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Phosphorus is a necessary element in all biological organisms due to its indispensability in energy supply, DNA, RNA, and phospholipid biosynthesis. 1) However, organisms must replenish their phosphate supply from the external environment in order to survive. Organisms obtain phosphorus from the environment through different transport mechanisms. These differing mechanisms work interdependently to maintain the ecological balance of phosphorus in what is known as the phosphorus cycle.
Bacteria transport inorganic phosphorus across their cell membranes directly from the environment. The two most extensively studied prokaryotes in the field of phosphorus transport are E. coli and A. johnsonni. However, both these organisms have been shown to have significant similarities with other prokaryotes and can be viewed as models for a conserved prokaryotic mechanism of phosphorus transport. There are two primary mechanisms for prokaryotic transport of phosphorus from the external environment into the cell: the phosphate specific transport system (Pst) and the inorganic phosphate transport (Pit). 2)
Pit is a secondary transport protein whose mechanism is driven by the differential proton gradient between the alkaline cytoplasm of the cell and the acidic external environment. Thus, the transport of phosphorus into the cell cytoplasm via Pit always occurs concurrently with the transport of an H+ ion. The Pit transport mechanism is undertaken by one membrane spanning protein and allows for both the influx and efflux of phosphorus. 4)
The functionality of Pit has been found to be dependent upon the presence of divalent metal cations in the external environment such as Ca2+, Mg2+, Co2+, or Mn2+. Thus, inorganic phosphorus is transported through the Pit primarily in the form of neutral soluble metal chelates (MeHPO4)). This chelation involves the formation of coordinate bonds between a metal cation and one of the organic derivatives of phosphorus which allows the transport of the phosphorus itself to be net-neutral in its effect of the cell's internal pH (this is excluding the affect of the symport of H+ with phosphorus).5)
Pit has a low affinity for phosphorus in comparison with Pst, with a Km value of 25uM. Furthermore, it has low substrate specificity with nearly the same affinity for arsenate, the toxic chemical analogue of phosphate, as for phosphate itself. Essentially, the Pit fails to distinguish between phosphorus and arsenic and can thus contribute to transporting toxic forms of arsenic into the cell. Although the Pit has a lower affinity and specificity for phosphate than the Pst, it still affords two major advantages to the cell. First, it is resistant to osmotic shock. Second, it requires no energy input and thus can still function in phosphorus intake even when the cell is in a state of energy starvation. 6)
Pst is a primary active transporter that hydrolyzes ATP to ADP in order to supply the energy needed to transport phosphorus from the external environment to the interior of the cell. Pst is made up of four sub-units: one phosphate-binding protein in the periplasmic space, two membrane spanning proteins (each made up of six alpha helices), and one dimeric ATP binding protein located on the interior surface of the cell membrane. 8)
Pst has a much higher affinity for phosphorus than Pit, with an apparent Km of approximately 0.7um. Instead of transporting phosphorus in the form of a neutral metal chelate, Pst has been shown to translocate H2PO4- and HPO42- from the exterior to the interior of the cell, only allowing for the influx (and not efflux) of phosphorus. Pst also has a much higher specificity for phosphate than for arsenate.9)
Pst is highly regulated by the pstS genes which form an operon with the ability to up-regulate or down-regulate the activity of Pst. When inorganic phosphate (in the form of H2PO4- and HPO42-) is present in large quantities outside the cell, Pst is down-regulated to a low level of activity. However, if inorganic phosphorus in the environment becomes scarce, Pst can undergo a 100 fold increase in phosphorus transport activity, a process often referred to as “phosphorus scavenging”. 10)
Seeing as many eukaryotes are complex multicellular organisms, the issue of transporting phosphorus from the environment is complicated. Different classes of eukaryotic organisms utilize different transport proteins and mechanisms to accomplish this task, with most species using several different types of transport proteins. However, in common with the prokaryotes, they all involve the co-transport of a positive ion. However, in the case of Eukaryotes, the symport of a proton is not the driving energetic force for phosphorus transport, but instead serves to maintain the internal pH of the cell which would be affected by the negative ionic forms of phosphorus often found in nature.
In mammals, inorganic phosphate (Pi) must be replenished through ingesting phosphate sources in the diet. Special transport proteins within mammalian cells called Na/Pi transporters, so named because they require the co-transport of an Na+ ion in order to absorb Pi, accomplish this task in mammalian cells. There are three identified types of Na/Pi transporters. Type I and Type II Na/Pi transporters are located in the epithelial cells of the proximal convoluted tubule of the kidney. These cells handle the reabsorption of Pi in mammals 11). The reabsorption of Pi in the kidneys maintains adequate circulating levels of Pi in mammalian systems, however, mammalian cells in other tissues utilize Type III Na/Pi transporters in order to obtain the phosphorus supply needed in order to maintain normal cellular function (ie use in metabolites, DNA, RNA, repair). 12)
All three types of Na/Pi transporters exhibit similar affinities for Pi even though their primary sequences are heterologous 13). Other differences include that Type III transporters are blocked by viral infections and that Type II transporters have been shown to be catalyzed by N-linked glycosylation at residues Asn-298 and Asn-328. 14). While it is known that the three types of Na/Pi co-transporters have differing structures and regulatory mechanisms, conclusive results regarding these subjects have yet to be discovered. 15)
Plants face the unique obstacle of absorbing Pi directly from the soil, in which it is present in low concentrations. Nonetheless, the membranes in plant root cells absorb phosphorus directly from the soil. The precise mechanics of this membrane transport are unknown, however, it is hypothesized that it involves the co-transport of one or more protons into the cell along with phosphorus. Furthermore, there appears to be at least two types of transporters within plants: one that is active in Pi rich conditions with a lower-affinity for Pi and another with a higher affinity for Pi that is activated following Pi starvation. The Km for the low-affinity transporter is estimated to be in the range of 50uM-330uM while the Km for the high-affinity transporter is estimated to be in the range of 3uM-7uM. Most plants form symbiotic relationships with either fungi or bacteria to assist in nutrient uptake. However, the role of fungi in the absorption of Pi by plants is still poorly understood. Nonetheless, experimental results show that plants that have formed a relationship with mychorrizal fungi (the most common fungal symbiont in plants) do not have higher levels of Pi intake than those plants who lack this relationship. 16)