N-Butyryl CoA

Besides MMCM and GM, two other coenzyme B -dependent carbon skeleton mutases are known. These are (1) methylene glutarate mutase (MGM) from the anaerobe Eubacterium (Clostridium) barkeri, which catalyzes the equilibration of 2-methylene-glutarate with (R)-3-methylitaconate as part of a degradative path of nicotinic acid [175,199] and (2) isobutyryl-CoA mutase (ICM), which is observed in species of gram-positive bacteria Strep-tomyces and catalyzes the reversible rearrangement of iso-butyryl-CoA and n-butyryl-CoA [177]. The isomerization of iso-butyryl-CoA and n-butyryl-CoA in ICM is relevant in the biosynthesis of polyketide antibiotics [177]. 

The role of valine as a precursor to butyrate units in streptomycete metabolism has been observed in the biosynthesis of many secondary metabolites, such as tylosin and monensin (36,37). In these pathways, the valine is thought to be degraded to isobutyryl CoA, which is then isomerized to n-butyryl CoA. The stereochemistry of this latter isomerization has been investigated in vivo in Snreptomyces ctniurnionemis, the producer of the antibiotic monensin A (38). 

Butyryl-CoA mutase interconverts isobutyryl-CoA and n-butyryl-CoA (Table 1, entry 4) in an adenosyl-cobamide-dependent way and is presumed to involve the rearrangement of radicals in a mechanism similar to that of methylmalonyl-CoA mutase (10,81). 

For every step of the P oxidation sequence there is a small family of enzymes with differing chain length preferences.6 7 For example, in liver mitochondria one acyl-CoA dehydrogenase acts most rapidly on M-butyryl and other short-chain acyl-CoA a second prefers a substrate of medium chain length such as n-octanoyl-CoA a third prefers long-chain substrates such as pal-mitoyl-CoA and a fourth, substrates with 2-methyl branches. A fifth enzyme acts specifically on isovaleryl-CoA. Similar preferences exist for the other enzymes of the P oxidation pathway. In Escherichia coli... 

The DEBS 1-TE multienzyme was purified to 90-95% homogeneity and then used in another series of experiments to establish the extent to which alternative starter units could be used by the polyketide synthase [36], Substantial amounts of lactones were obtained in the presence of acetyl-, n-butyryl-, and isobutyryl-CoA, illustrating that the loading didomain exhibits a relaxed specificity for the starter unit (Fig. 10). The utilization of acetyl-CoA and -butyryl-CoA by DEBS 1 + TE was demonstrated in a cell-free system [39], Additionally, in the absence of the reducing cofactor NADPH, cell-free DEBS 1+TE converted... 

Figure 17 Design of assays to evaluate decarboxylation of methylmalonyl-CoA catalyzed by DEBS 1-TE. It was anticipated that a decarboxylase activity would incorporate deuterium into the starter unit of propionyl lactone and that such labeling would be visible by GC-MS analysis. As decarboxylation has been reported in the presence of primers other than propionyl-CoA, assays I—III included n-hutyryl-CoA as a starter unit. Assay IV was designed to evaluate the possibility that -butyryl-CoA suppresses decarboxylation. GC-MS analysis gave no evidence for labeling of the side chain in any assay. Therefore, decarboxylation is not a significant reaction of KSt under these conditions.

Polyketides. Collective name for natural products produced biosynthetically by way of poly(/5-oxo-carboxylic acids). The name was derived in 1907 by Collie on the basis of the hypothesis that natural prt ucts may be formed by multiplication of ketene (HjCsC o) units. The P. chains are constructed on multienzyme complexes (polyketide synthases) from acetyl- and malonyl-CoA ( acetogenins) or also by use of propio-nyl- and/or butyryl-CoA. Depending on the number of building blocks the natural products ate classified as triketides (n=3), tetraketides (n=4), etc. 

Boynton, Z.L., Bennett, G.N., and Rudolph, EB. (1996) Cloning, sequencing, and expression of clustered genes encoding -hydroxybutyryl-coenzyme A (CoA) dehydrogenase, crotonase, and butyryl-CoA dehydrogenase from Clostridium acetobutylicum ATCC 824. J. Bacteriol, 178, 3015-3024. 

The general basic mechanism of the carboxylation reactions (acetyl-, propionyl-, butyryl-CoA and pyruvate) is the fixation of HCOg in an ATP dependent reaction to the I -N of the biotin molecule ... 

One approach, first examined by Atsumi et al. (2008b), is the metabolic engineering of the relatively easily manipulated bacteria E. coli to make n-butanol, not characteristic for this species. The first part of the metabolic pathway used E. coli metabolism from glucose to acetyl-CoA, and the second followed the engineered 1-butanol production pathway from C. acetobutylicum, consisting of six enzymatic steps from acetyl-CoA to n-butanol. In order to achieve the required transformations in these steps, it was necessary to express genes that encode the synthesis of acetoacetyl-CoA, 3-hydroxybutryl-CoA, crotonyl-CoA, butyryl-CoA, butyralde-hyde and, lastly, n-butanol. 

Malonyl-ACP, formed from acetyl-CoA (shuttled out of mitochondria) and CO2, condenses with an acetyl bound to the Cys—SH to yield acetoacetyl-ACP, with release of CO2. This is followed by reduction to the n-/3-hydroxy derivative, dehydration to the trans-t -unsaturated acyl-ACP, and reduction to butyryl-ACP. NADPH is the electron donor for both reductions. Fatty acid synthesis is regulated at the level of malonyl-CoA formation. 

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