Three phosphate groups on one nucleoside

Adenosine triphosphate consists of a sugar unit, a base, and a chain of three phosphate units. That last part of the description is the reason why a macromineral with atomic number 15 shows up in nearly every biochemical equation of human metabolism.

Elemental phosphorus does not occur in the body. What is taken in through food and processed further in cells is always a phosphate ion: one phosphorus atom surrounded by four oxygen atoms, carrying a negative charge of one to three units depending on the acidity of its surroundings. Chemically, it is the anion of orthophosphoric acid.

This anion can be attached to other molecules and removed again without breaking apart the carrier molecule. It is exactly this property that makes phosphate the most widely used add-on in cell biochemistry — and it explains why the European Union has authorized wording for this element that refers to energy-yielding metabolism.

The following sections first set out the chemistry, then compile the key figures for the element, and finally reproduce the authorized wording exactly as it appears in the annex of the regulation.

Anhydride bonds and what their cleavage releases

When two phosphate groups are directly linked, they share one oxygen atom. This linkage is called a phosphoric anhydride. It is under strain because the neighboring negative charges repel each other and the bonding electrons cannot spread out evenly.

When such a bond is split by water, the fragments move into an energetically more favorable state. The difference between the starting and final state is the amount textbooks list as the standard free energy of hydrolysis; for adenosine triphosphate under cellular conditions it is about 50 kilojoules per mole. This number describes a thermodynamic quantity, not a sensation and not a physical condition.

In technical language, the term energy-yielding metabolism covers all the reaction pathways in which such transfers take place: glycolysis, the citric acid cycle, the respiratory chain, and the synthesis and breakdown pathways coupled to them. Phosphate takes part as a reactant in every one of these pathways.

ATP, ADP, and AMP each differ by one group

Adenosine monophosphate carries one phosphate unit, adenosine diphosphate two, adenosine triphosphate three. Between the first and second group, and between the second and third, lie the anhydride bonds mentioned above; the bond between the sugar and the first group, by contrast, is an ordinary ester.

At any given moment, the body holds only a few dozen grams of these compounds. The turnover is many times higher, because the same molecules are continuously split and reassembled. A single molecule goes through this cycle many thousands of times over the course of a day.

Substrate-level phosphorylation and the route through ATP synthase

Two processes are available for rebuilding. In the first, a phosphate group that is already bound is transferred directly from an intermediate of the breakdown pathway onto adenosine diphosphate — this happens twice in glycolysis and once in the citric acid cycle. It requires no membrane system, only a suitable enzyme.

In the second process, a proton gradient is first built up across the inner mitochondrial membrane. As the protons flow back through the channel section of ATP synthase, its movable part rotates, and the enzyme links inorganic phosphate to adenosine diphosphate. By far the greater part of daily regeneration runs along this route.

These relationships are what the one entry the European Commission has authorized for phosphorus in this context refers to: “Phosphorus contributes to normal energy-yielding metabolism.” Per Regulation (EU) No 432/2012. The sentence is tied to this single element and describes a contribution to a process that takes place anyway when intake is adequate.

Inorganic phosphate as the counterpart of every cleavage

Each time an anhydride bond is split, a free phosphate ion forms alongside the shortened nucleotide. In technical terms it is referred to as inorganic phosphate and written with the abbreviation Pi. It stays dissolved in the cell water and is ready for the next building step.

This leads to a peculiarity that is easily overlooked in reference works: the phosphate ion is not used up in these reactions but shuttled back and forth between a bound and a free state. What the body takes in through food does not make up for consumption, but for losses via the kidneys and gut and for incorporation into hard tissue and newly formed cell components.

• Sugar phosphates stand at the start and in the middle of glucose breakdown.
• Nucleotides carry the groups on which the transfers take place.
• Phosphorylated proteins switch between two states by attaching and detaching a group.
• Inorganic phosphate shuttles between these forms and the free cell water.