title: Proteomic analysis of Clostridium thermocellum core metabolism: relative protein expression profiles and growth phase-dependent changes in protein expression.
abstract: BACKGROUND
  Clostridium thermocellum produces H2 and ethanol, as well as CO2, acetate, formate, and lactate, directly from cellulosic biomass. It is therefore an attractive model for biofuel production via consolidated bioprocessing. Optimization of end-product yields and titres is crucial for making biofuel production economically feasible. Relative protein expression profiles may provide targets for metabolic engineering, while understanding changes in protein expression and metabolism in response to carbon limitation, pH, and growth phase may aid in reactor optimization. We performed shotgun 2D-HPLC-MS/MS on closed-batch cellobiose-grown exponential phase C. thermocellum cell-free extracts to determine relative protein expression profiles of core metabolic proteins involved carbohydrate utilization, energy conservation, and end-product synthesis. iTRAQ (isobaric tag for relative and absolute quantitation) based protein quantitation was used to determine changes in core metabolic proteins in response to growth phase.
  
  
  RESULTS
  Relative abundance profiles revealed differential levels of putative enzymes capable of catalyzing parallel pathways. The majority of proteins involved in pyruvate catabolism and end-product synthesis were detected with high abundance, with the exception of aldehyde dehydrogenase, ferredoxin-dependent Ech-type [NiFe]-hydrogenase, and RNF-type NADH:ferredoxin oxidoreductase. Using 4-plex 2D-HPLC-MS/MS, 24% of the 144 core metabolism proteins detected demonstrated moderate changes in expression during transition from exponential to stationary phase. Notably, proteins involved in pyruvate synthesis decreased in stationary phase, whereas proteins involved in glycogen metabolism, pyruvate catabolism, and end-product synthesis increased in stationary phase. Several proteins that may directly dictate end-product synthesis patterns, including pyruvate:ferredoxin oxidoreductases, alcohol dehydrogenases, and a putative bifurcating hydrogenase, demonstrated differential expression during transition from exponential to stationary phase.
  
  
  CONCLUSIONS
  Relative expression profiles demonstrate which proteins are likely utilized in carbohydrate utilization and end-product synthesis and suggest that H2 synthesis occurs via bifurcating hydrogenases while ethanol synthesis is predominantly catalyzed by a bifunctional aldehyde/alcohol dehydrogenase. Differences in expression profiles of core metabolic proteins in response to growth phase may dictate carbon and electron flux towards energy storage compounds and end-products. Combined knowledge of relative protein expression levels and their changes in response to physiological conditions may aid in targeted metabolic engineering strategies and optimization of fermentation conditions for improvement of biofuels production.
authors.affiliation: Department of Microbiology, University of Manitoba, Winnipeg, MB R3T 2N2, Canada.
authors.forename: Thomas
authors.initials: T
authors.lastname: Rydzak
authors.forename: Peter D
authors.initials: PD
authors.lastname: McQueen
authors.forename: Oleg V
authors.initials: OV
authors.lastname: Krokhin
authors.forename: Vic
authors.initials: V
authors.lastname: Spicer
authors.forename: Peyman
authors.initials: P
authors.lastname: Ezzati
authors.forename: Ravi C
authors.initials: RC
authors.lastname: Dwivedi
authors.forename: Dmitry
authors.initials: D
authors.lastname: Shamshurin
authors.forename: David B
authors.initials: DB
authors.lastname: Levin
authors.forename: John A
authors.initials: JA
authors.lastname: Wilkins
authors.forename: Richard
authors.initials: R
authors.lastname: Sparling
mesh_terms: D001426:Bacterial Proteins / Q000032:analysis*; D002475:Cellobiose / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D048013:Acetivibrio thermocellus / Q000737:chemistry* / Q000254:growth & development / Q000378:metabolism*; D003470:Culture Media / Q000737:chemistry; D015180:Electrophoresis, Gel, Two-Dimensional; D020869:Gene Expression Profiling; D020543:Proteome / Q000032:analysis*; D053719:Tandem Mass Spectrometry
chemical_list: D001426:Bacterial Proteins; D003470:Culture Media; D020543:Proteome; D002475:Cellobiose