisms, but here we have another source (Figure 19.14). 

The studies of the individual enzymes of fatty acid synthesis in higher plants has shown that the two reductive steps, p-ketoacyl ACP reductase and enoyl ACP reductase have different cofactor requirements. As a result the synthesis of fatty acids depends on the availability of both NADH and NADPH. While the provision of NADPH can be attributed to the photosynthetic reactions, the source of NADH in the chloroplast is less certain. Takahama etal (8) have demonstrated that the content of NADPH in the chloroplast is influenced by illumination as expected, but there is no such fluctuation of the oxidation state of NAD/NADH. The production of NADH to be utilized in fatty acid synthesis would therefore appear to depend on dark reactions. One possibility would be by the action of pyruvate dehydrogenase, which would generate not only the NADH required for reduction in fatty acid synthesis but also the precursor acetyl CoA. 

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. 

The major source of NADPH used in fatty acid synthesis is the pentose phosphate pathway (PPP). 

Schmidt and Katz [45] suggested an alternative cyclic process involving malate, which would bypass the citrate cleavage enzyme and transfer intramitochondrial reducing equivalents to the cytosol without producing acetyl units. Were this short-circuit of the malate transhydrogenation cycle found to play a major role in adipose tissue, it could supply more than 50% of the NADPH required by the synthetase. Another potential source of extramitochrondial NADPH is isocitrate dehydrogenase however, it does not appear to be of major importance in fatty acid synthesis, as will be discussed later. 

The best-defined role of niacin is in oxidation and reduction reactions, as the functional nicotinamide part of the coenzymes NAD and NADP (section 2.4.1.3). In general, NAD is involved as an electron acceptor in energy-yielding metabolism, being oxidized by the mitochondrial electron transport chain (section 3.3.1.2), whereas the major coenzyme for reductive synthetic reactions is NADPH. An exception to this general rule is the pentose phosphate pathway of glucose metabolism (section 5.4.2), which results in the reduction of NADP to NADPH and is a major metabolic source of reductant for fatty acid synthesis (section 5.6.1). 

The biosynthesis of fatty acids such as palmitate thus requires acetyl-CoA and the input of chemical energy7 in two forms the group transfer potential of ATP and the reducing power of NADPH. The ATP is required to attach C02 to acetyl-CoA to make malonyl-CoA the NADPH is required to reduce the double bonds. We return to the sources of acetyl-CoA and NADPH soon, but first let s consider the structure of the remarkable enzyme complex that catalyzes the synthesis of fatty acids. 

Adipose tissue can metabolize glucose by means of the HMP, thereby producing NADPH, which is essential for fat synthesis (see p. 184 and Figure 24.5, ). However in humans, de novo synthesis is not a major source of fatty acids in adipose tissue. 

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