Trans- A-2-enoyl-CoA

Acyl CoA + E-FAD trans- A 2 -enoyl CoA + E- Acyl CoA dehydrogenases (several isozymes having different... 

Consider the oxidation of palmitoleate. This Cjg unsaturated fatty acid, which has one double bond between C-9 and C-10, is activated and transported across the inner mitochondrial membrane in the same way as saturated fatty acids. Palmitoleoyl CoA then undergoes three cycles of degradation, which are carried out by the same enzymes as in the oxidation of saturated fatty acids. However, the cis-A 3-enoyl CoA formed in the third round is not a substrate for acyl CoA dehydrogenase. The presence of a double bond between C-3 and C-4 prevents the formation of another double bond between C-2 and C-3. This impasse is resolved by a new reaction that shifts the position and configuration of the cis-A double bond. An isomerase converts this double bond into a trans- A double bond. The subsequent reactions are those of the saturated fatty acid oxidation pathway, in which the trans- A 2-enoyl CoA is a regular substrate. 

Elegant inversion. Peroxisomes have an alternative pathway for oxidizing polyunsaturated fatty acids. They contain a hydratase that converts d-3-hydroxyacyl CoA into trans-A. 2-enoyl CoA. How can this enzyme be used to oxidize CoAs containing a cis double bond at an even-numbered carbon atom (e.g., the cis-A double bond of linoleate) ... 

In the fifth round of P oxidation, cw-A 2-enoyl CoA is formed. Dehydration by the classic hydratase yields d-3-hydroxyacyl CoA, the wrong isomer for the next enzyme in P oxidation. This dead end is circumvented by a second hydratase that removes water to give trans-A 2-enoyl CoA. The addition of water by the classic hydratase then yields 1-3- hydroxyacyl CoA, the appropriate isomer. Thus, hydratases of opposite stereospecificities serve to epimerize (invert the configuration of) the 3-hydroxyl group of the acyl CoA intermediate. 

Oxidation of unsaturated fatty acids The oxidation of unsatu rated fatty acids provides less energy than that of saturated fatly I acids because they are less highly reduced and, therefore, tom I reducing equivalents can be produced from these structured Oxidation of monounsaturated fatty acids, such as 18 1(9) (deb acid) requires one additional enzyme, 3,2-enoyl CoA isomeras which converts the 3-cis derivative obtained after three rounds (3-oxidation to the 2-trans derivative that can serve as a substra for the hydratase. Oxidation of polyunsaturated fatty acids, sue ... 

Polyunsaturated fatty acids are also degraded by [3 oxidation, but the process requires enoyl-CoA isomerase and an additional enzyme, 2,4-dienoyl-CoA reductase (fig. 18.6). For example, the degradation of linoleoyl-CoA (18 2A9-12) begins, like that of oleoyl-CoA, with three rounds of /3 oxidation and results in a A3-cis unsaturated fatty acyl-CoA that is not a substrate for acyl-CoA dehydrogenase. Isomerization of the double bond to the A2-trans position by enoyl-CoA isomerase allows the resumption of... 

Hoffmeister M, Piotrowski M, Nowitzki U, Martin W (2005) Mitochondrial trans-2-enoyl-CoA reductase of wax ester fermentation from Euglena gracilis defines a new family of enzymes involved in lipid synthesis. J Biol Chem 280 4329-4338... 

The hydration of enoyl CoA is stereospecific. Only the 1 isomer of 3-hydroxyacyl CoA is formed when the trans-A 2 double bond is hydrated. The enzyme also hydrates a cis-A 2 double bond, but the product then is the d isomer. We shall return to this point shortly in considering how unsaturated fatty acids are oxidized. 

Enoyl-CoA hydratase catalyses the hydration of unsaturated acyl-CoA. This enzyme has broad specificity and can act on a-, P- (or A -) unsaturated CoA in trans or cis configuration. The products formed are... 

Fig. 8. P-Oxidation of fatty acids in E. coli. Long-chain fatty acids are transported into the cell by FadL and converted to their CoA thioesters by FadD (not shown). The acyl-CoAs are substrates for the (1) acyl-CoA dehydrogenase (YafH) to form a trans-2-enoyl-CoA. The double bond is reduced by (2) rrans-2-enoyl-hydratase (crotonase) activity of FadB. The P-hydroxyacyl-CoA is then a substrate for the NADP -dependent dehydrogenase activity of FadB (3). A thiolase, FadA (4), releases acetyl-CoA from the P-ketoacyl-CoA to form an acyl-CoA for subsequent cycles. (5) Polyunsaturated fatty acyl-CoAs are reduced by the 2,4-dienoyl-CoA reductase (FadH). (6) FadB also catalyzes the isomerization of c/s-unsaturated fatty acids to trans. (7) The epimerase activity of FadB converts O-P-hydroxy thioesters to their L-enantiomers via the /rans-2-enoyl-CoA.

The discovery of A -A -enoyl-CoA isomerase gave rise to the proposal that metabolites with odd-numbered double bonds underwent only a Z-cis to 2-trans isomerization before re-entering the p-oxidation spiral (Fig. 2). However, an additional pathway that reduces the double bond in an NADPH-dependent maimer was recently described by Tsemg and Jin." The starting metabolite in this reaction sequence is trans-2-cis-5-dxtmy -CoA, which can either complete the P-oxidation cycle to yield cw-3-enoyl-CoA or can be converted to tran -3,c -5-dienoyl-CoAbyA A -enoyl-CoA isomerase. A novel enzyme, a3,5 A2.<.dienoyl-CoA isomerases then catalyzes the shift of both double bonds to pro ce a trans-2,A-trans-ditnoy -CoP which is a substrate of 2,4-dienoyl-CoA reductase and, hence, can be reduced to tram--3-enoyl-CoA. This is then converted to rrans-2-enoyl-CoA by A A -enoyl-CoA isomerase (Fig. 2). 

Polyunsaturated fatty acids pose a slightly more complicated situation for the cell. Consider, for example, the case of linoleic acid shown in Figure 24.24. As with oleic acid, /3-oxidation proceeds through three cycles, and enoyl-CoA isomerase converts the cA-A double bond to a trans-b double bond to permit one more round of /3-oxidation. What results this time, however, is a cA-A enoyl-CoA, which is converted normally by acyl-CoA dehydrogenase to a trans-b, cis-b species. This, however, is a poor substrate for the enoyl-CoA hydratase. This problem is solved by 2,4-dienoyl-CoA reductase, the product of which depends on the organism. The mammalian form of this enzyme produces a trans-b enoyl product, as shown in Figure 24.24, which can be converted by an enoyl-CoA isomerase to the trans-b enoyl-CoA, which can then proceed normally through the /3-oxidation pathway. Escherichia coli possesses a... 

FIGURE 24.24 The oxidation pathway for polyunsaturated fatty adds, illustrated for linoleic add. Three cycles of /3-oxidation on linoleoyl-CoA yield the cis-A, d.s-A intermediate, which is converted to a tran.s-A, cis-A intermediate. An additional round of /S-oxi-dation gives d.s-A enoyl-CoA, which is oxidized to the trans-A, d.s-A species by acyl-CoA dehydrogenase. The subsequent action of 2,4-dienoyl-CoA reductase yields the trans-A product, which is converted by enoyl-CoA isomerase to the tran.s-A form. Normal /S-oxida-tion then produces five molecules of acetyl-CoA. 

Unsaturated fatty acids usually contain a cis double bond at position 9 or 12—e.g., linoleic acid (18 2 9,12). As with saturated fatty acids, degradation in this case occurs via p-oxida-tion until the C-9-ds double bond is reached. Since enoyl-CoA hydratase only accepts substrates with trans double bonds, the corresponding enoyl-CoA is converted by an iso-merase from the ds-A, cis- A isomer into the trans-A, cis-A isomer [1]. Degradation by p-oxidation can now continue until a shortened trans-A, ds-A derivative occurs in the next cycle. This cannot be isomerized in the same way as before, and instead is reduced in an NADPH-dependent way to the trans-A compound [2]. After rearrangement by enoyl-CoA isomerase [1 ], degradation can finally be completed via normal p-oxidation. 

Another enzyme, in addition to the isomerase, is required for the oxidation of polyunsaturated fatty acids which have a double bond at an even-numbered carbon atom. In this case the 2,4-dienoyl intermediate resulting from the action of acyl CoA dehydrogenase is acted on by 2,4-dienoyl CoA reductase to form c/s-A3-enoyl CoA (Fig. 4). This is then converted by the isomerase into the trans form which continues down the pathway. These reactions are important since over half the fatty acids of plant and animal lipids are unsaturated (and often polyunsaturated). 

After one further -oxidation cycle, a 4-cis-enoyl CoA intermediate is formed. It is acted upon by enoyl-CoA dehydrogenase to give 2-trans, 4-cis-dienoyl CoA. Further metabolism of this intermediate proceeds through one cycle of /3-oxidation and requires a second auxiliary enzyme, 2,4-dienoyl-CoA reductase which has high activity in mitochondria. Thus, nine molecules of acetyl-CoA are produced from the oxidation of linoleic acid. 

The first reaction in each round of degradation is the oxidation of acyl CoA by an acyl CoA dehydrogenase to give an enoyl CoA with a trans double bond between C-2 and C-3. 

Two new enzymes are required to handle these situations Enoyl-CoA isomerase (isomerizes a cis-3,4-double bond to a frflns-2,3-double bond), and 2,4-Dienoyl-CoA reductase (reduces the ds-4,5-double bond in the trans-2,3-ds-4,5-dienoyl-CoA derivative formed during feta-oxidation). The resulting products are then broken down by the feta-oxidation enzymes. 

Glutaric acidemia type II is caused by defects in the ETF/ETF-QO proteins. The clinical manifestations of these disorders are similar to medium-chain acyl-CoA dehydrogenase deficiency (discussed later). The double bond formed by the acyl-CoA dehydrogenase has a trans configuration. The double bonds in naturally occurring fatty acids are generally in the cis configuration. The oxidation of unsaturated m-fatty acids requires two auxiliary enzymes, enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase. 

Oxidation of unsaturated fatty acids requires A -cis-,A -trans-enoyl-CoA isomerase and NADPH-dependent 2,4-dienoyl-CoA reductase, in addition to the enzymes of y3-oxidation. The enoyl-CoA isomerase produces the substrate for the hydration step. The reductase catalyzes the reduction of A -frans-,A -cjs-decadienoyl-CoA to A -rrans-decenoyl-CoA. The latter is isomerized to A -trans-decenoyl isomerase, which is a normal -oxidation intermediate. These reactions are illustrated for oxidation of oleic and linoleic acids in Figures 18-7 and 18-8. 

Another problem arises with the oxidation of polyunsaturated fatty acids. Consider linoleate, a Cx polyunsaturated fatty acid with cis-A and cis-A double bonds (Figure 22.11). The cis-A double bond formed after three rounds of P oxidation is converted into a trans-A" double bond by the aforementioned isomerase. The acyl (ioA produced by another round of P oxidation contains a cis-A double bond. Dehydrogenation of this species by acy] CoA dehydrogenase yields a. 2,4-dienoyl intermediate, which is not a substrate for the next enzyme in the p-oxidation pathway. This impasse is circumvented by 2,4-dienoyl CoA reductase, an enzyme that uses to reduce the 2,4-dienoyl intermediate to trans-A -enoyl CoA. ds-A Enoyl CoA isomerase then converts trans-A -enoyl CoA into the trans-A form, a customary intermediate in the P-oxidation pathway. These catalytic... 

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