title: Immobilization of Acetobacter sp. CCTCC M209061 for efficient asymmetric reduction of ketones and biocatalyst recycling.
abstract: BACKGROUND
  The bacterium Acetobacter sp. CCTCC M209061 is a promising whole-cell biocatalyst with exclusive anti-Prelog stereoselectivity for the reduction of prochiral ketones that can be used to make valuable chiral alcohols such as (R)-4-(trimethylsilyl)-3-butyn-2-ol. Although it has promising catalytic properties, its stability and reusability are relatively poor compared to other biocatalysts. Hence, we explored various materials for immobilizing the active cells, in order to improve the operational stability of biocatalyst.
  
  
  RESULTS
  It was found that Ca-alginate give the best immobilized biocatalyst, which was then coated with chitosan to further improve its mechanical strength and swelling-resistance properties. Conditions were optimized for formation of reusable immobilized beads which can be used for repeated batch asymmetric reduction of 4'-chloroacetophenone. The optimized immobilized biocatalyst was very promising, with a specific activity of 85% that of the free-cell biocatalyst (34.66 μmol/min/g dw of cells for immobilized catalyst vs 40.54 μmol/min/g for free cells in the asymmetric reduction of 4'-chloroacetophenone). The immobilized cells showed better thermal stability, pH stability, solvent tolerance and storability compared with free cells. After 25 cycles reaction, the immobilized beads still retained >50% catalytic activity, which was 3.5 times higher than degree of retention of activity by free cells reused in a similar way. The cells could be recultured in the beads to regain full activity and perform a further 25 cycles of the reduction reaction. The external mass transfer resistances were negligible as deduced from Damkohler modulus Da < <1, and internal mass transfer restriction affected the reduction action but was not the principal rate-controlling step according to effectiveness factors η < 1 and Thiele modulus 0.3<∅ <1.
  
  
  CONCLUSIONS
  Ca-alginate coated with chitosan is a highly effective material for immobilization of Acetobacter sp. CCTCC M209061 cells for repeated use in the asymmetric reduction of ketones. Only a small cost in terms of the slightly lower catalytic activity compared to free cells could give highly practicable immobilized biocatalyst.
authors.affiliation: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China.
authors.forename: Xiao-Hong
authors.initials: XH
authors.lastname: Chen
authors.forename: Xiao-Ting
authors.initials: XT
authors.lastname: Wang
authors.forename: Wen-Yong
authors.initials: WY
authors.lastname: Lou
authors.forename: Ying
authors.initials: Y
authors.lastname: Li
authors.forename: Hong
authors.initials: H
authors.lastname: Wu
authors.forename: Min-Hua
authors.initials: MH
authors.lastname: Zong
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Smith
authors.forename: Xin-De
authors.initials: XD
authors.lastname: Chen
mesh_terms: D000091:Acetobacter / Q000378:metabolism*; D000464:Alginates / Q000737:chemistry; D000480:Alkynes / Q000737:chemistry / Q000378:metabolism; D055162:Biocatalysis; D000440:Butanols / Q000737:chemistry / Q000378:metabolism; D018914:Cells, Immobilized / Q000378:metabolism; D048271:Chitosan / Q000737:chemistry; D020723:Glucuronic Acid / Q000737:chemistry; D006603:Hexuronic Acids / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007659:Ketones / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction; D012997:Solvents / Q000737:chemistry; D013237:Stereoisomerism; D013696:Temperature; D002721:omega-Chloroacetophenone / Q000737:chemistry / Q000378:metabolism
chemical_list: C540507:3-butyn-2-ol; D000464:Alginates; D000480:Alkynes; D000440:Butanols; D006603:Hexuronic Acids; D007659:Ketones; D012997:Solvents; D002721:omega-Chloroacetophenone; D020723:Glucuronic Acid; D048271:Chitosan