Fusion of cellobiose phosphorylase and potato alpha-glucan phosphorylase facilitates substrate channeling for enzymatic conversion of cellobiose to starch.

We previously reported an in vitro enzymatic pathway for conversion of nonfood cellulose to starch (PNAS,110 (18): 7182-7187, 2013), in which the two sequential enzymes cellobiose phosphorylase (CBP) from Clostridium thermocellum and potato alpha-glucan phosphorylase (PGP) from Solanum tuberosum were the two key enzymes responsible for the whole conversion rate. In this work CBP and PGP were fused to form a large enzyme and it turned out that the fusion protein could exhibit a good bifunctionality when PGP moiety was put at the N-terminus and CBP moiety at the C-terminus (designated as PGP-CBP). Although the coupled reaction rate of PGP-CBP was decreased by 23.0% compared with the free enzymes, substrate channeling between the two active sites in PGP-CBP was formed, demonstrated by the introduction of the competing enzyme of PGP to the reaction system. The potential of PGP-CBP fusion enzyme being applied to the conversion of cellulose to amylose was discussed.