Crotonyl CoA

The degradation of pimelate is initiated by formation of the coenzyme A ester and is followed by a series of steps with the production of glutaryl-CoA that is decarboxylated to crotonyl-CoA... 

The kind of enantiomer [d-(-)- or l-(+)-] synthesized in the formation of the C4 intermediate varies. The acetoacetyl-CoA reductase (EC 1.1.1.36), which is NADPH-dependent, stereoselectively reduces acetoacetyl-CoA to d-(-)-3-hydroxybutyryl-CoA (R. eutropha [15]). The NADH-dependent reductase catalyzes the reduction of acetoacetyl-CoA to L-(+)-3-hydroxybutyryl-CoA. Afterwards two stereospecific crotonyl-CoA hydratases, l-(+)- and D-(-)-speci-fic, convert the L-(+)-3-hydroxybutyryl-CoA into the D-(-)-isomer (Rhodo-spirillum rubrum [16]). 

In Aeromonas caviae, 3-ketothiolase and acetoacetyl-CoA reductase are absent. In this species, the synthesis of poly(3HB) proceeds via an enoyl-CoA hy-dratase from either crotonyl-CoA or hexenoyl-CoA. The enoyl-CoA derivatives stem from the fatty-acid oxidation pathway [18]. 

Biotin (vitamin B ) is widespread in foods and is also synthesized by intestinal bacteria. It is a coenzyme for the carboxylation of pyruvate, acetyl-coenzyme-A (CoA), propionyl CoA, and /1-methyl-crotonyl CoA and is involved in fatty acid formation and in energy release from carbohydrates. In humans deficiencies only occur in patients with an abnormal gut flora and manifests itself as exfoliative dermatitis and alopecia. 

The energy of the butyryl-CoA linkage and of one of the acetyl-CoA linkages is conserved and utilized in the initial formation of crotonyl-CoA (Eq. 17-32). That leaves one acetyl-CoA which can be converted via acetyl-P to acetate with formation of ATP. 

The product of acetyl-CoA carboxylase reaction, malonyl-CoA, is reduced via malonate semialdehyde to 3-hydroxypropionate, which is further reductively converted to propionyl-CoA. Propionyl-CoA is carboxylated to (S)-methylmalonyl-CoA by the same carboxylase. (S)-Methylmalonyl-CoA is isomerized to (R)-methylmal-onyl-CoA, followed by carbon rearrangement to succinyl-CoA by coenzyme B 12-dependent methylmalonyl-CoA mutase. Succinyl-CoA is further reduced to succinate semialdehyde and then to 4-hydroxybutyrate. The latter compound is converted into two acetyl-CoA molecules via 4-hydroxybutyryl-CoA dehydratase, a key enzyme of the pathway. 4-Hydroxybutyryl-CoA dehydratase is a [4Fe-4S] cluster and FAD-containing enzyme that catalyzes the elimination of water from 4-hydroxybutyryl-CoA by a ketyl radical mechanism to yield crotonyl-CoA [34]. Conversion of the latter into two molecules of acetyl-CoA proceeds via normal P-oxidation steps. Hence, the 3-hydroxypropionate/4-hydroxybutyrate cycle (as illustrated in Figure 3.5) can be divided into two parts. In the first part, acetyl-CoA and two bicarbonate molecules are transformed to succinyl-CoA, while in the second part succinyl-CoA is converted to two acetyl-CoA molecules. 

The oxoester analog of crotonyl-CoA 26 shown in Fig. 5 was used as an alternative substrate for crotonase and exhibited about 300-fold decreased activity relative to the natural... 

Wu WJ, Tonge PJ, Raleigh DP. Stereospecific H and C NMR assignments of crotonyl CoA and hexadienoyl CoA conformational analysis and comparison with protein-CoA complexes. J. Am. Chem. Soc. 1998 120 9988-9994. 

Willadsen and Eggerer (75) have studied the stereochemistry of the enzyme acetyl CoA acetyltransferase, a key enzyme in both the terminal step in C-3 oxidation of fatty acids and the initial step in the biosynthesis of terpenes and steroids. The enzyme, when incubated separately with (2S)-[2-2Hi,2-3Hi]aceto-acetyl CoA and the (2R) isomer gave two moles of acetyl CoA as depicted in Scheme 17. Eggerer et al. (76) utilized the enzyme enoyl CoA hydratase to convert properly labeled crotonyl CoA, via syn addition, to the doubly isotopically labeled 3-hydroxyacyl CoA derivatives needed in this study. A discussion of this unique type of hydration has been presented by Rose (9). The labeled... 

Figure 37 Formation of 3(S)- and 3(fl)-hydroxybutyryl-CoA by ECH. Experimentally, the equilibrium constant is 7.5. Since the two enantiomers have the same energy, K2 must also be 7.5. Ab initio studies predict that frans-2-crotonyl-CoA is 12 kJ mol more stable than the c/s isomer, giving an equilibrium constant between the two enantiomers of 0.0079 (K4). K3, the equilibrium constant for the dehydration of 3(fl)-hydroxybutyryl-CoA to c/s-2-crotonyl-CoA, can then be calculated from the relationship K3 = K4/K2 = 0.001. Reproduced with permission from W. J. Wu Y. Feng X. Fie FI. S. Flofstein D. P. Raleigh P. J. Tonge, J. Am. Chem. Soc. 2000, 122, 3987.

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