apicoplast

An apicoplast is a non-photosynthetic plastid-like organelle found in most members of the phylum Apicomplexa, including Plasmodium and Toxoplasma. It originated through secondary endosymbiosis of a red algal ancestor, resulting in a four-membrane bound organelle that is separated from the parasite's mitochondrion and cytosol. In some apicomplexan lineages the organelle has been lost.

Unlike photosynthetic plastids, the apicoplast does not participate in photosynthesis. Its genome is a small, circular DNA molecule of about 30-35 kilobases that encodes a limited set of genes, including those for ribosomal RNAs and a handful of proteins involved in transcription and translation; most apicoplast proteins are nucleus-encoded and imported across four membranes.

Major biosynthetic roles include a type II fatty acid synthesis pathway (FAS II) and the MEP/DOXP pathway for isoprenoid precursor biosynthesis. The apicoplast also contributes to heme biosynthesis in many species, but the details vary and parts of heme synthesis are shared with other organelles. The pathways generate metabolites essential for parasite growth; loss of apicoplast function typically halts replication.

Because many of its pathways are distinct from those of the human host and essential for parasite survival, the apicoplast is a target for antiparasitic drugs. Antibiotics that inhibit plastid ribosomes (e.g., doxycycline, clindamycin) cause a delayed death phenotype by preventing duplication of the apicoplast, while other compounds target its metabolic pathways.

Terminology and research reflect the organelle’s origin in Apicomplexa; ongoing studies seek to elucidate its full functions and potential as a therapeutic target. The apicoplast is a relic of a secondary endosymbiosis with a red alga and serves as a notable example of non-photosynthetic plastids in eukaryotes.