Question

Collagen found in basement membranes has molecular arrangements and structure different from that found in other collagens.

1. Name two molecular/structural differences characteristic of basement membrane collagen.

2. What properties do these differences confer on basement membranes, and why is this important?

Answer 1

1.

• Collagen is a molecule that is composed of three polypeptide chains, each of which contains one or more regions characterized by the repeating amino acid motif (Gly-X-Y), where X and Y can be any amino acid. This motif allows the chains to form a right-handed triple-helical structure.
• Basement membranes are specialized structures found at tissue boundaries, underlying epithelial, endothelial, fat, muscle and nerve cells. Seen by electron microscopy, they are relatively thin sheets composed of collagen IV, laminins, heparin sulphate proteoglycans and nidogens. Collagen IV molecules are longer than the fibrillar collagens, and they contain several discontinuities in the (Gly-X-Y) repeat. There are six different collagen IV chains.
• The classical fibrillar collagens include: type I, found in skin, tendon, bone, cornea, lung and the vasculature; type II, limited essentially to cartilage; type III, found in embryonic skin, lung and blood vessels; type V, found in cornea; The common characteristic of the classical fibrillar collagens is a long central triple-helical region in each α chain, consisting of a continuous (Gly-X-Y)n repeat, where n is 337–343 (depending on collagen type)

2. Basement membranes are ubiquitous, thin, sheetlike structures found frequently under epithelial and endothelial cell linings but also surrounding many cell types such as muscle, nerve, and fat. The differnce in molecular structure provided different mechanical functions for each collagen based structure;

• tendons to have high tensile strength with oriented fibrils and high collagen low GAG
• cartilage to withstand compression with low collagen and high GAG content
• other collagen fibrils great strength in tension and weak in torsion and flexion
• The fibrillar collagens has extraordinary swelling capacity, tensile strength , unusual resistance from proteolytic attack.
• Basement membrane can withstand high tensile and compressile stresses. It acts as mechanical barrier preventing malignant cells from intrusion. Development of new blood vessels takes place in Basement membrane. Besides providing structural support and protection it also performs cell signalling. They form an important part in kidney glomeruli, lung alveoli.