The gut microbiota plays a major role in colonic health and homeostasis. The growth of beneficial bacteria can be promoted by the administration of prebiotics in the human diet. Resistant starch is a prebiotic that preferentially promotes the growth of specialized bacteria, including Bifidobacterium adolescentis. Type 2 resistant starches are raw, granular starches that are inaccessible to most enzymes due to their tightly packed semi-crystalline structure. To break down and utilize resistant starch as a nutrient source, a bacterium must encode proteins that bind to the starch granule components and hydrolyze glycosidic linkages. Starch- specific carbohydrate binding modules (CBMs) are commonly appended to glycoside hydrolase family 13 (GH13) domains, which hydrolyze the glucose linkages in starch. B. adolescentis encodes seven extracellular GH13-containing enzymes including BaAmy7, which is highly active on raw potato and corn starch. BaAmy7 encodes four predicted CBMs, three of which belong to CBM families previously shown to bind raw starch. This proposal aims to understand the mechanism of action of BaAmy7 and how the coordinated effort of these four CBMs permits resistant starch as a substrate. I hypothesize that the binding and proper spatial arrangement of each of the four CBMs is required for maximal catalytic activity. To test this hypothesis, I will (1) dissect the role of individual CBMs in resistant starch hydrolysis by BaAmy7 and (2) elucidate the structural arrangement of BaAmy7 domains. Completion of these aims will lead to a better understanding of the molecular features that set apart BaAmy7 in its ability to hydrolyze resistant starch.
Resistant starch is a promising prebiotic that preferentially facilitates the expansion of beneficial bacteria like Bifidobacterium adolescentis, in the human colon. B. adolescentis encodes a multi-domain ?-amylase, BaAmy7, which allows it to capture and degrade resistant starches. This project aims to elucidate the cooperative role of multiple starch binding domains encoded on BaAmy7 in degradation of this prebiotic substrate which will allow us to better target these beneficial organisms.
