Oxidation spiral

Each turn of the P-oxidation spiral splits off a molecule of acetyl-CoA. The process involves four enzymes catalysing, in turn, an oxidation (to form a double bond), a hydration, another oxidation (forming a ketone from a secondary alcohol) and the transfer of an acetyl group to coenzyme A (Figure 7.12). The process of P-oxidation operates as a multienzyme complex in which the intermediates are passed from one enzyme to the next, i.e. there are no free intermediates. The number of molecules of ATP generated from the oxidation of one molecule of the long-chain fatty acid pal-mitate (C18) is given in Table 7.4. Unsaturated fatty acids are also oxidised by the P-oxidation process but require modification before they enter the process (Appendix 7.3). 

When fatty acids are the fuel being combusted, the pathway of oxidation is termed the /1-oxidation spiral, which, following fatty acylCoA formation, involves four basic steps ... 

Figure 2.7. The complex pathways and processes involved in fat catabolism in vertebrate tissues such as cardiac and skeletal muscles. FFAs arrive at the cell boundary either via VLDL or albumin-associated and enter the cell either by simple diffusion or through transporters. In the cytosol, FFAs are bound by FABPs, which increase the rate and amount of FFA that can be transferred to sites of utilization. Shorter chain FFAs are converted to acetylCoA in peroxisomes longer chain FFAs are directly transferred to mitochondria (via a complex system involving acylcarnitines) as long-chain acylCoA derivatives these enter the /6-oxidation spiral and are released as acetylCoA for entrance into the Krebs or citric acid cycle in the mitochondrial matrix. Fatty acid receptors (FARs) in the nucleus bind to fatty acid response elements (FAREs) and in turn regulate the production of enzymes in their own metabolism. (Modified from Veerkamp and Maatman, 1995.)...

E) Two acetyl CoA molecules are produced in each turn of the p-oxidation spiral... 

The four steps just outlined constitute one cycle of /3-oxidation. During each later cycle, a two-carbon fragment is removed. This process, sometimes called the 13-oxidation spiral, continues until, in the last cycle, a four-carbon acyl-CoA is cleaved to form two molecules of acetyl-CoA. 

In the first of four reactions that constimte one cycle of the P-oxidation spiral, acyl-CoA is dehydrogenated to 2-tra/w-enoyl-CoA according to the following equation. 

The oxidation of fatty acids to acetyl CoA in the p-oxidation spiral conserves energy as FAD(2H) and NADH. FAD(2H) and NADH are oxidized in the electron transport chain, generating ATP from oxidative phosphorylation. Acetyl CoA is oxidized in the TCA cycle or converted to ketone bodies. 

Like the FAD in all flavoproteins, FAD(2H) bound to the acyl CoA dehydrogenases is oxidized back to FAD without dissociating from the protein (Fig. 23.8). Electron transfer flavoproteins (RTF) in the mitochondrial matrix accept electrons from the enzyme-bound FAD(2H) and transfer these electrons to ETF-QO (electron transfer flavoprotein -CoQ oxidoreductase) in the inner mitochondrial membrane. ETF-QO, also a flavoprotein, transfers the electrons to CoQ in the electron transport chain. Oxidative phosphorylation thus generates approximately 1.5 ATP for each FAD(2H) produced in the (3-oxidation spiral. 

The p-oxidation spiral uses the same reaction types seen in the TCA cycle when succinate is converted to oxaloacetate. 

Palmitic acid is 16 carbons long, with no double bonds, so it requires 7 oxidation spirals to be completely converted to acetyl-CoA. After 7 spirals, there are 7 FAD(2H), 7 NADH, and 8 acetyl-CoA. Each NADH yields 2.5 ATP, each FAD(2H) yields 1.5 ATP, and each acetyl-CoA yields 10 ATP as it is processed around the TCA cycle. This then yields 17.5 + 10.5 + 80.5 = 108 ATP. However, activation of palmitic acid to palmityl-CoA requires two high-energy bonds, so the net yield is 108 - 2, or 106 moles of ATP. 

Approximately one half of the fatty acids in the human diet are unsaturated, containing cis double bonds, with oleate (C18 1, A ) and linoleate (18 2,A ) being the most common. In 3-oxidation of saturated fatty acids, a trans double bond is created between the 2nd and 3rd (a and 3) carbons. For unsaturated fatty acids to undergo the 3-oxidation spiral, their cis double bonds must be isomerized to trans double bonds that will end up between the 2nd and 3rd carbons during 3-oxidation, or the double bond must be reduced. The process is illustrated for the polyunsaturated fatty acid hnoleate in Fig. 23.9. Linoleate undergoes 3-oxidation until one double bond is between carbons 3 and 4 near the carboxyl end of the fatty acyl chain, and the other is between carbons 6 and 7. An isomerase moves the double bond from the 3,4 position so that it is trans and in the 2,3 position, and 3-oxida-tion continues. When a conjugated pair of double bonds is formed (two double bonds separated by one single bond) at positions 2 and 4, an NADPH-dependent reductase reduces the pair to one trans double bond at position 3. Then isomerization and 3-oxidation resume. 

Dietary medium-chain-length fatty acids are more water soluble than long-chain fatty acids and are not stored in adipose triacylglyce. After a meal, they enter the blood and pass into the portal vein to the liver. In the liver, they enter the mitochondrial matrix by the monocarboxylate transporter and are activated to acyl CoA derivatives in the mitochondrial matrix (see Fig. 23.1). Medium-chain-length acyl CoAs, like long-chain acyl CoAs, are oxidized to acetyl CoA via the (3-oxidation spiral. Medium-chain acyl CoAs also can arise from the peroxisomal oxidation pathway. 

P-carbon during the p-oxidation spiral, and can no longer interfere with oxidation of the p-carbon. Peroxisomal p-oxidation thus can proceed normally, releasing pro-pionyl CoA and acetyl CoA with alternate turns of the spiral. When a medium chain length of approximately eight carbons is reached, the fatty acid is transferred to the mitochondrion as a carnitine derivative, and p-oxidation is resumed. 

Within the liver, they bind to fatty acid-binding proteins and are then activated on the outer mitochondrial membrane, the peroxisomal membrane, and the smooth endoplasmic reticulum by fatty acyl CoA synthetases. The fatty acyl group is transferred from CoA to carnitine for transport through the inner mitochondrial membrane, where it is reconverted back into fatty acyl CoA and oxidized to acetyl CoA in the (3-oxidation spiral (see Chapter 23). 

Each turn of the j8-oxidation spiral involves which of the following ... 

If it arises from pyruvate (Topic 14), the formation of acetyl CoA is pushed by the energetically favourable oxidative decarboxylation reaction. If, on the other hand, one starts from acetate, or similarly from a long-chain fatty acid waiting to enter the fatty oxidation spiral, the formation of the acyl CoA involves an activation reaction in which ATP is split. 

One property of the translocase is specially noteworthy its affinity for long-chain acylcamitine is very much higher than for carnitine itself. This suggests a basic role for carnitine, additional to the acetylation buffer already mentioned. Every fourth reaction of the P-oxidation spiral (the excision of an acetyl unit by a thiolase enz5mie) requires free CoASH, so that if the CoA of the mitochondrial matrix became over-acy-lated the whole process would come to a halt. If acyl-CoA were formed directly from fatty acids in the same compartment (the matrix) as oxidation this might readily happen. However, the carnitine system guards against this fail-nonsafe situation, because if the acyl-CoA/CoASH ratio (and hence the acylcamitine/camitine ratio) rises the translocase will selectively export acylcamitine from the matrix and import carnitine this will lower the ratio and restore P-oxidation. It remains (I think ) to be seen if this effect can be conclusively demonstrated. 

Although the fatty acid p-oxidation spiral comprises only four reactions, imder-standing of the complexity of fatty acid degradation has dramatically increased in recent years due to the discovery of a variety of new P-oxidation enzymes. This article will discuss the enzymes that catalyze the second and third steps of the P-oxidation pathway, an area of recent and substantial progress. 

The second step of the P-oxidation spiral is the reversible hydration of 2-trans-enoyl-CoA to yield L-3-hydroxyacyl-CoA, catalyzed by enoyl-CoA hydratase. However, in fungi 2-tra 5-enoyl-CoA is hydrated by peroxisomal D-3-hydroxyacyl-CoA dehydratase to form D-3-hydroxyacyl-CoA. Enoyl-CoA hydratases are usually associated with the N-terminal region of multifunctional proteins except for the mitochondrial matrix enoyl-CoA hydratase and the E. coli long-chain enoyl-CoA hydratase (see Table 1). D-3-hydioxyacyl-CoA dehydratases are located on the C-terminal domain of the peroxisomal D-specific bifunctional P-oxidation enzyme or the central domain of 1 P-hydroxysteroid dehydrogenase type IV. ... 

---