TheRuminococcus bromiiamylosome protein Sas6 binds single and double helical α-glucan structures in starch

Abstract

Resistant starch is a prebiotic with breakdown by gut bacteria requiring the action of specialized amylases and starch-binding proteins. The human gut symbiont Ruminococcus bromii expresses granular starch-binding protein Sas6 (Starch Adherence System member 6) that consists of two starch-specific carbohydrate binding modules from family 26 (RbCBM26) and family 74 (RbCBM74). Here we present the crystal structures of Sas6 and RbCBM74 with a double helical dimer of maltodecaose bound along an extended surface groove. Binding data combined with native mass spectrometry suggest that RbCBM26 binds short maltooligosaccharides while RbCBM74 can bind single and double helical α-glucans. Our results support a model by which RbCBM74 and RbCBM26 bind neighboring α-glucan chains at the granule surface. CBM74s are conserved among starch granule-degrading bacteria and our work provides molecular insight into how this structure is accommodated by select gut species.

Clostridium butyricum Prazmowski can degrade and utilize resistant starch via a set of synergistically acting enzymes

“…Modules of the CBM74 family have been identified as a driver for RS degradation and, further, have been shown to play an important role in granule pore formation during the fermentation process ( 33 ). Furthermore, the structure of a representative of this family from R. bromii was recently solved and demonstrates an architecture and binding capacity much different than other starch-binding CBM families, providing perhaps a unique advantage for RS degraders ( 34 ). However, this CBM family appears to be absent from the C. butyricum genome, and so our results would indicate that while the CBM74 may be playing an important role in RS degradation, it is not an absolute requirement.…”

Clostridium butyricum Prazmowski can degrade and utilize resistant starch via a set of synergistically acting enzymes

“…Modules of the CBM74 family have been identified as a driver for RS degradation, and further, have been shown to play an important role in granule pore formation during the fermentation process (55). Furthermore, the structure of a representative of this family from R. bromii was recently solved and demonstrates an architecture and binding capacity much different than other starch binding CBM families, providing perhaps a unique advantage for RS degraders (56). However, this CBM family appears to be absent from the C. butyricum genome and so our results would indicate that while the CBM74 may be playing an important role in RS degradation, it is not an absolute requirement.…”

Presence of digestible starch impacts in vitro fermentation of resistant starch

“…In contrast, only a few microbes are reported to be able to degrade and ferment RS type 2 and 3 directly, such as Ruminococcus bromii and Bifidobacterium adolescentis. 35,36 These microbes express, next to specific α-amylases, also specific carbohydrate binding modules (CBMs), such as CBM74, that are able to bind to insoluble starch particles 37 and therefore accelerate hydrolysis.…”