NADPH fatty acid synthesis

The coenzyme for the oxidation-reduction reactions of fatty acid synthesis is NADP /NADPH, whereas degradation involves the NAD /NADH couple. 

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. 

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... 

NADPH is a reducing agent that is reserved for biosynthetic pathways—notably fatty acid synthesis. Thus, the HMP pathway is called upon when reducing equivalents and fatty acid synthesis are turned on. Primarily, the regulation of the pathway is through the supply and demand of NADPH. 

Numerous experiments were then performed with mitochondria incubated with acetate, CoA, ATP, etc., in attempts to detect fatty acid synthesis. In 1957, Lynen and his colleagues reported the presence in mitochondria of a system which catalyzed the elongation of caproyl CoA to octanoyl CoA by the addition of an acetate unit. NADH and NADPH had to be present. The existence of this mitochondrial system was confirmed by Wakil et al. in 1961 who showed the 12C acid could be extended to 16C by successive additions of 2C fragments. 

By 1960 it was clear that acetyl CoA provided its two carbon atoms to the to and co—1 positions of palmitate. All the other carbon atoms entered via malonyl CoA (Wakil and Ganguly, 1959 Brady et al. 1960). It was also known that 3H-NADPH donated tritium to palmitate. It had been shown too that fatty acid synthesis was very susceptible to inhibition by p-hydroxy mercuribenzoate, TV-ethyl maleimide, and other thiol reagents. If the system was pre-incubated with acetyl CoA, considerable protection was afforded against the mercuribenzoate. In 1961 Lynen and Tada suggested tightly bound acyl-S-enzyme complexes were intermediates in fatty acid synthesis in the yeast system. The malonyl-S-enzyme complex condensed with acyl CoA and the B-keto-product reduced by NADPH, dehydrated, and reduced again to yield the (acyl+2C)-S-enzyme complex. Lynen and Tada thought the reactions were catalyzed by a multifunctional enzyme system. 

Experiments with chloroplasts showed an apparent inhibition of fatty acid synthesis by PAN (at 72 ppm for 10 min). The result is difficult to interpret the inhibition could be attributed to inactivation of one of the enzymes of the multistep system or to oxidation of the reductant (reduced NADP, or NADPH) required in the chain elongation process. 

Figure 11.4 Condensation, dehydration and reduction reactions in fatty add synthesis. These reactions constitute the major components of the pathway of fatty acid synthesis and are all catalysed by fatty acid synthase. The reduction reactions, indicated by addition of 2H in the diagram, involve the conversion of NADPH to NADP . (The re-conversion of NADP back to NADPH occurs in the pentose phosphate pathway.) The condensation reaction results in an increase in size of acyl-ACP by two carbon units in each step. The two carbons for each extension are each provided by malonyl-CoA. ACP - acyl carrier protein.

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 ... 

Again, the resulting high [NADPH]/[NADP+] ratio provides the reducing environment that favors reductive anabolic processes such as fatty acid synthesis. 

Synthesis of Fatty Acids from Glucose After a person has ingested large amounts of sucrose, the glucose and fructose that exceed caloric requirements are transformed to fatty acids for triacylglycerol synthesis. This fatty acid synthesis consumes acetyl-CoA, ATP, and NADPH. How are these substances produced from glucose ... 

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. 

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. 

In addition to its importance in providing cytosolic acetyl-CoA and NADPH, citrate also serves as a major regulator of the rate of fatty acid synthesis. As we shall see (chapter 18) citrate is a strong positive modifier of the first reaction in fatty acid synthesis. It should be remembered (see chapter 12) that citrate also is a negative modifier of phosphofructokinase and thereby exerts a negative effect on glycolysis, which also occurs in the cytosol. 

The Reduction Reactions. The object of the next three reactions (steps 4 to 6 in fig. 18.12a) is to reduce the 3-carbonyl group to a methylene group. The carbonyl is first reduced to a hydroxyl by 3-ketoacyl-ACP reductase. Next, the hydroxyl is removed by a dehydration reaction catalyzed by 3-hydroxyacyl-ACP dehydrase with the formation of a trans double bond. This double bond is reduced by NADPH catalyzed by 2,3-trans-enoyl-ACP reductase. Chemically, these reactions are nearly the same as the reverse of three steps in the j6-oxidation pathway except that the hydroxyl group is in the D-configuration for fatty acid synthesis and in the L-configuration for /3 oxidation (compare figs. 18.4a and 18.12a). Also remember that different cofactors, enzymes and cellular compartments are used in the reactions of fatty acid biosynthesis and degradation. 

Overview Fatty acid synthesis involves the condensation of two-carbon units, in the form of acetyl CoA, to form long hydrocarbon chains in a series of reactions. These reactions are carried out on the fatty acid synthase complex using NADPH as reductant. The fatty acids are covalently linked to acyl carrier protein (ACP) during their synthesis. 

The pathway The first committed step in fatty acid biosynthesis is the carboxylation of acetyl CoA to form malonyl CoA which is catalyzed by the biotin-containing enzyme acetyl CoA carboxylase. Acetyl CoA and malonyl CoA are then converted into their ACP derivatives. The elongation cycle in fatty acid synthesis involves four reactions condensation of acetyl-ACP and malonyl-ACP to form acetoacetyl-ACP releasing free ACP and C02, then reduction by NADPH to form D-3-hydroxybutyryl-ACP, followed by dehydration to crotonyl-ACP, and finally reduction by NADPH to form butyryl-ACP. Further rounds of elongation add more two-carbon units from malonyl-ACP on to the growing hydrocarbon chain, until the C16 palmitate is formed. Further elongation of fatty acids takes place on the cytosolic surface of the smooth endoplasmic reticulum (SER). 

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. 

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