title: A new model for the anaerobic fermentation of glycerol in enteric bacteria: trunk and auxiliary pathways in Escherichia coli.
abstract: Anaerobic fermentation of glycerol in the Enterobacteriaceae family has long been considered a unique property of species that synthesize 1,3-propanediol (1,3-PDO). However, we have discovered that Escherichia coli can ferment glycerol in a 1,3-PDO-independent manner. We identified 1,2-propanediol (1,2-PDO) as a fermentation product and established the pathway that mediates its synthesis as well as its role in the metabolism of glycerol. We also showed that the trunk pathway responsible for the conversion of glycerol into glycolytic intermediates is composed of two enzymes: a type II glycerol dehydrogenase (glyDH-II) and a dihydroxyacetone kinase (DHAK), the former of previously unknown physiological role. Based on our findings, we propose a new model for glycerol fermentation in enteric bacteria in which: (i) the production of 1,2-PDO provides a means to consume reducing equivalents generated in the synthesis of cell mass, thus facilitating redox balance, and (ii) the conversion of glycerol to ethanol, through a redox-balanced pathway, fulfills energy requirements by generating ATP via substrate-level phosphorylation. The activity of the formate hydrogen-lyase and F(0)F(1)-ATPase systems were also found to facilitate the fermentative metabolism of glycerol, and along with the ethanol and 1,2-PDO pathways, were considered auxiliary or enabling. We demonstrated that glycerol fermentation in E. coli was not previously observed due to the use of medium formulations and culture conditions that impair the aforementioned pathways. These include high concentrations of potassium and phosphate, low concentrations of glycerol, alkaline pH, and closed cultivation systems that promote the accumulation of hydrogen gas.
authors.affiliation: Department of Chemical and Biomolecular Engineering, Rice University, P.O. Box 1892, Houston, TX 77251-1892, USA. Ramon.Gonzalez@rice.edu
authors.forename: Ramon
authors.initials: R
authors.lastname: Gonzalez
authors.forename: Abhishek
authors.initials: A
authors.lastname: Murarka
authors.forename: Yandi
authors.initials: Y
authors.lastname: Dharmadi
authors.forename: Syed Shams
authors.initials: SS
authors.lastname: Yazdani
mesh_terms: D000693:Anaerobiosis / Q000502:physiology; D004926:Escherichia coli / Q000201:enzymology / Q000254:growth & development*; D029968:Escherichia coli Proteins; D000431:Ethanol / Q000378:metabolism; D005285:Fermentation / Q000502:physiology*; D005990:Glycerol / Q000378:metabolism*; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D010084:Oxidation-Reduction; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism; D011409:Propylene Glycols / Q000378:metabolism*; D006180:Proton-Translocating ATPases / Q000378:metabolism; D013401:Sugar Alcohol Dehydrogenases / Q000378:metabolism
chemical_list: D029968:Escherichia coli Proteins; D011409:Propylene Glycols; D000431:Ethanol; C041787:1,3-propanediol; D006859:Hydrogen; D013401:Sugar Alcohol Dehydrogenases; C020672:glycerol dehydrogenase; D017853:Phosphotransferases (Alcohol Group Acceptor); C023132:glycerone kinase; D006180:Proton-Translocating ATPases; D005990:Glycerol