Lactyl-CoA

Perhaps the most unusual of this group is lactyl-CoA dehydratase as isolated... 

Other enzymes in the aconitase family include isopropylmalate isomerase and homoaconitase enzymes functioning in the chain elongation pathways to leucine and lysine, both of which are pictured in Fig. 17-18.90 There are also iron-sulfur dehydratases, some of which may function by a mechanism similar to that of aconitase. Among these are the two fumarate hydratases, fumarases A and B, which are formed in place of fumarase C by cells of E. coli growing anaerobically.9192 Also related may be bacterial L-serine and L-threonine dehydratases. These function without the coenzyme pyridoxal phosphate (Chapter 14) but contain iron-sulfur centers.93-95 A lactyl-CoA... 

Acrylate and 3-MPA were readily metabolised by sediment microbes when they were present at micromolar concentrations but were not so easily degraded at millimolar levels. Acrylate is metabolised by a variety of bacteria. Escherichia coli converts acrylyl-CoA to pyruvate via lactyl-CoA, and some clostridia ferment acrylate, via CoA intermediates, to a mixture of acetate and propionate. Of particular relevance to the fate of DMSP was the isolation by Wagner and Stadtman (14) of a species of Clostridium (probably . propionicuml from river mud that fermented DMSP as follows ... 

Abeles and co-workers extended the work of Buckel on the properties and mechanism of the reductive dehydration of lactate to propionate in C. propionicum (234, 235). Studies by Abeles concluded that acrylate is indeed an intermediate in lactate dehydration, based on the observation of a primary kinetic isotope effect of 1.8 on the conversion of [3- H3]lactate to propionate, indicating that -hydrogen abstraction is at least partially rate limiting. The true substrates for the reaction were determined to be the CoA thioesters of lactate and acrylate, hence the enzyme has been aptly named lactyl-CoA dehydratase (234). The reaction was found to be mediated by two proteins, El and E2, and like the (R)-2-hydroxyglutarate-dehydrating enzyme from A. fermentans, both El and E2 from... 

Neither El nor E2, together or alone, mediated the formation of acrylyl- or lactyl-CoA from the respective acid plus CoA and acetyl phosphate or acetyl-CoA. However, a CoA transferase that catalyzes the formation of the CoA thioesters of lactate, acrylate, and propionate from acetyl-CoA has been isolated from C. propionicum (233). It is curious that acrylyl-CoA could not be isolated as a product with the crude or purified enzyme system, although acrylyl-CoA is readily converted to lactyl-CoA in the purified system. Abeles suggested that perhaps acrylyl-CoA exists as an enzyme-bound intermediate that requires the presence of a reductase for release as propionate. In this manner the organism is protected from buildup of toxic amounts of any acrylyl intermediates, which are known to undergo Michael addition reactions with biological nucleophiles. 

Scheme 54. Mechanism of lactyl-CoA dehydration proposed by Kuchta and Abeles (234).

There was a breakthrough when it was found that one mutant of PHA synthase was capable of incorporating lactic acid (LA) from its CoA form, lactyl-CoA (LA-CoA), into the polymer chain [67]. PHAs containing 2HA monomers, lactic acid (LA), glycolate (GL), and 2HB can be synthesized by engineered microbes in which the broad substrate specificities of PHA synthase and propionyl-CoA transferase (PCX) are critical factors for the incorporation of the monomers into the polymer chain. LA-based polymers, such as P[LA-co-3HB], have the properties of pliability and stretchiness which are distinctly different from those of the rigid poly(lactic acid) and P(3HB) homopolymers. 

Several synthetic pathways for 3-HP production from glucose at a theoretical yield of 100% have been patented by Cargill et al. ° In the representative pathway, glucose is first converted to lactate. Then, CoA transferase catalyzes the formation of lactyl-CoA,... 

I, pyruvate kinase 2, OS17 enzyme 3, 3-hydroxypropionyl-CoA dehydratase 4, lactate dehydrogenase 5, CoA transferase 6, 3-hyd roxy propiony l-CoA/3-hyd roxy i sobutryl-CoA hydrolase 7, lactyl-CoA dehydratase 8, pyruvate/glutamate transaminase 9, alanine dehydrogenase 10, alanine 2,3-aminomutase ... 

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