Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a glycolytic enzyme that also functions in transcriptional regulation， oxidative stress， vesicular trafficking， and apoptosis. Because GAPDH is required for the insertion of cellular heme into inducible nitric oxide synthase [Chakravarti， R.， et al. (2010) Proc. Natl. Acad. Sci. U.S.A. 107， 18004-18009]， we extensively characterized the heme binding properties of GAPDH. Substoichiometric amounts of ferric heme bound to GAPDH (one heme per GAPDH tetramer) to form a low-spin complex with UV-visible maxima at 362， 418， and 537 nm and when reduced to ferrous gave maxima at 424， 527， and 559 nm. Ferric heme association and dissociation rate co... More
Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a glycolytic enzyme that also functions in transcriptional regulation， oxidative stress， vesicular trafficking， and apoptosis. Because GAPDH is required for the insertion of cellular heme into inducible nitric oxide synthase [Chakravarti， R.， et al. (2010) Proc. Natl. Acad. Sci. U.S.A. 107， 18004-18009]， we extensively characterized the heme binding properties of GAPDH. Substoichiometric amounts of ferric heme bound to GAPDH (one heme per GAPDH tetramer) to form a low-spin complex with UV-visible maxima at 362， 418， and 537 nm and when reduced to ferrous gave maxima at 424， 527， and 559 nm. Ferric heme association and dissociation rate constants at 10 °C were as follows: k(on) = 17800 M(-1) s(-1)， k(off1) = 7.0 × 10(-3) s(-1)， and k(off2) = 3.3 × 10(-4) s(-1) (giving approximate affinities of 19-390 nM). Ferrous heme bound more poorly to GAPDH and dissociated with a k(off) of 4.2 × 10(-3) s(-1). Magnetic circular dichroism， resonance Raman， and electron paramagnetic resonance spectroscopic data on the ferric， ferrous， and ferrous-CO complexes of GAPDH showed that the heme is bis-ligated with His as the proximal ligand. The distal ligand in the ferric complex was not displaced by CN(-) or N(3)(-) but in the ferrous complex could be displaced by CO at a rate of 1.75 s(-1) (for >0.2 mM CO). Studies with heme analogues revealed selectivity toward the coordinating metal and porphyrin ring structure. The GAPDH-heme complex was isolated from bacteria induced to express rabbit GAPDH in the presence of δ-aminolevulinic acid. Our finding of heme binding to GAPDH expands the protein's potential roles. The strength， selectivity， reversibility， and redox sensitivity of heme binding to GAPDH are consistent with it performing heme sensing or heme chaperone-like functions in cells.
