Phosphate Acquisition in Protozoan Parasites: Transport Systems, and Extracellular Phosphate-Releasing Enzymatic Activities.

Inorganic phosphate (Pi) is an essential nutrient required for energy metabolism, macromolecule biosynthesis, and signal transduction. Protozoan parasites are exposed to pronounced fluctuations in nutrient availability throughout their life cycles and therefore rely on adaptive strategies to secure phosphate from diverse host and environmental niches. This review summarizes current knowledge on phosphate acquisition mechanisms in protozoan parasites, with emphasis on membrane transport systems and extracellular phosphate-releasing enzymatic activities. Phosphate transport systems energized by proton or sodium gradients have been functionally characterized in several protozoan species, and in many cases phosphate uptake capacity is modulated by extracellular Pi availability. In addition, ectophosphatases expressed at the parasite surface contribute to phosphate acquisition by hydrolyzing extracellular phosphorylated substrates and releasing inorganic phosphate that can be subsequently internalized and metabolically utilized. Although phosphate-dependent regulation of ectophosphatase activity has been demonstrated in a more limited number of species, available evidence supports a functional interplay between extracellular phosphate scavenging and transmembrane transport, particularly under phosphate-limiting conditions. Despite these advances, the molecular mechanisms underlying phosphate sensing and regulatory coordination in protozoan parasites remain poorly understood. This review provides a comparative overview of phosphate transport systems and extracellular phosphate-scavenging enzymes in protozoan parasites, highlighting current evidence and remaining knowledge gaps.