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Apicoplast

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Title: Apicoplast  
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Subject: Plasmodium falciparum biology, Apicomplexa, Algae, Plasmodium falciparum, Organelles
Collection: Articles with Inconsistent Citation Formats, Endosymbiotic Events, Malaria, Organelles
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Apicoplast

An apicoplast is a derived non-photosynthetic plastid found in most Apicomplexa, including malaria parasites such as Plasmodium falciparum, but not in others such as Cryptosporidium. It originated from an alga (there is debate as to whether this was a green or red alga) through secondary endosymbiosis. The apicoplast is surrounded by four membranes within the outermost part of the endomembrane system.[1]

Contents

  • Significance 1
  • Evolutionary origin 2
  • Architecture and distribution 3
  • Function 4
    • Fatty Acid Synthesis 4.1
    • Isoprenoid Synthesis 4.2
    • Heme synthesis 4.3
    • Iron-sulphur cluster synthesis 4.4
  • References 5

Significance

Apicoplasts are a relict, nonphotosynthetic plastid found in most protozoan parasites belonging to the phylum Chromalveolata.[2][3] Among the most infamous Chromalveolata parasites is Plasmodium falciparum, a causative agent of severe malaria. Because apicoplasts are vital to parasite survival, they provide an enticing target for antimalarial drugs.[4] Specifically, apicoplasts' plant-like properties provide a target for herbicidal drugs.[3] And, with the emergence of malarial strains resistant to current treatments it is paramount that novel therapies, like herbicides, are explored and understood.[4] Furthermore, herbicides may be able to specifically target the parasite's plant-like apicoplast and without any noticeable effect on the mammalian host's cells.

Evolutionary origin

Evidence suggests that the apicoplast is a product of secondary

  1. ^ a b c d Lim, L. & McFadden, G. (2010). "The evolution, metabolism and functions of the apicoplast". Philosophical Transactions of the Royal Society 365: 749–763.  
  2. ^ Biology 9th edition by Reece, Urry, Cain, Wasserman, Minorsky, Jackson
  3. ^ a b c d e f g h i j Maréchal, E.; et al. (2001). "The Apicoplast: a new member of the plastid family" (PDF).  
  4. ^ a b c d e Ralph, S.; et al. (2001). "The apicoplast as an antimalarial drug target" (– Scholar search).  
  5. ^ Ralph, S.; et al. (2004). "Evolutionary Pressures on Apicoplast Transit Peptides" (– Scholar search).  
  6. ^ Essential Cell Biology, 2nd ed. "The Eukaryotic Cell.". 
  7. ^ a b c d e f "Endosymbiosis and The Origin of Eukaryotes". 2006-05-24. 

References

Various iron-sulphur cluster biosynthetic enzymes including SufB or Orf470 have been identified in the apicoplast genome.[1]

Iron-sulphur cluster synthesis

The apicoplast has also been implicated with heme synthesis[4] and amino acid synthesis. It is also suggested to have a role in cell development. These functions, however, are merely postulations and are not yet conclusively supported by experimentation.[3]

Heme synthesis

The apicoplast is also thought to have a role in isoprenoid synthesis, which are prosthetic groups on many enzymes and also act as precursors to ubiquinones (involved in electron transport) and dolichols (involved in glycoprotein formation).[1] The apicoplast contains the DOXP pathway for isoprenoid synthesis and is the sole site for synthesis in the Plasmodium genome.[1]

Isoprenoid Synthesis

The destruction of the apicoplast does not immediately kill the parasite but instead prevents it from invading new host cells. This observation suggests that the apicoplast may be involved in lipid metabolism. If unable to synthesize sufficient fatty acids the parasite is unable to form the parasitophorous vacuole (PV) that is imperative to a successful invasion of host cells. This conclusion is supported by the discovery of Type II Fatty Acid Synthase (FAS) machinery in the apicoplast.[4]

Fatty Acid Synthesis

[3] The functions of apicoplasts have not been conclusively defined. However, it has been established that the apicoplast is a vital

Function

Most Apicomplexa contains a single ovoid shaped apicoplast that is found at the anterior of the invading parasitic cell.[3] The apicoplast is situated in close proximity to the cell's nucleus and always closely associated with a mitochondrion. The small plastid, only 0.15-1.5 μm in diameter,[3] is surrounded by four membranes.[7] The two inner membranes are called the outer (OEM) and inner envelope membrane (IEM) and are derived from the plastid envelope.[3] Within the apicoplast's membrane is a 35 kb long circular DNA strand that codes for approximately 30 proteins, tRNAs and some RNAs.[7] Particles suspected to be bacterial ribosomes are present.[4] The plastid, at least in the Plasmodium species, also contains "tubular whorls" of membrane that bear a striking resemblance to the thylakoids[3] of their chloroplast relatives.[7]

Architecture and distribution

[7] and the disappearance of a nucleomorph.[3] These losses of function are hypothesized to have occurred at an early evolutionary stage in order to have allowed sufficient time for the complete degradation of acknowledged photosynthetic relicts[7]

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