TPP_enzyme_M_2 is the middle domain of thiamine pyrophosphate in sequences not captured by Pfam:PF00205. This enzyme is necessary for the first step of the biosynthesis of menaquinone, or vitamin K2, an important cofactor in electron transport in bacteria [1].

1: Specificity and reactivity in menaquinone biosynthesis: the structure of Escherichia coli MenD (2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexadiene-1-carboxylate synthase). Dawson A, Fyfe PK, Hunter WN; J Mol Biol. 2008;384:1353-1368. PMID:18983854