title: Effect of pH on metabolic pathway shift in fermentation of xylose by Clostridium tyrobutyricum.
abstract: The effect of pH (between 5.0 and 6.3) on butyric acid fermentation of xylose by Clostridium tyrobutyricum was studied. At pH 6.3, the fermentation gave a high butyrate production of 57.9 g l(-1) with a yield of 0.38-0.59 g g(-1) xylose and a reactor productivity up to 3.19 g l(-1)h(-1). However, at low pHs (<5.7), the fermentation produced more acetate and lactate as the main products, with only a small amount of butyric acid. The metabolic shift from butyrate formation to lactate and acetate formation in the fermentation was found to be associated with changes in the activities of several key enzymes. The activities of phosphotransbutyrylase (PTB), which is the key enzyme controlling butyrate formation, and NAD-independent lactate dehydrogenase (iLDH), which catalyzes the conversion of lactate to pyruvate, were higher in cells producing mainly butyrate at pH 6.3. In contrast, cells at pH 5.0 had higher activities of phosphotransacetylase (PTA), which is the key enzyme controlling acetate formation, and lactate dehydrogenase (LDH), which catalyzes the conversion of pyruvate to lactate. Also, PTA was very sensitive to the inhibition by butyric acid. Difference in the specific metabolic rate of xylose at different pHs suggests that the balance in NADH is a key in controlling the metabolic pathway used by the cells in the fermentation.
authors.affiliation: Department of Chemical Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, OH 43210, USA.
authors.forename: Ying
authors.initials: Y
authors.lastname: Zhu
authors.forename: Shang-Tian
authors.initials: ST
authors.lastname: Yang
mesh_terms: D000084:Acetate Kinase / Q000378:metabolism; D000085:Acetates / Q000378:metabolism; D019149:Bioreactors; D002087:Butyrates / Q000378:metabolism; D048014:Clostridium tyrobutyricum / Q000254:growth & development / Q000378:metabolism*; D005285:Fermentation / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007770:L-Lactate Dehydrogenase / Q000378:metabolism; D019344:Lactic Acid / Q000378:metabolism; D010709:Phosphate Acetyltransferase / Q000378:metabolism; D017851:Phosphotransferases (Carboxyl Group Acceptor) / Q000378:metabolism; D014994:Xylose / Q000378:metabolism*
chemical_list: D000085:Acetates; D002087:Butyrates; D019344:Lactic Acid; D014994:Xylose; D007770:L-Lactate Dehydrogenase; D010709:Phosphate Acetyltransferase; D017851:Phosphotransferases (Carboxyl Group Acceptor); D000084:Acetate Kinase; C019285:butyrate kinase