Production of recombinant cholesterol oxidase containing covalently bound FAD in Escherichia coli

Abstract

Background: Cholesterol oxidase is an alcohol dehydrogenase/oxidase flavoprotein that catalyzes the dehydrogenation of C(3)-OH of cholesterol. It has two major biotechnological applications, i.e. in the determination of serum (and food) cholesterol levels and as biocatalyst providing valuable intermediates for industrial steroid drug production. Cholesterol oxidases of type I are those containing the FAD cofactor tightly but not covalently bound to the protein moiety, whereas type II members contain covalently bound FAD. This is the first report on the over-expression in Escherichia coli of type II cholesterol oxidase from Brevibacterium sterolicum (BCO).

Results: Design of the plasmid construct encoding the mature BCO, optimization of medium composition and identification of the best cultivation/induction conditions for growing and expressing the active protein in recombinant E. coli cells, concurred to achieve a valuable improvement: BCO volumetric productivity was increased from approximately 500 up to approximately 25000 U/L and its crude extract specific activity from 0.5 up to 7.0 U/mg protein. Interestingly, under optimal expression conditions, nearly 55% of the soluble recombinant BCO is produced as covalently FAD bound form, whereas the protein containing non-covalently bound FAD is preferentially accumulated in insoluble inclusion bodies.

Conclusions: Comparison of our results with those published on non-covalent (type I) COs expressed in recombinant form (either in E. coli or Streptomyces spp.), shows that the fully active type II BCO can be produced in E. coli at valuable expression levels. The improved over-production of the FAD-bound cholesterol oxidase will support its development as a novel biotool to be exploited in biotechnological applications.

Figure 1

Sequence alignment of the BCO forms expressed in E. coli. fBCO: Protein obtained by subcloning the fBCO-cDNA into pET24b(+) using the NdeI/XhoI restriction sites; mBCO: protein obtained by subcloning the mBCO-cDNA into pET24b(+) using the NdeI/XhoI restriction sites. For comparison, the sequence of the "mature" form of BCO (as determined from the 3-D structure, PDB code 1I19) is reported.

Figure 2

Production of mBCO in 2 L-batch fermentation of recombinant E. coli cells under optimized conditions. Top panel: time course of pH (down triangle), pO₂ (up triangle), temperature (square), and growth curve measured as OD_{600 nm} (circle). Bottom panel: production of mBCO measured as specific activity (U/mg, filled bars) and volumetric productivity (U/L, empty bars). The arrow indicates the moment of IPTG addition.

Figure 3

Determination of the flavinylated vs. non-covalent recombinant mBCO by means of SDS-PAGE analysis. SDS-PAGE Gel was analyzed for flavin fluorescence (panel A) and subsequently stained for total proteins with Coomassie-blue (panel B). Conditions: recombinant E. coli cells grown in LB medium, added of 1 mM IPTG at OD_{600 nm} = 0.9, then incubated at 25°C and collected after 18 hours. I, Insoluble fraction (pellet after cell disruption and centrifugation); S, soluble fraction (soluble crude extract). An amount of sample corresponding to 0.1 mL of fermentation broth was loaded in each lane. BCOcov and BCOncov: purified flavinylated and non-covalent mBCO forms, respectively; the amount of enzyme loaded are indicate in micrograms. M: molecular weight markers (indicated in kDa on the right side of panel B).

Figure 4

Purified total soluble mBCO (top panel) and its percentage as flavinylated covalent mBCO (bottom panel). Recombinant E. coli cells were grown at 37°C, added of 1 mM IPTG at OD_{600 nm} = 1.5-2, and harvested after 18 hours at the temperature indicated below each bar. TB+20G: TB medium in the presence of 20 mL/L of glycerol: SB+8G; SB medium in the presence of 8 mL/L of glycerol. Data variability is <10%.