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Monomer-Supplying Enzymes for Polyhydroxyalkanoate Biosynthesis
Published in Martin Koller, The Handbook of Polyhydroxyalkanoates, 2020
Maierwufu Mierzati, Takeharu Tsuge
Three-ketoacyl-CoA thiolase (acetyl-CoA acetyltransferase; EC 2.3.1.9) is an enzyme that catalyzes the thiolytic cleavage of 3-oxoacyl-CoA into acyl-CoA (shortened by two carbon atoms and generates acetyl-CoA), which happens to be the final step of fatty acid β-oxidation. In contrast, the reverse reaction is catalyzed by PhaA to form acetoacetyl-CoA via the condensation of two acetyl-CoA molecules in the first step of poly(3-hydroxybutyrate) [P(3HB); a.k.a. PHB] biosynthesis. Therefore, thiolases can function either as degradative in the β-oxidation pathway of fatty acids or biosynthetic agents. Additionally, PhaA has a narrow substrate specificity in the range of C3–C5 monomers chain length [46]; therefore, PhaA is specialized for scl-PHA biosynthesis. There were two 3-ketothiolases found in R. eutropha, encoded as PhaA and BktB, that can act in the biosynthetic pathway of PHA synthesis with the difference in substrate specificity [47]. All of the bacterial PHA biosynthetic 3-ketothiolases are classified as homotetramers [48–52], and the molecular weights of PhaA and BktB are 44 and 46 kDa, respectively [16,17]. BktB is proven to have substrate specificity mainly in longer monomers (C4–C10) compared with PhaA [53] and is also considered responsible for synthesizing 3-hydroxyvalerate (3HV) monomers in poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] biosynthesis [17].
Biological Process for Butanol Production
Published in Jay J. Cheng, Biomass to Renewable Energy Processes, 2017
Maurycy Daroch, Jian-Hang Zhu, Fangxiao Yang
Acetyl-CoA Condensation: Condensation of two molecules of acetyl-CoA is the first step of chain elongation required for the formation of higher alcohols such as butanol or isopropanol. In solventogenic Clostridia this reaction is catalyzed by thiolase (acetyl-CoA: acetyl-CoA C-acetyltransferase, E.C. 2.3.1.19, Enzyme 1 in Figure 8.3). Thiolase catalyzes the condensation of two acetyl-CoA molecules into an acetoacetyl-CoA which is the precursor for formation of higher alcohols and acetone (Wiesenborn, 1988). The reaction liberates one molecule of free coenzyme A that can be used for another phosporoclastic cleavage of pyruvate to maintain carbon flow through metabolism.
Production of Butanol from Corn
Published in Shelley Minteer, Alcoholic Fuels, 2016
Thaddeus C. Ezeji, Nasib Qureshi, Patrick Karcher, Hans P. Blaschek
The central core of both the acidogenic and solventogenic pathways is the series of reactions that produces butyryl-CoA from acetyl-CoA. Thiolase condenses two molecules of acetyl-CoA into one molecule of acetoacetyl-CoA. Acetoacetyl-CoA is reduced to 3-hydroxybutyryl-CoA by hydroxybutyryl-CoA dehydrogenase. From this, crotonyl-CoA is formed by dehydration, catalyzed by crotonase. The carbon-carbon double bond in crotonyl-CoA is reduced with NADH to produce butyryl-CoA. This last step is catalyzed by butyryl-CoA dehydrogenase (Bennett and Rudolph, 1995).
Biosynthesis of butyric acid by Clostridium tyrobutyricum
Published in Preparative Biochemistry and Biotechnology, 2018
Jin Huang, Wan Tang, Shengquan Zhu, Meini Du
Butyric acid has been produced from C. tyrobutyricum using various hexoses and pentoses, with acetic acid, CO2, and H2 formed as the major fermentation by-products.[2] The metabolic pathways of butyric acid production by C. tyrobutyricum with glucose and xylose have been revealed little by little (Figure 1). When glucose is used as the substrate, glucose (1 mol) is catabolized to pyruvate (2 mol) with ATP (2 mol) and NADH (2 mol) through Embden–Meyerhof–Pamas (EMP) pathway.[32] In the xylose pathway, the xylose substrate is catabolized to fructose 6-phosphate and glyceraldehyde-3-phosphate through the pentose phosphate pathway followed by phosphorylation, epimerization, then pyruvate formation under a series of enzymes; through that process, 5 mol pyruvate along with 5 mol ATP and 5 mol NADH is generated from 3 mol xylose.[28] In both pathways, pyruvate is then oxidized to acetyl-CoA with the release of CO2 and generated the reduced ferredoxin, which is reoxidized by hydrogenase as electron transfer to hydrogen ions to form H2. Under certain conditions, pyruvate may also be converted to lactate by lactate dehydrogenase (LDH) . Acetyl-CoA is converted either to acetate by PTA and AK or to acetoacetyl-CoA under the catalysis of thiolase, which is followed by conversion to butyryl-CoA through a series of enzymes (β-hydroxybutyryl-CoA dehydrogenase, crotonase, and butyryl CoA dehydrogenase). Butyryl-CoA, which is the key precursor in the formation of butyric acid, is further converted to butyryl-P by PTB and finally to butyric acid by BK.[323334] Glucose and xylose fermentation by C. tyrobutyricum follow the stoichiometric equations below: