This family is found in bacteria and archaea, and is approximately 100 amino acids in length. There is a conserved NGPCGG sequence motif. This family is the C terminal of methylene-tetrahydrofolate reductase. This protein reduces FAD using the reducing equivalents from reduced FAD, subsequently reduces tetrahydrofolate. The C terminal of MTHFR contains the FAD binding site and is the catalytic portion of the enzyme.

1: Functional Role for the Conformationally Mobile Phenylalanine 223 in the Reaction of Methylenetetrahydrofolate Reductase from Escherichia coli. Lee MN, Takawira D, Nikolova AP, Ballou DP, Furtado VC, Phung NL, Still BR, Thorstad MK, Tanner JJ, Trimmer EE; Biochemistry. 2009; [Epub ahead of print] PMID:19610625