Figure 4. Predicted SFB metabolic pathways.

Overview of SFB metabolic pathways. SFB are highly auxotrophic and have a few complete essential pathways mostly for utilization of glycans and monosaccharides. They have complete glycolysis and pentose phosphate pathways, but lack the TCA cycle. Although fatty acid biosynthesis pathways are present, fatty acid metabolism pathways are absent. Absent as well are most pathways for co-factor and amino acid biosynthesis with the exception of the interrelated pathways for lysine, aspartate, glutamate, asparagine, and glutamine as noted in the figure. In contrast, multiple oligosaccharide and metal ion (in particular iron) transport and utilization mechanisms are present in SFB, including PTS, ABC, and other transporters, as well as extracellular peptidases and glycosyl hydrolases, which are shown interacting with extracellular glycans. SFB appear to be able to digest the glycoprotein components of the mucus layer and import sugars released in the process via ABC transporters and TCS. Once imported, several enzymes prepare these substrates for glycolysis. All these pathways provide SFB with the ability to acquire multiple metabolites from the surrounding environment and the host. Within the cell, essential pathways leading from import of polysaccharides through production of peptidoglycan, fatty acids, reduced ferredoxin, and acetate are shown.