title: Thermodynamic evaluation on H2 production in glucose fermentation.
abstract: The normal maximum H2 yield in mesophilic biohydrogen (bioH2) fermentation is approximately 2 mol of H2/(mol of glucose). Thermodynamics could be the most fundamental control for bioH2 formation, since proton reduction is strongly energy consuming (+79.4 kJ/(mol of H2)). However, most of the electron equivalents in glucose do not accumulate in H2 but in a range of organic acids and alcohols. Thus, evaluating the hypothesis of thermodynamic control requiresthe full stoichiometry of the fermentation. We carried out batch bioH2 reactions with a range of pH values that yielded H2 yields from 0 to approximately 2 mol of H2/(mol of glucose). We constructed complete electron equivalent(e(-) equiv) balances for high or low H2 yield by measuring all e(-) sinks. The highest H2 yield occurred with pH approximately 4 and was coincident with major butyrate accumulation; ethanol or lactate correlated to reduced H2 yields at pH 7 and 10, respectively. Although the Gibb's free energies for all overall reactions were similar (-10.6 to -11.2 kJ/(e(-) equiv)), thermodynamics controlled the H2-producing reaction coupled to ferredoxin; this reaction was favorable at acidic pH but thermodynamically blocked at pH 10. Also, butyrate formation was the most thermodynamically favorable reaction that produced ATP after glycolysis.
authors.affiliation: Center for Environmental Biotechnology, The Biodesign Institute, Arizona State University, P. O. Box 875701, Tempe, Arizona 85287-5701, USA. hyungsool@asu.edu
authors.forename: Hyung-Sool
authors.initials: HS
authors.lastname: Lee
authors.forename: Michael B
authors.initials: MB
authors.lastname: Salerno
authors.forename: Bruce E
authors.initials: BE
authors.lastname: Rittmann
mesh_terms: D005285:Fermentation; D005947:Glucose / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D013816:Thermodynamics
chemical_list: D006859:Hydrogen; D005947:Glucose