What Lies Between the Cells

Connective tissue is recognized less by its cells than by what those cells release to the outside. Fibroblasts secrete proteins and long sugar compounds that arrange themselves beyond the cell membrane into a material of their own, the extracellular matrix. We come across it in the walls of blood vessels, in the capsules of internal organs and in the loose tissue that wraps organs and lets them glide against one another.

Seen mechanically, this matrix is a composite. Collagen fibrils take up pulling forces, elastic fibers spring back to their starting length after being stretched, and a water-rich filler made of proteoglycans keeps the gaps open. Lying loosely side by side, these components would accomplish little. They are chemically linked, dismantled and renewed, and several of those steps rely on enzymes that cannot catalyze anything without a particular metal ion.

Hemimorphitara follows two such enzymes all the way into their active site: a copper-containing enzyme that sets up cross-bridges between fiber proteins, and a manganese-containing enzyme that splits fragments left over from collagen breakdown back into building blocks. Each of the two elements has its own authorized statement from the European Union, and each is quoted separately here.

One Collagen Chain, from Cross-Linking to Breakdown

Collagen leaves the cell as a precursor molecule with bulky end pieces. Only after these are trimmed off outside the cell do the strands gather into fibrils. At that point they still lie next to each other without being joined. What turns them into one coherent material happens over the following hours and days on the surface of the growing fibril.

Allysine as a Chemical Anchor Point

Lysyl oxidase is secreted as an inactive precursor and switches on once a propeptide has been split off. It singles out lysine residues at the untwisted ends of collagen molecules. Consuming oxygen, it detaches the amino group of the side chain, and what remains is a reactive aldehyde group. Ammonia and hydrogen peroxide form as by-products.

The next step needs no enzyme at all. The aldehyde reacts with an amino group or with a second aldehyde on the neighboring molecule. Two-way links form first, and with time they react further into three-way bridges such as pyridinoline. The copper ion stays inside the enzyme the whole time; without it, lysyl oxidase has no catalytic activity. For copper, the EU has authorized the following statement:

The Two-Metal Center of Prolidase

Collagen is broken down and built anew on a continuous basis. Collagenases and other proteases chop the strands into smaller and smaller pieces until only two-unit fragments are left. When such a dipeptide ends in proline or hydroxyproline, the stiff ring of that amino acid locks the peptide bond in a position that ordinary dipeptidases cannot get at.

Prolidase is cut out for precisely this bond. Its binding pocket houses two manganese ions linked through a shared hydroxide ion. That hydroxide attacks the carbon atom of the peptide bond, and the dipeptide comes apart into its two amino acids. The proline can then be built into a collagen chain once again. For manganese, the authorized statement reads:

Sugar Chains for the Water-Rich Filler

Manganese ions show up in a second place, too. In the Golgi apparatus, glycosyltransferases lengthen the sugar chains of proteoglycans by adding activated sugar units one by one. Several of these enzymes bind their sugar donor only when a divalent metal ion, often manganese, lines up the donor’s phosphate groups.

The enzymes described above are scientific background. The regulation itself names none of them; it words a separate sentence about connective tissue for each element, and every one of those sentences holds only for the element it names.