The Tree's Knees

Knees Are Like Trees, Except...

Bend down and take a handful of that dark, rich soil. (I am thinking Iowa here, not desert Idaho.) There are yards of humus-laden soil beneath your feet. The dark color and nutrient-absorbing properties are provided by the second most abundant biological macromolecule on Earth, lignin. Lignin is part of the secondary walls of plants and is produced by free radical polymerization. So, unlike other macromolecules, proteins, polysaccharides and nucleic acids, lignin cannot be hydrolyzed and thus accumulates in soil. The only reason that it is number two in abundance is that the first place goes to cellulose, which is not degraded because it is present in dehydrated fibers that are not readily attacked by microbe enzymes.
Lignins make trees stiff and resistant to compression. That is why leaves can suck on xylem vessels and the vessels don’t collapse. The lignin molecules, which can reach the dimensions of a Sequoia are rigid and non-compressible. But since they are so stiff and neither extend nor shrink, plants can’t jump.
If you want to jump, you need cartilage that can stretch and bounce back when compressed. Articular cartilage is made up of hyaluronan, a polysaccharide with uronic acid residues related in structure to those of the pectin (polygalacturonic acid) in plant walls; collagen fibers, similar in rigidity to the cellulose fibers of plant walls; and proteogylcans, similar to the hemi-celluloses in plant walls. The bounceable difference is that joint cartilage has sulfated polysaccharides (heparan sulfate, chondroitin sulfate, dermatan sulfate) attached to the protein backbone (aggrecan) of its proteoglycans. The sulfates make all the difference -- no sulfate, no bounce. Sulfates have a net negative charge and lots of oxygens for hydrogen bonding. So, instead of just having a simple hydroxyl (OH-) on the sugars, there is a sulfate that forms a huge, multilayered shell of water molecules. In fact, the articular cartilage doesn’t have a lot of free water, so that if you squeeze it, it is like closed-cell foam and not a water-saturated sponge.
Sulfation does for knee cartilage what lignin does for trees.

ECM Basics -- Cartilage Components

Cartilage, and all of the stuff that cells dump onto their outer surfaces by secretion, is made up of proteins and polysaccharides. The polysaccharides, polymers of simple sugars, are of two types, big acidic (negatively charged) polymers (hyaluronans) and backbone proteins decorated with medium-sized acidic polysaccharides (heparin sulfate and keratan sulfate). [The acidic quality (-COOH) of the polysaccharides will become more significant later because of its ability to form ester linkages through borates.] The proteins are also of two types, huge hairy fibers of collagen and tri-headed, hydras that bind to the other polysaccharides and proteins.
Collagens make up most of the protein of the external material, the extracellular matrix. If meat, muscle tissue, is heated for a prolonged time, the connective tissue (not the actin and myosins of the muscle fibers) collagens are dissolved. If the collagen solution is cooled, the collagen molecules stick together in a disorganized way and instead of reforming thick, long fibers of hundreds of bundles of extended collagen protein molecules, a sponge of millions of cross-linked proteins is formed -- gelatin. There are about three dozen different collagen proteins, each coded by a different gene, but they are all produced according to the same basic pattern. Different cell types produce a different set of collagens and as a consequence the cells become embedded in a different extracellular matrix.
Collagen proteins are synthesized on ribosomes in the cytoplasm of most cells. As the collagen is synthesized by the addition of one amino acid at a time, the growing protein chain is pulled into the endoplasmic reticulum. The full length procollagen molecules are swept into vesicles and transported through a series of membrane structures where the ends of the proteins are cut off (the pieces removed are important) by enzymes (metaloproteinases) and amino acids along the protein are oxidized. Vitamin C is essential in the oxidation of proline to hydroxyproline, which takes place during transit through the Golgi. [Insufficient vitamin C can lead to the deficiency disease called scurvy. One of the characteristics of scurvy is disruption of connective tissue -- loose teeth, bleeding. Similarly, heart disease may be a form of chronic, subsymptomatic scurvy due to vitamin C deficiency and inadequate repair of repetitively stretched coronary arteries.] Reteated sequences of glycine, proline, hydroxyproline, along the collagen, permit the protein to twist in triplets to form the collagen units that stack up into long, thick fiber after the matrix materials are secreted onto the surface of a cell. Different collagen proteins have slightly different amino acid sequences and are modified at different rates and assemblee in different ways into the fibers. Some are clipped, modified and stacked neatly into the fiber without loose ends, while others retain their ends and limit the size of the fibers as they form a shaggy surface layer. The loose ends can bind to other molecules in the matrix. Still other collagens retain their heads and form a hydra-like or three-fold caducious structure, with a twisted gly, pro, hyp repeat shaft. The heads bind the acidic polysaccharides and other components of the matrix.
The matrix components must be made, stored, secreted, assembled into functional matrix, change conformations in response to mechanical and chemical changes in the environment and be disassembled to permit replacement. Enzymes facilitate these processes and the proteolytic enzymes involved can be activated or inactivated by metal ions, hence they are named metaloproteinases and are typically called MMPs or matrix metaloproteinases. The metals involved are Zn++, Ca++, and Mg++. To further complicate the picture, the structures of the heads of the collagens are also dependent on the binding of divalent cations. Thus, it can be expected that the activity of the MMPs as well as their substrate collagens will be altered by the ion environment of the intracellular organelles, the region immediately next to the cytoplasmic membrane outside the cell (the pericellular region) and the more distant extracellular region. The mechanical stresses applied to the extracellular matrix may change the shapes of the collagen components and alter their ion binding properties with subsequent changes in the activity of the MMPs embedded in the matrix.
Clipping off the ends of collagens by MMPs releases small peptides of 40-400 amino acids in length. Many of these peptides have sequences that are related to sequences of peptide hormones, such as the cytokines. It is possible that the processing peptides released during matrix secretion, pericellular modifications and response to environmental changes my provide signals to neighboring cells embedded in the matrix. Connective tissue diseases may result from any abnormalities in the production of signalling molecules by the matrix cells.