Eighty kilograms of ATP a day — and not a gram of it in reserve

Why cells cannot store energy

At any given moment the organism carries only a few dozen grams of the molecule from which it meets its work: adenosine triphosphate, ATP for short. Measured against turnover that is a minute amount. Over the course of a quiet day an adult breaks this molecule down and builds it up again on an order of magnitude approaching their own body weight; under physical strain the figure rises markedly above that. There is no depot. What is needed must arise in the same minute.

The greater part of this resupply takes place in the mitochondria, those enclosed spaces inside the cell whose inner membrane is laid into deep folds. A liver cell contains a few hundred of them, a heart muscle cell many times more, a mature red blood cell none. At this inner membrane runs the process called the respiratory chain in physiology: electrons from food are passed along a series of protein complexes until they finally meet oxygen.

Involved in this are not only proteins but also metal ions and other cofactors that belong firmly to the enzymes. Two of the minerals named in this connection are carried by the Union list with an entry of their own: Magnesium contributes to normal energy-yielding metabolism — Pursuant to Regulation (EU) No 432/2012. And in a second, independent entry: Iron contributes to normal energy-yielding metabolism — Pursuant to Regulation (EU) No 432/2012. Both statements apply each to their element and cannot be combined into a joint claim.

How a charge gradient arises at the inner membrane

The breakdown pathways of sugar, fatty acids, and amino acids all end at the same place: at two carrier molecules that have taken up electrons and must give them up again. This release happens at the inner mitochondrial membrane, and it happens not in one step but in small portions across several protein complexes connected in series. At each transfer, part of the energy released is used to move protons from the inner space into the intermembrane space.

Four complexes and the path of the electrons

At the start stands a complex that takes electrons from NADH; a second receives them from the breakdown of succinic acid. Both pass them on to a small fat-soluble molecule that wanders around in the membrane and brings them to the third complex. From there it goes via a water-soluble transport protein to the fourth and last, which transfers the electrons to oxygen. From four electrons, four protons, and one oxygen molecule, two molecules of water arise.

The proton gradient built up in the process drives a fifth protein: a kind of molecular rotor that reassembles adenosine diphosphate and free phosphate into ATP. The coupling of electron flow and phosphate transfer via a membrane potential is regarded as one of the fundamental insights of twentieth-century biochemistry.

Where the iron sits in this chain

Three of the four complexes contain iron-bearing components. Some of them are heme groups, as also occur in the blood pigment; others are assemblies of iron and sulfur atoms embedded in the protein framework. In both cases the function rests on the same trick: the atom takes up an electron, passes it on, and returns to its initial state without being consumed itself. The first complex alone carries eight such assemblies. It is at this level that the authorized statement applies: Iron contributes to normal energy-yielding metabolism — Pursuant to Regulation (EU) No 432/2012.

Why the enzyme does not grip the free nucleotide

ATP carries three phosphate groups with strongly negative charge, which repel one another. In the cell the molecule is therefore hardly ever present alone, but together with a divalent magnesium ion that lays itself between two of the phosphate groups and shields the charge. Only this association fits into the binding pocket of the enzymes that pass phosphate on. The rotor that assembles ATP at the inner membrane also works with this form. On this point the authorized claim: Magnesium contributes to normal energy-yielding metabolism — Pursuant to Regulation (EU) No 432/2012.

What releases the fiber again after tensing

Muscle tissue is among the consumers that change their requirement fastest. On tensing, two filament systems slide into one another; on releasing, calcium must be carried back into an internal store, and this pump works with the same nucleotide-magnesium association as the enzymes of metabolism. Around a quarter of the body's magnesium stock therefore lies in the musculature. The European Union carries a separate entry on this, distinct from energy metabolism: Magnesium contributes to normal muscle function — Pursuant to Regulation (EU) No 432/2012. A corresponding claim relating to musculature does not exist for iron.