Carbohydrate Binding Module 74 (CBM74) Is a Specialized Resistant Starch Binding Domain

Abstract

Starch is a polymer of glucose that is used for energy storage in plants and is a major constituent of the Western diet. Starch that is not digestible by human amylases is termed resistant starch (RS) and acts as a prebiotic that preferentially promotes the growth of specialized beneficial microbes. The consumption of dietary RS has been linked to a lower incidence of colorectal cancer and intestinal inflammation, but it can only be processed by select gut bacteria in the human colon. Bifidobacterium adolescentis and Ruminococcus bromii are constituents of the healthy gut microbiota that can utilize RS as a sole carbon source and increase in abundance during host RS consumption. However, the genes involved in RS degradation as well as how the encoded proteins synergize to attack this insoluble fiber have not yet been elucidated. Uncooked starch granules (type 2 RS) are resistant to the action of human amylases due to their supramolecular structure in which adjacent starch chains form double helices which are packed together in crystalline layers, alternating with amorphous layers rich with inter-chain branch points. To access granular starch, bacteria need a way to bind at the granule surface and hydrolyze the glycosidic bonds of the constitutive amylose and amylopectin. These functions can occur on either the same multi-domain protein as in B. adolescentis or on multi-protein complexes like the R. bromii amylosome. Both human and bacterial amylases contain glycoside hydrolase family 13 (GH13) domains, which catalyze the breakdown of starch. However, bacterial amylases differ from human enzymes in that they encode discrete domains known as carbohydrate binding modules (CBMs). One notable CBM family, CBM74, was first characterized as part of an