title: Cellulose utilization by Clostridium thermocellum: bioenergetics and hydrolysis product assimilation.
abstract: The bioenergetics of cellulose utilization by Clostridium thermocellum was investigated. Cell yield and maintenance parameters, Y(X/ATP)True = 16.44 g cell/mol ATP and m = 3.27 mmol ATP/g cell per hour, were obtained from cellobiose-grown chemostats, and it was shown that one ATP is required per glucan transported. Experimentally determined values for G(ATP)P-T (ATP from phosphorolytic beta-glucan cleavage minus ATP for substrate transport, mol ATP/mol hexose) from chemostats fed beta-glucans with degree of polymerization (DP) 2-6 agreed well with the predicted value of (n-2)/n [corrected] (n = mean cellodextrin DP assimilated). A mean G(ATP)(P-T) value of 0.52 +/- 0.06 was calculated for cellulose-grown chemostat cultures, corresponding to n = 4.20 +/- 0.46. Determination of intracellular beta-glucan radioactivity resulting from 14C-labeled substrates showed that uptake is different for cellulose and cellobiose (G2). For 14C-cellobiose, radioactivity was greatest for G2; substantially smaller but measurable for G1, G3, and G4; undetectable for G5 and G6; and n was approximately 2. For 14C-cellulose, radioactivity was greatest for G5; lower but substantial for G6, G2, and G1; very low for G3 and G4; and n was approximately 4. These results indicate that: (i) C. thermocellum hydrolyzes cellulose by a different mode of action from the classical mechanism involving solubilization by cellobiohydrolase; (ii) bioenergetic benefits specific to growth on cellulose are realized, resulting from the efficiency of oligosaccharide uptake combined with intracellular phosphorolytic cleavage of beta-glucosidic bonds; and (iii) these benefits exceed the bioenergetic cost of cellulase synthesis, supporting the feasibility of anaerobic biotechnological processing of cellulosic biomass without added saccharolytic enzymes.
authors.affiliation: Thayer School of Engineering, Dartmouth College, Hanover, NH 03755, USA.
authors.forename: Yi-Heng Percival
authors.initials: YH
authors.lastname: Zhang
authors.forename: Lee R
authors.initials: LR
authors.lastname: Lynd
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D019149:Bioreactors; D002250:Carbon Radioisotopes / Q000378:metabolism; D002475:Cellobiose / Q000378:metabolism; D002482:Cellulose / Q000378:metabolism*; D048013:Acetivibrio thermocellus / Q000378:metabolism* / Q000502:physiology; D004734:Energy Metabolism / Q000502:physiology*; D005285:Fermentation; D005936:Glucans / Q000378:metabolism; D006868:Hydrolysis; D013997:Time Factors
chemical_list: D002250:Carbon Radioisotopes; D005936:Glucans; D002475:Cellobiose; D000255:Adenosine Triphosphate; D002482:Cellulose