Fatty-acyl-CoA

FIGURE 21.V The fatty acyl-CoA dehydrogenase reaction, emphasizing that the reaction involves reduction of enzyme-bonnd FAD (indicated by brackets). 

This is a crucial point because (as we will see) proton transport is coupled with ATP synthesis. Oxidation of one FADHg in the electron transport chain results in synthesis of approximately two molecules of ATP, compared with the approximately three ATPs produced by the oxidation of one NADH. Other enzymes can also supply electrons to UQ, including mitochondrial 5w-glyc-erophosphate dehydrogenase, an inner membrane-bound shuttle enzyme, and the fatty acyl-CoA dehydrogenases, three soluble matrix enzymes involved in fatty acid oxidation (Figure 21.7 also see Chapter 24). The path of electrons from succinate to UQ is shown in Figure 21.8. 

FIGURE 24.8 The mechanism of the acyl-CoA synthetase reaction involves fatty acid carboxylate attack on ATP to form an acyl-adenylate intermediate. The fatty acyl CoA thioester product is formed by CoA attack on this intermediate. 

All of the other enzymes of the /3-oxidation pathway are located in the mitochondrial matrix. Short-chain fatty acids, as already mentioned, are transported into the matrix as free acids and form the acyl-CoA derivatives there. However, long-chain fatty acyl-CoA derivatives cannot be transported into the matrix directly. These long-chain derivatives must first be converted to acylearnitine derivatives, as shown in Figure 24.9. Carnitine acyltransferase I, located on the outer side of the inner mitochondrial membrane, catalyzes the formation of... 

In essence, this series of four reactions has yielded a fatty acid (as a CoA ester) that has been shortened by two carbons, and one molecule of acetyl-CoA. The shortened fatty acyl-CoA can now go through another /3-oxidation cycle, as shown in Figure 24.10. Repetition of this cycle with a fatty acid with an even number of carbons eventually yields two molecules of acetyl-CoA in the final step. As noted in the first reaction in Table 24.2, complete /3-oxidation of palmitic acid yields eight molecules of acetyl-CoA as well as seven molecules of FADHg and seven molecules of NADFI. The acetyl-CoA can be further metabolized in the TCA cycle (as we have already seen). Alternatively, acetyl-CoA can also be used as a substrate in amino acid biosynthesis (Chapter 26). As noted in Chapter 23, however, acetyl-CoA cannot be used as a substrate for gluco-neogenesis. 

Dephospho-acetyl-CoA carboxylase (Low [citrate] activates, high [fatty acyl-CoA] inhibits)... 

FIGURE 25.5 The activity of acetyl-CoA carboxylase is modulated by phosphorylation and dephosphoryladon. The dephospho form of the enzyme is activated by low [citrate] and inhibited only by high levels of fatty acyl-CoA. 

FIGURE 25.12 Elongation of fatty acids in mitochondria is initiated by the thiolase reaction. The /3-ketoacyl intermediate thus formed undergoes the same three reactions (in reverse order) that are the basis of /3-oxidation of fatty acids. Reduction of the /3-keto group is followed by dehydration to form a double bond. Reduction of the double bond yields a fatty acyl-CoA that is elongated by two carbons. Note that the reducing coenzyme for the second step is NADH, whereas the reductant for the fourth step is NADPH. 

FIGURE 25.16 Regulation of fatty acid synthesis and fatty acid oxidation are conpled as shown. Malonyl-CoA, produced during fatty acid synthesis, inhibits the uptake of fatty acylcarnitine (and thus fatty acid oxidation) by mitochondria. When fatty acyl CoA levels rise, fatty acid synthesis is inhibited and fatty acid oxidation activity increases. Rising citrate levels (which reflect an abundance of acetyl-CoA) similarly signal the initiation of fatty acid synthesis. 

FIGURE 25.20 Triacylglycerols are formed primarily by the action of acyltransferases on mono- and diacylglycerol. Acyltransferase in E. coli is an integral membrane protein (83 kD) and can utilize either fatty acyl-CoAs or acylated acyl carrier proteins as substrates. It shows a particular preference for palmitoyl groups. Eukaryotic acyltransferases nse only fatty acyl-CoA molecnles as substrates. 

Write a balanced, stoichiometric reaction for the synthesis of phosphatidylethanolamine from glycerol, fatty acyl-CoA, and ethanolamine. Make an estimate of the AG° for the overall process. 

Step 1 of Figure 29.3 Introduction of a Double Bond The /3-oxidation pathway begins when a fait)7 acid forms a thioester with coenzyme A to give a fatty acyl Co A. Two hydrogen atoms are then removed from C2 and C3 of the fatty acyl CoA by one of a family of acyl-CoA dehydrogenases to yield an a,/3-unsaturated acyl CoA. This kind of oxidation—the introduction of a conjugated double bond into a carbonyl compound—occurs frequently jn biochemical pathways and usually involves the coenzyme flavin adenine dinucleotide (FAD). Reduced FADH2 is the by-product. 

Q Nucleophilic addition of coenzyme A to the keto group occurs, followed by a retro-Claisen condensation reaction. The products are acetyl CoA and a chain-shortened fatty acyl CoA. 

This pathway (the microsomal system ) elongates saturated and unsaturated fatty acyl-CoAs (from Cjg upward) by two carbons, using malonyl-CoA as acetyl donor and NADPH as reductant, and is catalyzed by the microsomal fatty acid elongase system of enzymes (Figure 21-5). Elongation of stearyl-CoA in brain increases rapidly during myehnation in order to provide C22 and C24 fatty acids for sphingoEpids. 

Reaction 3 is analogous to the dehydrogenation of fatty acyl-CoA thioesters (see Figure 22—3). In isovaleric acidemia, ingestion of protein-rich foods elevates isovalerate, the deacylation product of isovaleryl-CoA. Figures 30-20, 30-21, and 30-22 illustrate the subsequent reactions unique to each amino acid skeleton. 

Once fatty acids have been made 16 carbons long, they can be lengthened by adding 2 carbon atoms at a time with malonyl-CoA in a reaction that looks a lot like the first step of fatty acid synthesis. However, the elongation reaction is carried out on the fatty acyl-CoA and by an enzyme that is different from fatty acid synthase.4 ... 

Notice that fatty acid synthesis makes the fatty acid rather than the fatty acyl-CoA as a prod-... 

Fatty acyl-CoA in, acetyl-CoA and NADH, FADH2 out From triglycerides through hormone-sensitive lipase... 

A somewhat newer pathway, which may be the real pathway in many cells, has been discovered recently. This may or may not be described in your text. In this pathway, the fatty acyl-CoA is metabolized... 

Lynen had studied chemistry in Munich under Wieland his skill as a chemist led to the successful synthesis of a number of fatty acyl CoA derivatives which proved to be substrates in the catabolic pathway. Many of these C=0 or C=C compounds had characteristic UV absorption spectra so that enzyme reactions utilizing them could be followed spectrophotometrically. This technique was also used to identify and monitor the flavoprotein and pyridine nucleotide-dependent steps. Independent evidence for the pathway was provided by Barker, Stadtman and their colleagues using Clostridium kluyveri. Once the outline of the degradation had been proposed the individual steps of the reactions were analyzed very rapidly by Lynen, Green, and Ochoa s groups using in the main acetone-dried powders from mitochondria, which, when extracted with dilute salt solutions, contained all the enzymes of the fatty acid oxidation system. 

The desaturation process is particularly interesting as it provides an example of a microsomal (as opposed to mitochondrial) electron transport system. The enzymes responsible, fatty acyl-CoA desaturases, are examples of mixed function oxidases... 

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