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Acidocalcisome biochemistry and osmoregulation

The role of acidocalcisomes in T. cruzi metabolism

The acidocalcisome is an acidic organelle rich in phosphorus, calcium, and other cations.183-186 Phosphorus is present as pyrophosphate and polyphosphate (polyP) and is complexed with calcium, and other cations. PolyP is linear polymer of phosphate linked by high-energy phosphoanhydride bonds that could have from a few to several hundreds phosphate units. The acidocalcisome membrane in T. cruzi contains a number of pumps (Ca2+-ATPase, V-H+-ATPase, and H+-PPase), and at least a channel (aquaporin), while its matrix contains enzymes related to pyrophosphate and polyP metabolism183-186 (Fig. 17.7). Acidocalcisomes in T. cruzi are also rich in basic amino acids such as arginine, ornithine, and lysine, probably also complexed with polyP.122 Some of the functions of acidocalcisomes in T. cruzi are the storage of cations, phosphorus, and amino acids, pyrophosphate and polyphosphate metabolism, and osmoregulation. The finding of novel enzymes in this organelle that are absent from mammalian cells has led to the discovery of novel targets for drug action.183-186

Scheme of a T. cruzi acidocalcisome

Figure 17.7 Scheme of a T. cruzi acidocalcisome. The matrix contains Pi, PPi, and polyP combined with a variety of cations (Ca21, Mg21, K1, Na1, and Zn21) and basic amino acids, and also a pyrophosphatase (PPase) and exopolyphosphatase (PPX) activities. The membrane possesses proton pumps (V-H1-ATPase and V-H1-PPase), a calcium pump (Ca21-ATPase), a water channel (aquaporin), and a polyP kinase complex (VTC complex). Other transporters for cations, Pb PPi, and basic amino acids are also probably present (dashed lines).

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