Collagen Synthesis

The biosynthetic pathway responsible for collagen production is a very complex one.[4,8] Each specific collagen type is encoded by a specific gene; the genes for all of the collagen types are found on a variety of chromosomes. As the messenger RNA (mRNA) for each collagen type is transcribed from the gene, or DNA "blueprint," it undergoes many processing steps to produce a final code for that specific collagen type. This step is called mRNA processing. Once the final pro-alpha chain mRNA is produced, it attaches to the site of actual protein synthesis. This step of the synthesis is called translation. This site of pro-alpha chain mRNA translation is found on the membrane-bound ribosomes also called the rough endoplasmic reticulum or rER. Like most other proteins that are destined for function in the extracellular environment, collagen is also synthesized on the rER (Figure 2, step 1).

(Enlarge Image) Figure 2. The intracellular and extracellular events involved in the formation of a collagen fibril. Copyright 1994 from Molecular Biology of the Cell, Third Edition , by Alberts, Bray, Lewis, Raff, Roberts, Watson (eds). Reproduced by permission of Routledge, Inc., part of The Taylor & Francis Group.

A precursor form of collagen called procollagen is produced initially.[9] Procollagen contains extension proteins on each end called amino and carboxy procollagen extension propeptides. These nonhelical portions of the procollagen molecule make it very soluble and therefore easy to move within the cell as it undergoes further modifications. As the collagen molecule is produced, it undergoes many changes, termed post-translational modifications.[4,8] These modifications take place in the Golgi compartment of the ER.

Collagen, like most proteins that are destined for transport to the extracellular spaces for their function or activity, is produced initially as a larger precursor molecule called procollagen.[9] Procollagen contains additional peptides at both ends that are unlike collagen. On one end of the molecule, called the amino terminal end, special bonds called disulfide bonds are formed among three procollagen chains and insure that the chains line up in the proper alignment. This step is called registration. Once registration occurs, the three chains wrap around each other forming a string-like structure.

One of the first modifications to take place is the very critical step of hydroxylation of selected proline and lysine amino acids in the newly synthesized procollagen protein (Figure 2, step 2). Specific enzymes called hydroxylases are responsible for these important reactions needed to form hydroxyproline and hydroxylysine. The hydroxylase enzymes require Vitamin C and Iron as cofactors.[10] If a patient is Vitamin C deficient, then this reaction will not occur. In the absence of hydroxyproline, the collagen chains cannot form a proper helical structure, and the resultant molecule is weak and quickly destroyed.[11] The end result is poor wound healing, and the clinical condition is called scurvy.[12] The current recommended daily allowance for Vitamin C is 60mg; however, 200mg may be optimal.[13,14]

Some of the newly formed hydroxylysine amino acids are glycosylated by the addition of sugars, such as galactose and glucose.[15] The enzymes that catalyze the glycosylation step, galactosyl and glucosyl transferases, require the trace metal manganese (Mn+2). The glycosylation step imparts unique chemical and structural characteristics to the newly formed collagen molecule and may influence fibril size.[16] It is of interest to note that the glycosylation enzymes are found with the highest activities in the very young and decrease as we age.[17]

While inside the cell and when the procollagen peptides are intact, the molecule is about 1,000 times more soluble than it is at a latter stage when the extension peptides are removed.[18] This high degree of solubility allows the procollagen molecule to be transported easily within the cell where it is moved by means of specialized structures called microtubules to the cell surface where it is secreted into the extracellular spaces.[19]

As the procollagen is secreted from the cell, it is acted upon by specialized enzymes called procollagen proteinases that remove both of the extension peptides from the ends of the molecule.[20] Portions of these digested end pieces are thought to re-enter the cell and regulate the amount of collagen synthesis by a feed-back type of mechanism.[21,22] The processed molecule is referred to as collagen and now begins to be involved in the important process of fiber formation.

In the extracellular spaces, another post-translational modification takes place as the triple helical collagen molecules (Figure 1) line up and begin to form fibrils and then fibers. This step is called crosslink formation and is promoted by another specialized enzyme called lysyl oxidase (Figure 3).[23] This reaction places stable crosslinks within (intramolecular crosslinks) and between the molecules (intermolecular crosslinks). This is the critical step that gives the collagen fibers such tremendous strength. On a per weight basis, the strength of collagen approaches the tensile strength of steel!

(Enlarge Image) Figure 3. The intramolecular and intermolecular cross-links formed within a collagen fibril. Copyright 1994 from Molecular Biology of the Cell, Third Edition , by Alberts, Bray, Lewis, Raff, Roberts, Watson (eds). Reproduced by permission of Routledge, Inc., part of The Taylor & Francis Group.

One can visualize the ultrastructure of collagen by thinking of the individual molecules as a piece of sewing thread. Many of these threads are wrapped around one another to form a string (fibrils). These strings then form cords; the cords associate to form a rope, and the ropes interact to form cables. The structure is just like the steel rope cables on the Golden Gate bridge. This highly organized structure is what is responsible for the strength of tendons, ligaments, bones, and dermis.

When the normal collagen in our tissues is injured and replaced by scar collagen, the connective tissue does not regain this highly organized structure. That is why scar collagen is always weaker than the original collagen. The maximum regain in tensile strength of scar collagen is about 70 to 80 percent of the original.[24] Collagen synthesis and remolding (see below) continue at the wound site long after the injury. The body is constantly trying to remodel the scar collagen to achieve the original collagen ultrastructure that was present before the injury. This remodeling involves ongoing collagen synthesis and collagen degradation. Anything that interferes with protein synthesis will cause the equilibrium to shift, and collagen degradation will be greater than collagen synthesis. For example, patients who are malnourished or patients receiving chemotherapy may experience wound dehiscence, because the wound site will become weak due to a shift in the balance toward collagen degradation. It is of interest to note that when wounds in the fetus heal, they do so in such a manner that the original collagen ultrastructure is achieved.[25] If only we understood more about the biology and mechanisms responsible for the rapid and optimal wound healing response seen in the fetus, we would have greater insight into the management of adult wounds.[26]