Fig. 5.

Proposed formyl-THF synthesis pathway in the presence and absence of formate. Formyl-THF may be synthesized directly from formate via formate-THF ligase (Fhs). In the absence of formate, we propose that serine is a precursor for formyl-THF synthesis. While no annotated phosphoserine phosphatase (PSPH) or serine-pyruvate aminotransferase (SPT) is annotated in C.thermocellum, alanine-glyoxylate transaminase (AGAT) may function as a SPT. Annotated gene numbers (Clo1313) are indicated in brackets. Enzyme E.C. numbers are provided for enzymes which are not annotated in C. thermocellum. THF tetrahydrofolate; GK glycerate kinase; GDH glycerate dehydrogenase; PGDH 3-phosphoglycerate dehydrogenase; PSAT phosphoserine aminotransferase); SGAT serine-glyoxylate aminotransferase; GlyA serine hydroxymethyltransferase; MetF methylene-THF reductase; FolD bifunctional methylene-THF dehydrogenase/methylene-THF cyclohydrolase; Pfl pyruvate:formate lyase