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heme oxygenase in eukaryotes and some bacteria This subfamily contains heme oxygenase (HO, EC 1.14.14.18) found in eukaryotes as well as some proteobacteria, including cyanobacteria. Heme oxygenase (HO) catalyzes the rate limiting step in the degradation of heme to biliverdin in a multi-step reaction. HO is essential for recycling of iron from heme which is used as a substrate and cofactor for its own degradation to biliverdin, iron, and carbon monoxide. In vertebrates, HO plays a role in heme homeostasis and oxidative stress response, and cellular signaling in mammals that include isoforms HO-1, HO-2 and HO-3. HO-1 is ubiquitously expressed after induction while HO-2 expression is constitutive, mostly limited to certain organs, such as the brain, testes, and the vascular system. HO-3 is non-functional in humans, suggesting that the Hmox3 gene is a pseudogene derived from HO-2 transcripts. In higher plants and cyanobacteria, heme oxygenase is required for the synthesis of light-harvesting pigments, which contain tetrapyrrols derived from biliverdin. Candida albicans expresses a heme oxygenase that is required for the utilization of heme as a nutritional iron source, whereas Saccharomyces cerevisiae responds to iron deprivation by increasing Hmx1p transcription, which is controlled by the major iron-dependent transcription factor, Aft1p, and promotes both the re-utilization of heme iron and the regulation of heme-dependent transcription during periods of iron scarcity. In pathogenic bacteria, HO is part of a pathway for iron acquisition from host heme. In Leptospira interrogans, a pathogenic spirochete that causes leptospirosis, HO is required for iron utilization when hemoglobin is the sole iron source, thus making HO an interesting target for novel antimicrobial agents. HO shares tertiary structure similarity to methane monooxygenase (EC 1.14.13.25), ribonucleotide reductase (EC 1.17.4.1) and thiaminase II (EC 3.5.99.2), but shares little sequence homology. |
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