Fatty acids P-oxidation pathway

In the liver, there is an alternative mechanism for regenerating CoA from acetyl-CoA two acetyl-CoA molecules react to form a 4-carbon compound called a ketone body. (Naming a class of molecules bodies is a quaint hangover from the past, but in reality these compounds embody the molecule of acetyl-CoA.) The enzyme in the hepatocyte that catalyzes the relevant reaction is hydroxymethylglutaryl CoA synthase. The consequent release of CoA from acetyl-CoA enables P-oxidation to continue rapidly. The ketone body that is formed first is acetoacetate, it is reduced to -hydroxybutyrate by NADH and the oxidation of NADH further stimulates flux via the fatty acid P-oxidation pathway. Recall that P-oxidation requires regeneration of NAD+ (Sec. 10.5). 

Compared with controls, 22 6n-3 biosynthesis was normal in cells from patients with deficiencies of the mitochondrial fatty acid p-oxidation enzymes, vay-long-chain acyl-CoA dehydrogenase (VLCAD) or medium-chain acyl-Co A dehydrogenase (MCAD) (Table 2). These findings confirmed that retro-conversion of 24 6n-3 to 22 6n-3 is via the peroxisomal, but not mitochondrial fatty acid p-oxidation pathway. 

Production of MCE PHAs was first investigated in recombinant E. coli by Langenbach et al (1997). A number of strategies have been developed to improve MCL PHA productivity by providing PHA precursors from the fatty acid P-oxidation pathway (Fig. 3 Park et al. 2004). The P-oxidation pathway has been engineered by the overexpression of enoyl-CoA hydratase (Fiedler et al. 2002 Fukui and Doi 1998) or 3-ketoacyl-ACP reductase (Park et al. 2002 Ren et al. 2000 ... 

Abstract Synthesis of polyhydroxyalkanoates (PHAs) in crop plants is viewed as an attractive approach for the production of this family of biodegradable plastics in large qnantities and at low costs. Synthesis of PHAs containing various monomers has so far been demonstrated in the cytosol, plastids, and peroxisomes of plants. Several biochemical pathways have been modified to achieve this, including the isoprenoid pathway, the fatty acid biosynthetic pathway, and the fatty acid P-oxidation pathway. PHA synthesis has been demonstrated in a number of plants, including monocots and dicots, and np to 40% PHA per gram dry weight has been demonstrated in Arabidopsis thaliana. Despite some successes, production of... 

Figure 2. Schematic diagram of the second reaction of the fatty acid P-oxidation pathway catalyzed by co/i enoyl-CoA hydratase. Compounds 1 and 2 are 2-mmr-enoyl-CoA and L-3-hydroxyacyl-CoA, respectively. The protonated Glu transfers a proton to the a carbon of the substrate on the re face, whereas the deprotonated Glu" attracts a proton from water whose oxygen makes a nudeopbilic attack on the P-carbon of the substrate. The amino group of Gly" in the peptide backbone acts as a l rogen donor to form a hydrogen bond with the carbonyl oxygen so that an electronic rearrangement occurs in the acryloyl portion of the sul trate. The transition state is shown in the square brackets The product, L-3-hydroxyacyl-CoA, can then leave the active site. Two general acid-base functional groups, the ycarboxyl groups of Glu and Glu , play a major part in the hydratase catalysis...

Gulevich, A., Skorokhodova, A., Sukhozhenko, A., Shakulov, R., and Debabov, V. (2012) Metabolic engineering of Escherichia coli for 1-butanol biosynthesis through the inverted aerobic fatty acid P-oxidation pathway. Biotechnol. Lett, 34, 463-469. 

Synthesis of PHAMCL from fatty acids such as octanoic acid or from the corresponding alkanes such as octane was first detected in P. oleovorans [119]. The alkanes are oxidized to the fatty acids the latter are activated by thiokinases and then degraded via the fatty acid /1-oxidation pathway. Obviously intermediates of this pathway accumulate under conditions favorable for the synthesis of PHA and are subsequently converted into substrates for the PHA synthase. Many reactions for the conversion of an intermediate of the -oxidation cycle into R-(-)-3-hydroxyacyl-CoA were considered. These were ... 

The existence of a mitochondrial pathway for de novo fatty acid synthesis was first reported 50 years ago, when it was generally assumed that fatty acid synthesis proceeded by reversal of the mitochondrial pathway for fatty acid P-oxidation (F. Lynen, 1957). However, the discovery of the cytosolic malonyl-CoA pathway (R.O. Brady, 1958 S.J Wakil, 1958) casted doubt on these claims and interest in this system waned until the discovery that mitochondria of both Neurospora crassa and Saccharomyces cerevisiae contain nuclear-encoded mitochondrial proteins that function as a type II FAS system. Disruption of the genes encoding these enzymes in both N. crassa and S. cerevisiae produces respiratory-deficient phenotypes and in S. cerevisiae cellular lipoic acid is reduced to less than 10% of that of the wild-type strain (R. Schneider, 1995 E. Schweizer, 1997). These observations suggested that in fungi one of the roles of this pathway might be to generate the lipoyl moieties required for mitochondrial function. 

Further biotransformations of A VPA involve both the liver microsomal cytochrome P-450 enzymes and the fatty acid 3-oxidation pathway (Fig. 32.28). The mixed-function oxidase system metabolizes the unsaturated metabolite to a y-butyrolactone derivative through a chemically reactive entity that is a suicide-substrate inhibitor of cytochrome P-450. The alkylation of the prosthetic haem by means of the radical occurs prior to the formation of the epoxide. Thus the epoxide is not involved in the cytochrome P-450 inhibition. 

The results presented here support the hypothesis that the 22 6n-3 synthetic pathway involves elongation and desaturation of 18 3n-3 to 24 6n-3, processes most likely carried out in microsomes then 24 6n-3 is transported to peroxisomes for one cycle of fatty acid P-oxidation to yield 22 6n-3. We showed that peroxisomal straight-chain acyl-Co A oxidase (AOxl), DBF, and 3-oxoacyl-CoA thiolase or sterol carrier protein X are involved in the P-oxidation of 24 6n-3 to 22 6n-3. 

In addition to HMG-CoA synthase, involved in ketogenesis, genes involved in fatty acid activation, like acyl-CoA synthetase," or in p-oxidation, like medium-chain acyl-CoA dehydrogenase are also target of PPAR. This transactivation pathway is reminiscent of a prokaryotic operon organization, in which fatty acids induce the expression of the genes responsible for their metabolism. We speculated that the main control step in fatty acid P-oxidation, the outer membrane component of carnitine palmitoyltransferase enzyme system, CPT I, could also be a PPAR target. 

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. 

Pathways II and III, found in various fluorescent pseudomonads, supply mainly md-(R)-3HA monomers from fatty acid P-oxidation and fatty acid biosynthesis intermediates, respiec-tively. The intermediates in these pathways are effectively converted by specialized enzymes to generate (R)-3HA-CoA monomers for polymerization. As shown in Figure 3, (R)-spedfic enoyl-CoA hydratase (PhaJ) and (R)-3-HA-ACP-CoA transferase (PhaG ACP, aryl carrier protein) function as metabolic suppliers of (R)-3HA-CoA from fr[Pg.160]

The second PHA synthesis pathway (pathway II) is related to fatty acid uptake by microorganisms. After fatty acid P-oxidation, acyl-CoA enters the PHA monomer synthesis process. Enzymes including 3-ketoacyl-CoA reductase, epimerase, (I )-enoyI-CoA hydratase/enoyl-CoA hydratase I, acyl-CoA oxidase (putative), and enoyl-CoA hydratase I (putative) were found to be involved in supplying the PHA precursor 3-hydroxyacyl-CoA for PHA synthesis. Pseudomonas putida, Pesudomonas aeruginosa, and A. hydrophila are able to use pathway n to synthesize medium-chain-length (mcl) PHA or copolymers of (/ )-3-hydroxybutyrate (R3HB) and (R)-3-hydroxyhexanoate (PHBHHx). 

Fig. 2 Pathways for medium-chain-length polyhydroxyalkanaote synthesis. Synthesis of medium-chain-length polyhydroxyalkanaote in bacteria can be accomplished through the use of intermediates either of the fatty acid P-oxidation cycle (left) or of the de novo fatty acid biosynthetic pathway (right)...

As outlined above, the mitochondrial fatty acid p-oxidation machinery relies on a variety of enzymes, most of which are strictly dependent on the incorporation of FAD as cofactor for proper functioning. Dietary riboflavin deficiency, or impaired metabolic pathways for the biosynthesis of FAD, is thus... 

Aliphatic and cycloaliphatic hydrocarbons may be used as substrates, whose decomposition leads to the growth of cells. The main pathway of biodegradation of alkenes is monoterminal oxidation that proceeds via the formation of the corresponding alcohol, aldehyde, and fatty acid. P-Oxidation of the fatty acids leads to formation of acetyl-CoA that is incorporated to the intermediary metabolism. 

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