Abstract
Heme and other tetrapyrroles, often called “the colors of life”, belong to the most important molecules of almost all extant organisms. They are synthesized by a common multistep pathway that is highly conserved throughout the tree of life [1]. One of the tetrapyrrole products is chlorophyll, the green pigment of plants and other phototrophs, which captures the energy of the sun. Vitamin B12, the most complex tetrapyrrole, is involved in DNA synthesis and energy metabolism [2]. The major product of tetrapyrrole biosynthesis in non-photosynthetic organisms is heme, an iron-coordinated porphyrin with the capacity to transfer electrons and bind diatomic gases. Here we summarize the current understanding of different aspects of heme metabolism in parasitic eukaryotes, including the synthesis and uptake of heme and its detoxification. A differential need for heme in distinct parasitic groups and the suitability of heme metabolism as a drug target for treating parasite-borne diseases are also discussed. First, however, let us review the functions of heme in various cellular processes.
Highlights
Heme and other tetrapyrroles, often called ‘‘the colors of life’’, belong to the most important molecules of almost all extant organisms
Primary heterotrophic eukaryotes clearly combined the original synthetic pathway of the pre-eukaryotic host with that of the a-proteobacterial predecessor of mitochondrion, resulting in an evolutionary mosaic pathway [10]. This pathway spans both the cytosol and the mitochondrion, where heme is mostly needed for respiratory cytochromes (Figure 1A). Photosynthetic eukaryotes acquired another tetrapyrrole synthesis pathway from the engulfed cyanobacterium transformed to a primary plastid, or from a eukaryotic alga already possessing a plastid evolving into a secondary plastid
The plastidial pathway serves mainly for the synthesis of chlorophyll, which is required in much greater amounts than heme
Summary
Most organisms are able to synthesize their own heme in a pathway in which the first committed precursor, d-aminolevulinic acid (ALA), is converted to heme by seven universally conserved enzymatic steps. Primary heterotrophic eukaryotes clearly combined the original synthetic pathway of the pre-eukaryotic host with that of the a-proteobacterial predecessor of mitochondrion, resulting in an evolutionary mosaic pathway [10]. This pathway spans both the cytosol and the mitochondrion, where heme is mostly needed for respiratory cytochromes (Figure 1A). The plastidial pathway serves mainly for the synthesis of chlorophyll, which is required in much greater amounts than heme Both the mitochondrion-cytosolic and the plastidial pathways co-exist in Euglena gracilis, which acquired its plastid through a secondary endosymbiosis relatively recently (Figures 1A, 1C, and 2) [11]. The plastidial pathway took over tetrapyrrole synthesis and the original pathway of the host cell disappeared [10,11,12]
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