This entry represents a subgroup of the YjeF N-terminal domain-containing proteins from eukaryotes, bacteria and archaea, including NAXE and YJEFN3 from humans []. NAXE is an NAD(P)H-hydrate epimerase that has been shown to regulate cholesterol efflux from endothelial cells []. YJEFN3 may play a role in spermiogenesis and oogenesis [].In bacteria or archaea, YjeF N-terminal domains are often fused to a YjeF C-terminal domain []. It is a bifunctional enzyme that catalyses the epimerisation of the S- and R-forms of NAD(P)HX and the dehydration of the S-form of NAD(P)HX at the expense of ADP, which is converted to AMP []. However, this entry represents the subgroup that only contain the YjeF N-terminal domain. The reduced forms of NAD and NADP, two major nucleotides playing a central role in metabolism, are continuously damaged by enzymatic or heat-dependent hydration. Eukaryotic NAD(P)H-hydrate epimerase catalyses the epimerisation of the S- and R-forms of NAD(P)HX, the damaged form of NAD(P)H. This is a prerequisite for the S-specific NAD(P)H-hydrate dehydratase to allow the repair of both epimers of NAD(P)HX []. This family also includes yeast YNL200C, which is a NAD(P)H-hydrate epimerase that catalyses the epimerisation of the S- and R-forms of NAD(P)HX, at the expense of ATP, which is converted to ADP []. In Arabidopsis, AtPPOX (At5g49970) or PDX3, a fusion protein with the YjeF N-terminal domain and a C-terminal domain that functions as a pyridoxine/pyridoxamine 5'-phosphate oxidase, has been identified. AtPPOX is involved in the vitamin B6 salvage pathway [].
