In birds, scales, beaks, claws and feathers also contain β-keratin of the avian family. Phylogenetic studies of β-keratin sequences show that feather β-keratins evolved from scale β-keratins. The scale β-keratins form the basal group in avians. Duplication and divergence events then led to claw β-keratin genes, and further recombination resulted in new feather and feather-like avian β-keratin genes. Evidence for these duplication events comes from the correlation of feather β-keratin clade structure with their genomic loci.
Changes in β-keratins may have also influenced the development of powered flight. A recent study using molecular dating methods to link the evolution of β-keratin genes to that of feathers reveals that the avian β-keratin family began diverging from the crocodile family about 216 million years ago. But the feather β-keratin family did not begin diverging until 125 million years ago, a date consistent with the adaptive radiation of birds during the Cretaceous. β-keratins found in modern feathers have increased elasticity, a factor that may have contributed to their role in flight. Thus, the feathered ancestors of birds including Anchiornis and Archaeopteryx, whose flight capabilities have been questioned, would have had avian, but not feather β-keratins.
The small alvarezsaurid dinosaur Shuvuuia deserti showed evidence of a featherlike skin covering. Analysis by Schweitzer et al. (1999) showed that these featherlike structures consisted of beta-keratin.
- Dalla Valle L, Nardi A, Belvedere P, Toni M, Alibardi L (July 2007). "Beta-keratins of differentiating epidermis of snake comprise glycine-proline-serine-rich proteins with an avian-like gene organization". Dev. Dyn. 236 (7): 1939–53. PMID 17576619. doi:10.1002/dvdy.21202.
- Dalla Valle L, Nardi A, Toffolo V, Niero C, Toni M, Alibardi L (February 2007). "Cloning and characterization of scale beta-keratins in the differentiating epidermis of geckoes show they are glycine-proline-serine-rich proteins with a central motif homologous to avian beta-keratins". Dev. Dyn. 236 (2): 374–88. PMID 17191254. doi:10.1002/dvdy.21022.
- Greenwold, M.J.; Sawyer, R.H. (2010). "Genomic organization and molecular phylogenies of the beta (β) keratin multigene family in the chicken (Gallus gallus) and zebra finch (Taeniopygia guttata): implications for feather evolution". BMC Evolutionary Biology 10. doi:10.1186/1471-2148-10-148.
- Greenwold, M.J.; Sawyer, R.H. (2011). "Linking the molecular evolution of avian beta (β) keratins to the evolution of feathers". Journal of Experimental Zoology 316B: 609–616. doi:10.1002/jez.b.21436.
- Nudds, R.L.; Dyke, G.J. (14 May 2010). "Narrow Primary Feather Rachises in Confuciusornis and Archaeopteryx Suggest Poor Flight Ability". Science 328: 887–889. PMID 20466930. doi:10.1126/science.1188895.
- .Schweitzer, Mary Higby, Watt, J.A., Avci, R., Knapp, L., Chiappe, L, Norell, Mark A., Marshall, M. (1999). "Beta-Keratin Specific Immunological reactivity in Feather-Like Structures of the Cretaceous Alvarezsaurid, Shuvuuia deserti Journal of Experimental Biology (Mol Dev Evol) 255:146-157
There are two main forms of keratin, alpha-keratin and beta-keratin. Alpha-keratin is seen in humans and other mammals, beta-keratin is present in birds and reptiles. Beta-keratin is harder than alpha-keratin. Structurally alpha-keratin have alpha-helical coiled coil structure while beta-keratin have twisted beta sheet structure.bIn the case of β-sheets, this allows sterically-unhindered hydrogen bonding between the amino and carboxyl groups of peptide bonds on adjacent protein chains, facilitating their close alignment and strong binding. Fibrous keratin molecules can twist around each other to form helical intermediate filaments.
SILK:- The secondary structure of silk is an example of the beta pleated sheet. In this structure, individual protein chains are aligned side-by-side with every other protein chain aligned in an opposite direction.The chains are antiparallel, with an alternating C → N orientation. The protein chains are held together by intermolecular hydrogen bonding, that is hydrogen bonding between amide groups of two separate chains. This intermolecular hydrogen bonding in the beta-pleated sheet is in contrast to the intramolecular hydrogen bonding in the alpha-helix.
The hydrogen on the amide of one protein chain is hydrogen bonded to the amide oxygen of the neighboring protein chain. The pleated sheet effect arises form the fact that the amide structure is planar while the "bends" occur at the carbon containing the side chain.
Fortunately, the "side" chain R groups in silk are not very bulky. The basic primary structure of silk consists of a six amino acid unit that repeats itself. The sequence where every other unit is glycine in silk is: -gly-ala-gly-ala-gly-ala-. Although glycine and alanine make up 75-80% of the amino acids in silk, another 10-15% is serine and the final 10% contain bulky side chains such as in tyr, arg, val, asp, and glu.