Phosphorus Biochemistry

Phosphorus oxidation states range from -3 to +5. However, in nature phosphorus almost exclusively exists in the +5 oxidation state and plays a significant role in living organisms. Prebiotic phosphorus

chemistry plays a central role in attempting to shed light on the origin of life on Earth. Phosphorus-rich minerals such as brushite [CaHPO₄·2H₂O], apatite [Ca₅(PO₄)₃OH], struvite [MgNH₄PO₄·6H₂O], luneb- urgite [Mg₃B₂(PO₄)₂(OH)₂·8H₂O], and schreibersite [(Fe,Ni)₃P] were sources of ortho[PO₄^3-], pyro [P₂O₇^4-] and tripoly [P₃O₁₀^5-] phosphates necessary for incorporation into organic molecules in the early stages of life creation.

Westheimer reviewed the role of phosphorus in living systems emphasizing the importance of phosphoric acid and its role in nucleic acids. Phosphates are ionized at physiological pH due to a low first pK_a ~ 2, which allows conservation of phosphate metabolites within a cell membrane. Organic phosphate esters are involved in biochemical reactions as energy carriers, coenzymes, or intermediates. They are involved in photosynthesis, carbohydrate and lipid metabolism, the nitrogen cycle, and other biochemical reactions involving energy transfer. Organic phosphate species belong to a few broad compound classes. For example, orthophosphate monoesters, which have one ester linkage, include sugar phosphates, mononucleotides, and inositol phosphates.

Adenosine triphosphate (ATP) is the most important energy carrier and is present in all life forms as a metabolic mediator between high-energy phosphate donors and low-energy phosphate acceptors. Creatine phosphate and phosphoenol pyruvate also belong to main reservoirs of biochemical energy. Nutrient degradation with highly exergonic phosphoryl-transfer reactions is coupled to the formation of ATP from adenosine diphosphate (ADP) and orthophosphate with the help of enzymes known as kinases. Kinases catalyze the transfer of phosphoryl groups between ATP and other molecules.

Phosphates required for the phosphorylation of adenosine 5'-diphosphate (ADP) to ATP in algae can be stored in the cells by conversion to polyphosphates. Orthophosphate diesters have two esters per phosphate and are represented by phospholipids, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).

The sugar-phosphate backbone is responsible for the primary structure of the nucleic acids. A phosphate from one nucleotide monomer connected to the sugar (deoxyribose or ribose) of the following nucleotide continue in this alternating sequence with long chains in DNA and RNA structure. The cell membranes also consist of phospholipids, which are bound together by divalent cations. Animal bone formation, buffering function in urine and blood are also metabolic functions of inorganic phosphates. Catabolic processes in which nutrients are broken down release energy which is then transferred to phosphorus-containing energy carriers.

The stored energy is subsequently used during anabolic biosynthetic processes where the products from catabolism are reassembled back into proteins, lipids, polysaccharides, and other molecules. Inorganic polyphosphates contain a phosphoanhydride bond, which is energetically similar to that of ATP and is used as a phosphate reserve. Orthophosphate is transported to the cell by the uptake system and converted into ATP by adding a phosphate ion to ADP.

The excess of orthophosphate is stored as polyphosphate inside the cell. Microorganisms accumulate reserve phosphorus compounds in the form of polyphosphates, and in some archaea and bacteria phosphorus is stored even as magnesium phosphate during phosphorus excess periods in the environment. Some strains of cyanobacteria have the ability to store phosphorus as polyphosphates for the events of external phosphorus shortage. Such ability gives the cyanobacteria a competitive advantage over other phytoplankton when the phosphorus availability is low.

Phosphorus is an important factor in the structure of certain cells. For instance, carbohydrate monomers and phosphate form covalent ester links creating a rigid structure of the prokaryotes cells, where the organically bound phosphate is found in the cell walls. Pyrophosphate is a waste product of the macromolecular biosynthesis. The incorporation of phosphorus into the cell takes place in several steps. Alkaline phosphatase converts organic and inorganic phosphoesters outside of the cell, at the cell surface, or in the periplasm.

Another important role of polyphosphates in living organisms are chelation of metal ions (e.g. Cd²+), decreasing their toxicity and also as a buffer against alkali ions.