Cloning and expression of a heme binding protein from the genome of Saccharomyces cerevisiae

Heme binding properties of glyceraldehyde-3-phosphate dehydrogenase

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.

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

Cisplatin upregulatesSaccharomyces cerevisiae genes involved in iron homeostasis through activation of the iron insufficiency-responsive transcription factor Aft1

The response of Saccharomyces cerevisiae to cisplatin was investigated by examining variations in gene expression using cDNA microarrays and confirming the results by reverse transcription polymerase chain reaction (RT-PCR). The mRNA levels of 14 proteins involved in iron homeostasis were shown to be increased by cisplatin. Interestingly, the expression of all 14 genes is known to be regulated by Aft1, a transcription factor activated in response to iron insufficiency. The promoter of one of these genes, FET3, has been relatively well studied, so we performed a reporter assay using the FET3 promoter and showed that an Aft1 binding site in the promoter region is indispensable for induction of transcription by cisplatin. The active domain of Aft1 necessary for activation of the FET3 promoter by cisplatin is identical to the one required for activation by bathophenanthroline sulfonate, an inhibitor of cellular iron uptake. Furthermore, we found that cisplatin inhibits the uptake of (55)Fe(II) into yeast cells. These findings suggest that cisplatin activates Aft1 through the inhibition of iron uptake into the cells, after which the expression of Aft1 target genes involved in iron uptake might be induced.

Hemoglobin regulates expression of an activator of mating-type locus alpha genes in Candida albicans

Phenotypic switching from the white to the opaque phase is a necessary step for mating in the pathogenic fungus Candida albicans. Suppressing switching during vascular dissemination of the organism may be advantageous, because opaque cells are more susceptible to host defenses. A repressor of white-opaque switching, HBR1 (hemoglobin response gene 1), was identified based on its specific induction following growth in the presence of exogenous hemoglobin. Deletion of a single HBR1 allele allowed opaque phase switching and mating competence, accompanied by a lack of detectable MTL alpha1 and alpha2 gene expression and enhanced MTLa1 gene expression. Conversely, overexpression of Hbr1p or exposure to hemoglobin increased MTLalpha gene expression. The a1/alpha2 repressed target gene CAG1 was derepressed in the same mutant in a hemoglobin-sensitive manner. Regulation of CAG1 by hemoglobin required an intact MTLa1 gene. Several additional Mtlp targets were perturbed in HBR1 mutants in a manner consistent with commitment to an a mating phenotype, including YEL007w, MFalpha, HST6, and RAM2. Therefore, Hbr1 is part of a host factor-regulated signaling pathway that controls white-opaque switching and mating in the absence of allelic deletion at the MTL locus.

Heme oxygenase in Candida albicans is regulated by hemoglobin and is necessary for metabolism of exogenous heme and hemoglobin to alpha-biliverdin

Candida albicans is an opportunistic pathogen that has adapted uniquely to life in mammalian hosts. One of the host factors recognized by this yeast is hemoglobin, which binds to a specific cell surface receptor. In addition to its regulating the expression of adhesion receptors on the yeast, we have found that hemoglobin induces the expression of a C. albicans heme oxygenase (CaHmx1p). Hemoglobin transcriptionally induces the CaHMX1 gene independent of the presence of inorganic iron in the medium. A Renilla luciferase reporter driven by the CaHMX1 promoter demonstrated rapid activation of transcription by hemoglobin and (cobalt protoporphyrin IX) globin but not by apoglobin or other proteins. In contrast, iron deficiency or exogenous hemin did not activate the reporter until after 3 h, suggesting that induction of the promoter by hemoglobin is mediated by receptor signaling rather than heme or iron flux into the cell. As observed following disruption of the Saccharomyces cerevisiae ortholog, HMX1, a CaHMX1 null mutant was unable to grow under iron restriction. This suggests a role for CaHmx1p in inorganic iron acquisition. CaHMX1 encodes a functional heme oxygenase. Exogenous heme or hemoglobin is exclusively metabolized to alpha-biliverdin. CaHMX1 is required for utilization of these exogenous substrates, indicating that C. albicans heme oxygenase confers a nutritional advantage for growth in mammalian hosts.

Regulation of intracellular heme levels by HMX1, a homologue of heme oxygenase, in Saccharomyces cerevisiae

Saccharomyces cerevisiae responds to iron deprivation by increasing the transcription of genes involved in the uptake of environmental iron and in the mobilization of vacuolar iron stores. HMX1 is also transcribed under conditions of iron deprivation and is under the control of the major iron-dependent transcription factor, Aft1p. Although Hmx1p exhibits limited homology to heme oxygenases, it has not been shown to be enzymatically active. We find that Hmx1p is a resident protein of the endoplasmic reticulum and that isolated yeast membranes contain a heme degradation activity that is dependent on HMX1. Hmx1p facilitates the capacity of cells to use heme as a nutritional iron source. Deletion of HMX1 leads to defects in iron accumulation and to expansion of intracellular heme pools. These alterations in the regulatory pools of iron lead to activation of Aft1p and inappropriate activation of heme-dependent transcription factors. Expression of HmuO, the heme oxygenase from Corynebacterium diphtheriae, restores iron and heme levels, as well as Aft1p- and heme-dependent transcriptional activities, to those of wild type cells, indicating that the heme degradation activity associated with Hmx1p is important in mediating iron and heme homeostasis. Hmx1p promotes both the reutilization of heme iron and the regulation of heme-dependent transcription during periods of iron scarcity.