Notes on a redox-inert metal
Iron and copper hand off electrons in metabolism and pull them back again. Zinc does not. It occurs in a single charge state and comes out of every bond unchanged. This page follows the question of what that means for a cell — and where the European register entry on this element draws its line.
To the full ConspectusA zinc ion carries two positive charges and keeps them, no matter what surroundings it finds itself in. The reason lies in the build of its electron shell: the shell in question is completely filled. There is nothing there to give away and no free space to take anything on. Inorganic chemistry calls a particle like that redox-inert.
That leaves this element fairly alone in its group. Iron swings between two states, copper does the same, manganese knows several. Exactly that switching makes them carriers of electrons, and exactly that explains why their unbound form stays unwelcome inside a cell. Zinc can sit in the same kind of pocket without any electrons changing hands.
Covellinera writes for adults who want to get to the bottom of a European wording. Gathered here is what the technical literature sets out on the binding chemistry of this ion. At the end the register text stands beside it, uncut and with nothing added.
An adult body holds roughly two grams of this element, the largest part of it in skeletal muscle and bone tissue. That figure says nothing, though, about how much lies around unbound inside a single cell. For a long time it could not be established at all: every procedure that had to break a cell open tore bound metal out along with it and therefore returned numbers that were too high.
Usable values only arrived with glowing probes that can be introduced into a living cell and change their color as soon as an ion docks. Since then the figures have moved in the range of picomoles per liter. That is an order of magnitude at which, on paper, only single unbound particles fall to a whole cell; everything else hangs on proteins.
In most cases the ion is held in place by the amino acid cysteine. Its side arm ends in a sulfur group that is present as a thiolate; four such groups can enclose a zinc ion together. Mixed forms are frequently described as well, in which two sulfur partners and two nitrogen partners from histidine come together.
From that follows the point this page is about. When an oxidizing compound meets a holder of that kind, it reacts with the sulfur and not with the metal. The sulfur atoms link up with one another, the clamp opens, the ion comes free. It leaves the site in exactly the charge state in which it arrived there.
What stood in context up to here now stands one item below the other — three places where the description can be pinned down.
In numerous proteins the ion serves not as a tool but as a clamp: it holds two stretches of a chain at a fixed angle to each other. As long as it sits there, the pocket is taken. A metal that could adopt the same geometry but passes electrons along in the process finds no way in at that site any more.
Metallothionein consists of around sixty building blocks, and about a third of them are cysteine — a share that hardly occurs anywhere else at that height. Two domains separate from one another are described, holding up to seven zinc ions between them. When more is on hand, the cell makes more of this protein; that is set in motion through a reading factor the literature calls MTF-1.
Everything named so far comes from technical articles. It explains why this subject area was investigated, and is not itself a permissible statement about a food. Permissible is only the following line, exactly as the European register carries it:
“Zinc contributes to the protection of cells from oxidative stress”
EU-authorized wording · Regulation (EU) No 432/2012
An entry does not appear on this list by itself. Each one was preceded by an opinion from EFSA on exactly one pairing of substance and subject area; written out, that body is called the European Food Safety Authority. The line is carried under the identifiers 294 and 1758.
What is not written there can be listed briefly: no amount, no organ, no time span, no protein name. At no point does the text distinguish between men and women, and it knows no age below adulthood. Anyone using it may neither shorten it nor render it in their own words.
The full-length edition sets out the binding chemistry at its whole length, lays two tables beside it, answers reader questions that came in, and gives the register text in the original.
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