L-glyceraldehyde 3-phosphate reductase from Escherichia coli is a heme binding protein

Recently, we reported that YghZ from Escherichia coli functions as an efficient L-glyceraldehyde 3-phosphate reductase (Gpr). Here we show that Gpr co-purifies with a b-type heme cofactor. Gpr associates with heme in a 1:1 stoichiometry to form a complex that is characterized by a K(d) value of 5.8+/-0.2 microM in the absence of NADPH and a K(d) value of 11+/-1.3 microM in the presence of saturating NADPH. The absorbance spectrum of reconstituted Gpr indicates that heme is bound in a hexacoordinate low-spin state under both oxidizing and reducing conditions. The physiological function of heme association with Gpr is unclear, as the L-glyceraldehyde 3-phosphate reductase activity of Gpr does not require the presence of the cofactor. Bioinformatics analysis reveals that Gpr clusters with a family of putative monooxygenases in several organisms, suggesting that Gpr may act as a heme-dependent monooxygenase. The discovery that Gpr associates with heme is interesting because Gpr shares 35% amino acid identity with the mammalian voltage-gated K+ channel beta-subunit, an NADPH-dependent oxidoreductase that endows certain voltage-gated K+ channels with hemoprotein-like, O2-sensing properties. To date the molecular origin of O2 sensing by voltage-gated K+ channels is unknown and the results presented herein suggest a role for heme in this process.