Biochemical characterization and bioinformatic analysis of two large multi-domain enzymes from Microbacterium aurum B8.A involved in native starch degradation

Microbacterium aurum B8.A is a unique bacterium with the ability to degrade starch granules through pore formation. In this study two enzymes (MaAmyA and MaAmyB) which are involved in granular starch degradation and were specific for the M. aurum B8.A strain, have been characterized in detail. Both enzyme are unusually long and carry many additional domains (seven in MaAmyA and six in MaAmyB) including multiple Carbohydrate Binding Modules (CBM), in addition to the catalytic domain. MaAmyA was characterized as an α-amylase and MaAmyB as an exo-amylase with a preference to release maltohexaose. It is likely that both enzymes work together in vivo on the degradation of starch granules, where MaAmyA forms the initial pores through which MaAmyB can enter and degrade the granules from the inside out. This thesis also has studied the roles of the additional domains and it was shown that the two CBM25 domains are required for activity on granular starch. In addition it was shown that the FNIII domains most likely function as flexible linker regions. Finally a previously unknown domain of MaAmyA has been characterized as a new CBM which was designated CBM74. It is mainly present in extremely large and complex α-amylase enzymes and always combined with either CBM25 or CBM26. Many of these enzymes are produced by Bifidobacteria or other gut related bacteria, which is a strong indication that there is a link between CBM74 and the degradation (fermentation) of resistant starch in the large intestine of the human digestive tract.