Calcium works where membranes meet

A doubly positive particle and its docking sites

In solution, calcium carries two positive charges. Almost everything the element does in the body follows from this single property: a doubly charged cation can hold two negatively charged groups at the same time and thereby keep them at a fixed distance from one another. It acts like a clamp between two parts that would otherwise repel each other.

The organism needs exactly such clamps at cell membranes. Their outer surface is occupied by phosphate groups and is therefore negative. Proteins that are meant to dock there bring negative side chains of their own — and can only approach the membrane once a cation bridges the electrical resistance between them. Calcium fills this role at several sites in the body, and the European Union has assigned an authorized claim to two of them.

Analcimfeld is an editorial offering and does not sell any products for ingestion. What is offered is a text. The following sections describe biochemistry and food science; wherever a health-related statement appears, it is given in its fixed wording in a separate block and is marked as such.

From the binding site to the label

Four passes, in the order in which they build on one another. The first two each end with the sentence the regulation provides for that area.

1. Ca · Atomic number 20

   Gla domains: negatively charged pockets waiting for a cation

   Several proteins of the coagulation cascade carry, at their start, a region of ten to twelve unusual amino acids. It is called the Gla domain, after its building block gamma-carboxyglutamate. Each of these building blocks has not one but two carboxyl groups — and thus a pocket into which a calcium ion settles.

   Once these pockets are occupied, the domain folds over and turns a group of water-repellent side chains outward. Only in this form does the protein attach to the phospholipid surface on which the next steps take place. Without occupied pockets the molecule remains free in the plasma and does not find its workplace.

   “Calcium contributes to normal blood clotting”

   In accordance with Regulation (EU) No 432/2012

2. Ca · divalent cation

   The influx at the presynaptic terminal

   At the end of a nerve fiber, vesicles lie ready, filled with neurotransmitter. They wait for a single event: the brief opening of voltage-gated calcium channels in the membrane. Because the concentration outside the cell is many times higher than inside, a small amount of calcium flows in immediately.

   This influx stays tightly confined in space. The ions reach only a narrow zone immediately beneath the membrane — and there sits a protein designed precisely to detect them. It draws the vesicle membrane and the cell membrane together until the two fuse and the neurotransmitter is released into the cleft.

   “Calcium contributes to normal neurotransmission”

   In accordance with Regulation (EU) No 432/2012

3. Food science

   Calcium levels beyond the dairy counter

   In level tables, dairy products rank high, yet they are not the only rich group. Poppy seeds and unhulled sesame are, per hundred grams, considerably higher than hard cheese; tofu set with calcium sulfate matches yogurt on paper. Sardines are eaten together with their fine bones and bring the content of those bones with them.

   For herbs and leaves it is worth looking at the portion rather than the hundred-gram column: arugula reaches high table values but is rarely eaten in large amounts. This section reproduces compositions and deliberately contains no health-related claim, because none is provided for food science.

4. Reach of the wording

   Where the authorized wording ends

   Both quoted sentences are addressed to the general population and presuppose a sufficient intake. They quantify no effect, they name no amount, and they say nothing about what an additional intake would do in a person who is already well supplied.

   Nor may they be merged. Each claim in the Union Register belongs to one nutrient and one process; a joint formulation for both areas would not be covered by the regulation. That is why they appear on this page in two separate blocks and never in a single sentence.

Two paths that resemble each other physically

At first glance, a coagulation cascade in the plasma and a nerve ending in the brain have little in common. Yet if one considers what the calcium ion does in both cases, the same basic figure emerges: a protein is to attach to a membrane, and the ion provides the bridge for it. Only the location, the protein involved and the time scale differ.

The prothrombinase as an assembled unit

On the surface of activated platelets, several components assemble into a working unit: an enzyme, an accessory protein, the membrane itself and calcium ions as the connecting element. Only this combination converts prothrombin into thrombin, which then converts fibrinogen into fibrin. Isolated in a test tube, the same enzyme works orders of magnitude more slowly — the acceleration is due to the ordered attachment at the surface.

In laboratory protocols, calcium therefore still appears to this day under the historical designation Factor IV. The name dates from a time when the components of the cascade were numbered in the order of their discovery, and it persists in the specialist literature to the present day.

The sentence the European Commission has authorized for this area reads, unchanged: “Calcium contributes to normal blood clotting” — In accordance with Regulation (EU) No 432/2012. It describes a contribution to a process that takes place anyway, and not a change in its pace.

Synaptotagmin and the time window of a millisecond

At the nerve ending, the vesicle carries a protein called synaptotagmin, which can take up several calcium ions in two segments. Once these sites are occupied, the protein dips a loop into the opposing membrane and brings the two fatty layers so close together that the pre-tensioned filament proteins of the SNARE group complete the fusion.

The speed is remarkable. Less than a millisecond passes between the opening of the channels and the release of the neurotransmitter. This is possible because channel and vesicle are pre-sorted in space: the ion has to travel only a few dozen nanometers before it meets its sensor.

Here too the regulation prescribes the wording: “Calcium contributes to normal neurotransmission” — In accordance with Regulation (EU) No 432/2012. It makes no mention of attention, memory or mood, and the sentence cannot be extended in that direction either.

Where the comparison between the two paths ends

The similarity of the two mechanisms is a finding of basic research, not an admissible argument in the sense of labeling. It does not follow from it that one claim could be transferred to the other area. The European Food Safety Authority examined both relationships separately, and the Commission entered them separately into the Union list.

The biochemical description on this page is editorial text. It explains processes and places terms in context without promising any effect. Usable in the sense of the regulation are only the two quoted sentences, each in its fixed version and each assigned to calcium.