Secretory expression of functional barley limit dextrinase by Pichia pastoris using high cell-density fermentation

Heterologous production of large multidomain proteins from higher plants is often cumbersome. Barley limit dextrinase (LD), a 98 kDa multidomain starch and alpha-limit dextrin debranching enzyme, plays a major role in starch mobilization during seed germination and is possibly involved in starch biosynthesis by trimming of intermediate branched alpha-glucan structures.

Highly active barley LD is obtained by secretory expression during high cell-density fermentation of Pichia pastoris. The LD encoding gene fragment without signal peptide was subcloned in-frame with the Saccharomyces cerevisiae alpha-factor secretion signal of the P. pastoris vector pPIC9K under control of the alcohol oxidase I promoter.

Optimization of a fed-batch fermentation procedure enabled efficient production of LD in a 5-L bioreactor, which combined with affinity chromatography on beta-cyclodextrin-Sepharose followed by Hiload Superdex 200 gel filtration yielded 34 mg homogenous LID (84% recovery). The identity of the recombinant LD was verified by N-terminal sequencing and by mass spectrometric peptide mapping.

A molecular mass of 98 kDa was estimated by SDS-PAGE in excellent agreement with the theoretical value of 97419 Da. Kinetic constants of LD catalyzed pullulan hydrolysis were found to K-m,K-app = 0.16 +/- 0.02 mg/mL and k(cat,app) = 79 +/- 10 s(-1) by fitting the uncompetitive substrate inhibition Michaelis-Menten equation, which reflects significant substrate inhibition and/or transglycosylation.

The resulting catalytic coefficient, k(cat,app)/K-m,K-app = 488 +/- 23 mL/(mg s) is 3.5-fold higher than for barley malt LD. Surface plasmon resonance analysis showed alpha-, beta-, and gamma-cyclodextrin binding to LD with K-d of 27.2, 0.70, and 34.7 mu M, respectively.