A proteoglycan is assembled in the Golgi apparatus and ends up in the bone matrix. This long read sets out the steps at which glycosyltransferases need a manganese ion along the way, and it quotes the EU wording on manganese with nothing added.
Throughout life, bone is taken down and laid down again in small patches. Osteoclasts handle the removal; osteoblasts handle the new deposits. What these cells secrete first is not mineral but a soft organic material, osteoid. In humans it usually takes about ten days, sometimes a little longer, before that material mineralizes.
About nine-tenths of the organic bone matrix is a single fibrous protein. The remainder is split between non-fibrous proteins, osteocalcin and osteopontin among them, and proteoglycans. In sheer quantity proteoglycans are a minor share, yet their influence on how the matrix is arranged has been studied closely.
Both are counted among the small leucine-rich proteoglycans. Their protein core is fairly short. Decorin typically carries one glycosaminoglycan chain, biglycan two. In bone, those chains are mostly chondroitin sulfate or dermatan sulfate.
Work with cell cultures and mice indicates that both molecules help organize the matrix and shape the setting in which crystals later form. Stripped of their sugar chains, they lose much of that capacity.
A glycosaminoglycan chain cannot grow until it has a starting point on the protein core. For chondroitin, dermatan and heparan sulfate, that starting point is made of the same four sugars, a so-called tetrasaccharide. The table lists the enzymes involved in the order in which they act.
| Step | Unit added | Enzyme | Metal ion |
|---|---|---|---|
| 1 | Xylose on serine | Xylosyltransferase (XYLT1, XYLT2) | divalent ion; in the lab, manganese among others |
| 2 | Galactose | β4-galactosyltransferase 7 (B4GALT7) | manganese in the active site |
| 3 | Second galactose | β3-galactosyltransferase 6 (B3GALT6) | manganese-dependent |
| 4 | Glucuronic acid | Glucuronosyltransferase I (B3GAT3) | manganese seen in the crystal structure |
| 5 | Chain elongation | Chondroitin synthases | divalent ion required |
Many glycosyltransferases with the so-called GT-A fold contain a short run of aspartate, any amino acid, and aspartate again. The two acidic side chains hold the metal ion in place. The ion in turn binds both phosphate groups of the UDP-sugar and shields their negative charge. That loosens the bond between sugar and UDP, so the sugar can pass onto the growing chain.
In a test tube, some of these enzymes also accept other divalent ions. For several of them, though, manganese yields the highest activity, which is put down to the size of the ion and the binding geometry it favors. Which ion is actually bound inside a living cell also depends on concentrations in the Golgi apparatus, and those are set by dedicated membrane proteins.
The link between manganese and the skeleton first came to light in animal nutrition. In 1936, American researchers reported that chicks raised on low-manganese feed developed changes in their leg bones; the condition was named perosis. Later studies traced the connection in part to reduced production of chondroitin sulfate.
Trials with feed deliberately stripped of manganese say nothing about people who eat an ordinary diet. Marked manganese deficiency in humans has only been reported in isolated cases. The animal data explain the biology behind the topic, but they give no reason to expect anything from extra intake.
Within the European Union, a health claim about a food is allowed only if it appears on a closed list. That list was published as Regulation (EU) No 432/2012. Before it came out, EFSA checked each entry to see whether the scientific evidence for the link was sufficiently solid. For manganese and bone, this is the authorized text:
“Manganese contributes to the maintenance of normal bones”EU-authorized wording · Regulation (EU) No 432/2012
Only a food containing at least a significant amount of manganese may carry the claim. The yardstick is the EU nutrient reference value of 2 mg per day set out in Annex XIII of Regulation (EU) No 1169/2011; as a rule, one serving has to provide at least 15% of that figure.
The wording makes no statement about diseases, about complaints, or about groups of people with special requirements. Nor does it permit linking manganese to body functions that are covered by their own, separately assessed entries.
In 2013, EFSA derived an adequate intake of 3 mg of manganese per day for adults in the EU. That figure differs from the 2 mg EU nutrient reference value used on labels; the two numbers serve different purposes.
Only a small fraction of the manganese in food makes it into the blood; a few percent is the figure usually given. Whatever the body does not need, it mostly releases through the liver into bile.
| What you eat | Manganese, mg per 100 g, rounded |
|---|---|
| Wheat germ | approx. 13 |
| Hazelnuts | approx. 5 |
| Rolled oats | approx. 4 |
| Chickpeas, dried | approx. 2 |
| Spinach, raw | approx. 1 |
Actual content shifts with variety, soil and processing.
EFSA has so far not set a tolerable upper intake level for manganese, because the available data were not enough to do so. Anyone taking dietary supplements should keep to the serving size on the package.
Yes. A sizable share of the body’s total manganese is located in the skeleton. Whether the body taps that share as a deliberate reserve has received little study in humans.
Health claims are written for consumers. They name a body function, not the molecular route that leads to it. The enzyme chemistry on this page is there only to explain.
Nothing points that way. The enzymes need an adequate supply, but extra manganese on hand does not make them work any faster. The EU claim is about maintaining a normal state.
Adults who want to follow the biochemistry without wading through specialist journals. It does not take the place of one-on-one advice from a doctor or a nutrition professional.
What you see here is an excerpt. The document expands on each chapter and adds a glossary of technical terms plus the complete list of sources.
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