Question

Explain how growth and shrinkage of actin filaments differs from that of microtubules? Explain 1) the process by which collagen is synthesized and secreted, and 2) how loss of collagen affect the human body.

Answer 1

3. The basic mechanism of growth and shrinkage in both actin filaments and microtubules is similar. However, the factors that control the growth and shrinkage are different in both the cases.

Th growth and shrinkage of microtubules require α/β tubulin dimers and are mediated by ϒ-tubulin ring complex. The growth occurs away from microtubule organizing centres in the + direction. The growth or shrinkage depends on the critical concentration of α/β tubulin dimmers. Critical concentration is the concentration where there is no net growth or net shrinkage. If the concentration is greater than the critical concentration, it results in microtubule growth. If it is lower, vice versa i.e, shrinkage occurs.

Whereas the growth and shrinkage of actin filaments require G-actin molecules. The critical concentration of G-actin effects the growth and shrinkage of actin filaments in a similar fashion. If the concentration is higher than critical concentration at both + end and – end, growth occurs at both ends. If it is lower at both ends, shrinkage occurs at both ends.

If G-actin concentration is higher than the critical concentration at plus end and lower at minus end, growth occurs at plus end and shrinkage occurs at minus end. This is known as tread milling. The rate of growth and shrinkage varies that depends upon the respective concentrations.

If the speed of growth equals the speed of shrinkage, it is known as steady state tread milling.

4.

Collagen is produced by fibroblasts, odontoblasts and osteoblasts.

Collagen contains specific amino acids: Gly-Pro-X, where X may be any amino acid.

Synthesis of collagen occurs at ribosomes on rough endoplasmic reticulum. Two peptide chains, alpha 1 and alpha 2 chains along with signal peptides are formed. This is known as preprocollagen. Signal peptides are cleaved immediately when they are released into ER and now called pro alpha chains. Hydroxylation of lysine and proline occurs in lumen which requires vitamin C as a cofactor. Then glycosylation of hydorxylysine occurs. Two alpha 1 chains and one alpha 2 chain form triple helical structure in ER and now called procollagen. This is transported into golgi where it is packaged and secreted by exocytosis.

Collagen secreted out of the cell where procollagen peptidase cleaves some peptides and forms tropocollagen. Tropocollagen molecules gather to form collagen fibres.

Scurvy is the deficiency caused due to defective collagen. Vitamin C acts as cofactor in collagen synthesis. The deficiency of vitamin C results in synthesis of non functional collagen. It does not produse strong connective tissues.