Investigating enzymatic degradation of Streptococcal and Mycobacterial glycans

Abstract

Glycans are ubiquitous in biology and present in all kingdoms of life, playing significant roles in metabolism, protein modification and cell recognition. Glycans are also abundant in cell walls and capsules of bacteria, where they serve additional functions to provide structural support and contribute to virulence. Mycobacteria possess elaborate multi-layered cell walls with a plethora of glycans, requiring the co-ordinated action of a myriad of enzymes for its degradation. One key component of this complex cell wall is arabinogalactan, an unusual, branched and complex glycan containing the uncommon D-arabinan, for which degradative enzymes have not been previously identified in any organism. Conversely, Group A Streptococcus (GAS) is encapsulated with a common glycosaminoglycan known as hyaluronic acid (HA), which is identical in structure and composition to HA found in the extracellular matrix of all mammalian cells. This linear glycan has been extensively studied and the capsule has been identified as a significant virulence factor, specifically as an evasin due to HA providing immune system evasion as a result of its poor immunogenicity. Degradation of HA is performed by hyaluronidases, a class of widely characterised enzymes that have been identified to be expressed by numerous microbes, including GAS itself, as an additional virulence factor due to the invasive property of HA-penetrating enzymes. As copper stress inhibits central carbon metabolism, it is theorised GAS may use enzymes to degrade capsular HA to provide an alternative nutrient source during this stress. In this study, a range of putative enzymes thought to degrade either streptococcal HA or mycobacterial D-arabinan are investigated. Mining of the human gut microbiota discerned Dysgonomonas gadei as a degrader of D-arabinan, allowing identification and biochemical characterisation of novel exo-acting ᴅ-arabinofuranosidases belonging to the DUF2961 superfamily and the new GH172 family. Digestion of HA by GAS is also confirmed by identification of HA-degrading enzymes in the MGAS5005 genome, which demonstrate co ordinated degradation of polymeric HA into monosaccharides, bypassing the Embden Meyerhof-Parnas (EMP) pathway which is inhibited by copper. Together, this investigation uncovers the various routes in which microbial enzymes can orchestrate the digestion of cell wall and capsular glycans, as informed by structural and biochemical data obtained for these carbohydrate-active enzymes (CAZymes).