J3-oxidation pathway

STEPS 7-8 Regeneration of oxaloacetate. Catalyzed by the enzyme fumarase. conjugate miclcophilic addition of w ater to fumarate yields t-malate in a reaction simUar to tliat of step 2 in the fatty acid j3>oxidation pathway. Oxida-i m with NAD then, gives oxaloacetate in a step catalyzed by malate dehydrogenase, and the citric acid cycle has return to ita starting point, ready to revolve again. 

Write the equations for the remaining passages of the j3-oxidation pathway following those shown in Figure 29.3. 

Hydration and isomerization. Conjugate addition of water to the double bond of phosphoenolpyruvate takes place in a process similar to that of step 2 in the j3-oxidation pathway (Figure 29.2). Isomerization then occurs by transfer of a phosphate group from C2 to C3, 5helding 3-phospho-glycerate. 

Valproic acid is rapidly distributed and the plasma protein binding is concentration dependent (18). As previously noted, valproic acid is extensively metabolized, primarily in the liver, with about 30-50% of the drug excreted as the glucuronide (phase II metabolism) in the urine, about 30-40%by the phase I mitochondrial j3-oxidation pathway, and about 10-20% by microsomal cytochrome P450-mediated hy droxylation/dehydrogena-tion of the side chain that provides the major phase I metabolites (36). The metabolites of valproic acid have been thought to be the cause of a rare, but fatal hepatotoxicity (35). The synthetic ( )-2,4-diene VPA has been shown to induce the same hepatic microve-sicular steatosis seen in patients, in chronic administration studies in rats (36). The ultimate causative factor (s) of hepatoxicity of valproic acid currently remain undefined (28,29). 

Oxidation of TritiatedPalmitate Palmitate uniformly labeled with tritium ( H) to a specific activity of 2.48 X 10 counts per minute (cpm) per micromole of palmitate is added to a mitochondrial preparation that oxidizes it to acetyl-CoA. The acetyl-CoA is isolated and hydrolyzed to acetate. The specific activity of the isolated acetate is 1.00 X 10 cpm//j,mol. Is this result consistent with the j3-oxidation pathway Explain. What is the final fate of the removed tritium ... 

Figure 18.18 J3-Oxidation pathway for polyunsaturated fatty acids. 

Step D of Figure 29.3 Introduction of a Double Bond The j3-oxidation pathway begins when two hydrogen atoms are removed from C2 and C3 of the fatty acyl CoA by one of a family of acyl-CoA dehydrogenases to yield an a,j8-imsaturated acyl CoA. This kind of oxidation—the introduction of a conjugated double bond into a carbonyl compound—occurs frequently in biochemical pathways and usually involves the coenzyme flavin adenine dinucleotide (FAD). Reduced FADH2 is the by-product. 

It is interesting that a compound such as palmitic acid can be metabolized by both types of hydroxylation reaction. At the /5 position, it is oxidized by successive dehydrogenation-hydration-dehydrogenation steps which constitute the well-known j3-oxidation pathway to CO2 and water. In the middle of the hydrocarbon chain, however, far from the activating influence of the carboxylate group, it is apparently attacked by an aerobic hydroxylation type of reaction, a step which eventually leads to its conversion to a olefinic acid (Bloomfield and Bloch. 1958). 

Most fatty acids have an even number of carbon atoms, so none are left over after /3-oxidation. Those fatty acids with an odd number of carbon atoms yield the three-carbon propionyl CoA in the final j3-oxidation. Propionyl CoA is then converted to succinate by a multistep radical pathway, and succinate enters the citric acid cycle (Section 29.7). Note that the three-carbon propionyl group should properly be called propnnoyl, but biochemists generally use the non-systematic name. 

Oxidation of fatty acids consumes a precious fuel, and it is regulated so as to occur only when the need for energy requires it. In the liver, fatty acyl-CoA formed in the cytosol has two major pathways open to it (1) J3 oxidation by enzymes in mitochondria or (2) conversion into triacylglycerols and phospholipids by enzymes in the cytosol. The pathway taken depends on the rate of... 

Peroxisomes of plants and animals, and glyoxysomes of plants, carry out j3 oxidation in four steps similar to those of the mitochondrial pathway in animals. The first oxidation step, however, transfers electrons directly to O2, generating H202. Peroxisomes of animal tissues... 

The pheromone biosynthetic pathway of T. ni is well defined and is relatively simple compared to those of many other moth species (5) (Figure 2). The initial substrate is the common 16-carbon saturated fatty acid thioester of Coenzyme A (palmitoylrCoA), which is derived from the combined actions of acetyl-CoA carboxylase and fatty acid synthase. An acyl-CoA All desaturase acts upon palmitoykCoA to produce a Z double bond between carbon atoms 11 and 12 (Zll-16 CoA). The multienzyme -oxidation complex subsequently acts on this compound in two successive rounds of j3-oxidation to produce Z9-14 CoA followed by Z7-12 CoA. The active component of the T. ni pheromone, Z7-12 OAc, results from the sequential action on Z7-12 CoA of a reductase and an acetyltransferase. A minor pheromone component, Z5- 12 OAc is produced by the same enzymatic steps as... 

A convenient way to summarize the reactions of coenzyme A thioesters is by reviewing the )3-oxidation pathway for fatty adds". Fatty acid activation occurs by acylation of the coenzyme A thiol by way of an acyl adenylate. This is then dehydrogenated to an o,/3-enoyl acyl coenzyme A derivative by a flavin-dependent dehydrogenase. The ability of the adjacent carbonyl to provide resonance stabilization of the product appears to be an important aspect of this reaction. Such flavin-dependent dehydro- nations occur in other reaction sequences, but only where carbonyl resonance stabilization is possible. Water adds to the a,j8-enoyl thioester to generate a j8-hydroxy fatty acid derivative, a reaction facilitated by j8-carbonium ion stabilization in enoyl thioesters. The j3-hydroxyl is next... 

An efficient synthetic route to (10Z)- and (10 )-19-lluoro-la,25-dihydroxy vitamin D3 has been developed (488). The key feature of this pathway is the introduction of a 19-fluoromethylene group to a (5 )-19-nor-10-oxo-vitamin D derivative. The 10-oxo compound 445 has been obtained via a 1,3-dipolar cycloaddition reaction of (5 )-la,25-dihydroxyvitamin D with in situ generated nitrile oxide, followed by ring cleavage of the formed isoxazoline moiety with molybdenum hexacarbonyl. Conversion of the keto group of (5 )-19-nor-10-oxo-vitamin D to the E and Z fluoromethylene group has been achieved via a two-step sequence, involving a reaction of lithiofluoromethyl phenyl sulfone, followed by the reductive de-sulfonylation of the u-lluoro-j3-hydroxysulfone. The dye-sensitized photoisomerization of the (5 )-19-fluorovitamin D affords the desired (5Z)-19-fluorovitamin D derivatives, (10Z)- and (10 )-19-fluoro-la,25-dihydroxy-vitamin D3. 

By known pathways of metabolism, the j3-carbon of pyruvate forms CO2 only under conditions in which the entire moiety is oxidized to C02 and water. Therefore, by means of the Embden-Meyerhof scheme, the proportion of Ci in total CO2 should never exceed the proportion of Ci in glucose carbon or 1 6. However, the decarboxylation of 6-phosphoglu-conate must lead to a selective appearance of Ci in the total CO2. Under conditions in which this pathway is very active and some carbon is conserved, as in growth or other synthesis, this use of the pathway will be revealed by a higher proportion of Ci in total CO2 than its proportion in the initial carbohydrate carbon. 

The biosynthesis of thebaine in P. bracteatum follows the same pathway as in the opium poppy (Hodges et al. 1977, Brochmann-Hanssen and Wunderly 1978). In contrast to earlier studies, recent isolation work has revealed thatP. bracteatum contains small amounts of codeine, neopine and oripavine as well as 14-j8-hydroxycodeinone (16) and 14-j3-hydroxycodeine (17) and both isomers of thebaine N-oxide (Kiippers et al. 1976, Phillipson et al. 1976, Theuns et al. 1977, Meshulam and Lavie 1980). It appears that the 0-methyl oxidases in P. bracteatum are extremely sluggish in comparison with the corresponding enzymes in P. somniferum. This leads to accumul-... 

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