Biochemistry - The Chemical Reactions of Living Cells Volume 2 - D. Metzler 1980

Biosynthesis; how new molecules are formed
Polymer biosynthesis and modification
Hydroxylation and other modifications of connective tissue proteins

Collagen, the most abundant protein in the body, constitutes the major part of the organic mass of Skin, tendons, Blood Vessels, bones, the cornea, the vitreous body of the eye, and various membranes. Elastin, a protein with similar properties, has been found in the elastic fibrils of Connective Tissues located in ligaments and blood vessel walls. Collagen is synthesized by fibroblasts and secreted into the extracellular space, where it polymerizes to form a tough, long-lived material [38a]. The intracellular precursor of collagen, procollagen, just like mature collagen (Chap. 2, Sec. B, 3, c), contains three chains. The predominant form of collagen in most tissues of most species (type I collagen) contains two a₁(I) chains and one a₂ chain, and is therefore designated as [a₁(I)]₂a₂. Cartilage collagen (type II collagen) contains three a₁ chains and is designated as [a₁(II)]₃. Type III collagen, found in various tissues, particularly in embryos, has the Structure [a₁(III)]₃ [38b].

Each procollagen chain (mol. wt. 140,000) contains over 1,000 amino acid residues. Specific hydroxylases (Chap. 10, Sec. G, 2, d) convert Certain Proline and Lysine residues within the procollagen chains into 4-hydroxyproline and hydroxylysine [Eqs. (11-27) and (11-28)]. Small amounts of 3-hydroxyproline are also formed in this process [38c]. Hydroxylation begins while the growing peptide chains are still attached to Ribosomes situated on the rough Endoplasmic reticulum. The hydroxylases are likely localized within the vesicles of The endoplasmic reticulum (ER) [39].

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Galactosyl units are transferred to certain hydroxyl groups of hydroxylysine side chains, and glycosyl units are subsequently transferred to some of the galactosyl groups. The three procollagen chains associate with one another to form a triple helix prior to their secretion from fibroblasts. In the extracellular space, procollagen is acted upon by two specific Enzymes—procollagen peptidases—which cleave off a polypeptide with a Molecular Weight of approximately 35,000 from the C-terminus of each of the three chains, and a polypeptide with a molecular weight of approximately 20,000 from the N-terminus of each chain [40–41]. The Amino Acid Composition of the cleaved Polypeptides differs significantly from that of the remaining collagen monomer (also referred to as tropocollagen), which consists of one-third Glycine and is rich in proline. These terminal polypeptides are linked to each other in procollagen molecules by disulfide bridges formed prior to cellular secretion [41a].

Collagen monomers are three-stranded filaments approximately 1.5 nm in diameter and 300 nm in length. Upon reaching their final destination, they become cross-linked to one another to form collagen fibers [40, 42–44]. This process is initiated by the copper-oxidase-catalyzed oxidation of the amino groups in the side chains of certain lysine and hydroxylysine residues to aldehyde groups [Eq. (11-29); Addendum 10-3]. The aldehyde groups participate in various reactions that lead to cross-linking between tropocollagen units and The formation of insoluble fibrils. One such reaction is aldol Condensation, accompanied by the elimination of a Water molecule [Eq. (11-30), step a]. If one of the two aldehydes participating in the aldol condensation is derived from hydroxylysine, two isomeric products are formed. The aldol condensation product can react further: the imidazole group of a Histidine side chain can add to the carbon-carbon double bond, and a lysine side chain can form a Schiff base with a free aldehyde. The outcomes of these two processes are illustrated in Eq. (11-30) (step b). In the final product, histidino-hydroxymerodesmosine, four side groups from different amino acid residues are linked together. In other cases, simple Schiff bases are formed between the aldehyde and ε-amino groups, or only two of the three reactions shown in Eq. (11-30) take place.

The cross-links appear to be distributed not randomly, but at specific positions, frequently oriented toward the ends of the collagen monomers. Histidine residues have been found in The polypeptide chains exclusively at positions 89, 929, and 1034. The variety and number of cross-links vary among different species.

Unusual cross-links have been discovered in elastin. Three aldehyde groups interact with a single lysine amino group via aldol condensation, dehydration, and oxidation reactions, yielding desmosine and isodesmosine. In the native elastin molecule, all four amino and carboxyl groups appear to be involved in peptide bond formation. The reader can readily deduce the mechanisms of desmosine and isodesmosine Biosynthesis [43a].

Addendum 11-B

Genetic Defects in Collagen Structure

When studying any protein that plays a role in organismal function, we inevitably encounter genetic issues. In the case of collagen, the likelihood of deleterious Mutations is heightened because this protein is encoded by more than one set of genes^a,b. At least four types of collagen, characterized by distinct molecular-level differences, have been identified, and it has been demonstrated that different collagen genes are expressed differentially across various tissues. For instance, cartilage collagen molecules consist predominantly of three identical a-chains, which differ in Amino Acid Sequence from the a₁ and a₂ chains of tendon and bone collagen. The skin collagen of infants, as well as the collagen of Heart Valves and large Arteries, contains two Other types of polypeptide chains.

A well-defined inherited anomaly of collagen is dermatosparaxis in cattle, a condition in which the animal's skin becomes extremely fragile. The collagen chains in this disorder lack an orderly structure and are therefore incapable of forming fibrils. This defect stems from an abnormality in procollagen peptidase, the enzyme responsible for cleaving the Peptides from the N-termini of collagen chains^c.

A human collagen-related disorder is Ehlers-Danlos syndrome, which in some cases is accompanied by joint hypermobility and spinal curvature. One form of this condition is caused by a deficiency of procollagen peptidase^d, whereas another results from a deficiency of type III collagen^e. The hydroxylysine content in the resulting collagen is abnormally low, which hinders proper cross-linking^f. When animals ingest seeds of the common sweet pea (Lathyrus odoratus), they can develop a condition known as lathyrism. Because humans consume sweet peas, this condition also occurs in humans, typically leading to spinal deformities and aortic rupture. It has been established that the biochemical effect of sweet pea seeds is due to the presence of β-cyanoalanine and its decarboxylation product, β-aminopropionitrile^f:

Although the exact MECHANISM OF ACTION of this compound remains unclear, there is strong reason to believe that it acts as an inhibitor of lysyl oxidase, an enzyme essential for cross-linking in collagen or elastin^g. A similar hereditary disorder has been discovered in mice, hypothesized to result either from insufficient lysyl oxidase activity or from a defect in copper METABOLISM^e. Furthermore, work by ^d suggested that one form of Ehlers-Danlos syndrome in humans is caused by deficient lysyl oxidase activity^d.

Currently, researchers are investigating The impact of structural abnormalities in collagen on human joint health. For example, it has been demonstrated that in osteoarthritis, a widespread disease, cartilage contains collagen with a-chains exhibiting a reduced degree of glycosylation rather than the normal [a₁(II)]₃ collagen type^h. Conversely, in the brittle bones of individuals suffering from one form of Osteogenesis Imperfecta, type III collagen is found alongside the normally present type I bone collagen^d.

According to available data, the content of type I collagen in fibrous atherosclerotic plaques of human arteries is elevated compared to its level in normal arterial walls (which primarily contain type III collagen).

^a Bailey A. J., Compr. Biochem., 26B, 297–423 (1968).

^b Miller E. J., Matukas V. J., Fed. Proc., Fed. Am. Soc. Exp. Biol., 33, 1197–1204 (1974).

^c von der Mark K., Bornstein P., JBC, 248, 2285–2289 (1973).

^d Müller P. K., Lemmen C., Gay S., Meigel W. N., EJB, 59, 97–104 (1975).

^e Pope F. M. et al., PNAS, 72, 1314–1316 (1975).

^f Bornstein P., Annu. Rev. Biochem., 43, 567–603 (1974).

^ж Narayanan A. S., Siegel R. C., Martin G. R., BBRC, 46, 745—751 (1972).

^з Nimni M., Deshmukh K., Science, 181, 751—752 (1973).

^и McCullagh К. A., Balian G., Nature (London), 258, 73—75 (1975).