Enoyl-CoA-hydrase

The second reaction, catalyzed by enoyl-CoA hydrase, involves a hydration of... 

This feature of peroxisomal /3-oxidation is in sharp contrast to the mitochondrial pathway, which synthesizes ATP. The H202 produced when FADH2 is oxidized is converted to H20 by catalase. Second, the next two reactions in peroxisomal /3-oxidation are catalyzed by two enzyme activities (enoyl-CoA hydrase and 3-hydroxyacyl CoA dehydrogenase) found on the same protein molecule. Finally, the last enzyme in the pathway (/3-ketoacyl-CoA thiolase) has a different substrate specificity than its mitochondrial version. It does not efficiently bind medium-chain acyl-CoAs. 

The reaction product can then be hydrated by enoyl-CoA hydrase to L(+) j3-hydroxyacyl CoA and enters the /S-oxidation spiral. a,jS-cis unsaturated acyl-CoA esters are hydrated by enoyl-CoA hydrase to the D(—) enantiomeric form similar to isocrotonyl-CoA which is hydrated to D(—) j5-hydroxybutyryl-CoA (Wakil 1957). This D(—) form is epimerized by the D(—) jS-hydroxyacyl-CoA epimerase to the L(+) antipode which is the specific substrate for j5-hydroxyacyl-CoA dehydrogenase. Both enzymes have been isolated from mitochondria of rat liver and shown to be present in all organs, the concentration in brain however being very low. The two enzymes exhibit no specificity for dilferent chain lengths. As an example the reaction sequence in the / -oxidation of linolyl-CoA is summarized in the next figure (Stoffel et al. 1965 a, b) ... 

Crotonase (unsaturated fatty acid acyl-CoA hydrase, or enoyl hydrase) is the only enzyme known to catalyze the hydration of the unsaturated acyl-CoA esters. First partially purified from beef mitochondria, the enzyme was later crystallized. The crude enzyme acts on all hydroxy-CoA or enoyl-CoA esters with carbon chains ranging from four to twelve carbons. The crystallized enzyme also acts on fatty acid derivatives with chain lengths up to nine carbons, but the activity on longer carbon chains has not been investigated with the crys-... 

The methacrylyl CoA formed by this reaction is hydrated by crude enoyl hydrase (crotonase) of pig heart and the crystalline enz3nme to form j8-hydroxyisobutyryl CoA (87). Methacrylyl CoA also undergoes hydration spontaneously but at a much slower rate. 

The oxidation of isovaleryl CoA to /8-methylcrotonyl CoA (Fig. 6, reaction 3) is analogous to the corresponding reaction with straight chain fatty acids (see Volume I, Chapter 7). Evidence for the dehydrogenation of isovaleryl CoA was secured by incubating this compound with dialyzed rat liver extract in an incubation mixture containing triphenyltetrazolium chloride (116). The oxidation-reduction indicator was reduced at the rate of 0.25 /xmoles per hour. While it was not feasible to identify the /8-methyl-crotonyl CoA formed because of the inability to separate it from the isovaleryl CoA, further evidence of this reaction was obtained by showing the conversion of the isovaleryl CoA to -hydroxyisovaleryl CoA due to presence of enoyl hydrase (crotonase) in the enzyme preparation. 

Hydration of tiglyl CoA by these tissue preparations was readily demonstrated spectrophotometrically, resulting from the disappearance of the ultraviolet absorption band of the double bond. The enzyme responsible for the hydration is probably enoyl hydrase (crotonase). 

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