Structure and topology of the linkers in the conserved lepidosaur β-keratin chain with four 34-residue repeats support an interfilament role for the central linker

Keratin from Animal By-Products: Structure, Characterization, Extraction and Application-A Review

Keratin is a structural fibrous protein and the core constituent of animal by-products from livestock such as wool, feathers, hooves, horns, and pig bristles. This natural polymer is also the main component of human hair and is present at an important percentage in human and animal skin. Significant amounts of keratin-rich animal tissues are discarded worldwide each year, ca. 12 M tons, and the share used for keratin extraction and added-value applications is still very low. An important stream of new potential raw materials, represented by animal by-products and human hair, is thus being lost, while a large-scale valorization could contribute to a circular bioeconomy and to the reduction in the environmental fingerprint of those tissues. Fortunately, scientific research has made much important progress in the last 10-15 years in the better understanding of the complex keratin architecture and its variability among different animal tissues, in the development of tailored extraction processes, and in the screening of new potential applications. Hence, this review aims at a discussion of the recent findings in the characterization of keratin and keratin-rich animal by-product structures, as well as in keratin recovery by conventional and emerging techniques and advances in valorization in several fields.

General aspects on skin development in vertebrates with emphasis on sauropsids epidermis

General cellular aspects of skin development in vertebrates are presented with emphasis on the epidermis of sauropsids. Anamniote skin develops into a multilayered mucogenic and soft keratinized epidermis made of Intermediate Filament Keratins (IFKs) that is reinforced in most fish and few anurans by dermal bony and fibrous scales. In amniotes, the developing epidermis in contact with the amniotic fluid initially transits through a mucogenic phase recalling that of their anamniotes progenitors. A new gene cluster termed EDC (Epidermal Differentiation Complex) evolved in amniotes contributing to the origin of the stratum corneum. The EDC contains numerous genes coding for over 100 types of corneous proteins (CPs). In sauropsids 2-8 layers of embryonic epidermis accumulate soft keratins (IFKs) but do not form a compact corneous layer. The embryonic epidermis of reptiles and birds produces small amount of other, poorly known proteins in addition to IFKs and mucins. In the following development, a resistant corneous layer is formed underneath the embryonic epidermis that is shed before hatching. The definitive corneous epidermis of sauropsids is mainly composed of CBPs (Corneous beta proteins, formerly indicated as beta-keratins) derived from the EDC. CBPs belong to a gene sub-family of CPs unique for sauropsids, contain an inner amino acid region formed by beta-sheets, are rich in cysteine and glycine, and make most of the protein composition of scales, claws, beaks and feathers. In mammalian epidermis CPs missing the beta-sheet region are instead produced, and include loricrin, involucrin, filaggrin and various cornulins. Small amount of CPs accumulate in the 2-3 layers of mammalian embryonic epidermis and their appendages, that is replaced with the definitive corneous layers before birth. Differently from sauropsids, mammals utilize KAPs (keratin associated proteins) rich in cysteine and glycine for making the hard corneous material of hairs, claws, hooves, horns, and occasionally also scales.

Insights of Keratin geometry from Agro-industrial wastes: A comparative computational and experimental assessment

Understanding the structural properties of keratin is of great importance to managing their potential application in keratin-inspired biomaterials and its management of wastes. In this work, the molecular structure of chicken feather keratin 1 was characterized by AlphaFold2 and quantum chemistry calculation. The predicted IR spectrum of the N-terminal region of feather keratin 1, consisting of 28 amino acid residues, was used to assign the Raman frequencies of the extracted keratin. The MW of experimental samples were 6 & 1 kDa while the predicted MW (∼10 kDa) of β-keratin. Experimental analysis shows the magnetic field treatment could affect the functional and surface structural properties of keratin. The particle size distribution curve illustrates the dispersion of particle size concentration, while TEM analysis demonstrates the reduction of particle diameter to 23.71 ± 1.1 nm following treatment. High-resolution XPS analysis confirmed the displacement of molecular elements from their orbital.

Keratin intermediate filament chains in the European common wall lizard (Podarcis muralis) and a potential keratin filament crosslinker

On the basis of sequence homology with mammalian α-keratins, and on the criteria that the coiled-coil segments and central linker in the rod domain of these molecules must have conserved lengths if they are to assemble into viable intermediate filaments, a total of 28 Type I and Type II keratin intermediate filament chains (KIF) have been identified from the genome of the European common wall lizard (Podarcis muralis). Using the same criteria this number may be compared to 33 found here in the green anole lizard (Anole carolinensis) and 25 in the tuatara (Sphenodon punctatus). The Type I and Type II KIF genes in the wall lizard fall in clusters on chromosomes 13 and 2 respectively. Although some differences occur in the terminal domains in the KIF chains of the two lizards and tuatara, the similarities between key indicator residues - cysteine, glycine and proline - are significant. The terminal domains of the KIF chains in the wall lizard also contain sequence repeats commonly based on glycine and large apolar residues and would permit the fine tuning of physical properties when incorporated within the intermediate filaments. The H1 domain in the Type II chain is conserved across the lizards, tuatara and mammals, and has been related to its role in assembly at the 2-4 molecule level. A KIF-like chain (K80) with an extensive tail domain comprised of multiple tandem repeats has been identified as having a potential filament-crosslinking role.

Structures of the ß-Keratin Filaments and Keratin Intermediate Filaments in the Epidermal Appendages of Birds and Reptiles (Sauropsids)

The epidermal appendages of birds and reptiles (the sauropsids) include claws, scales, and feathers. Each has specialized physical properties that facilitate movement, thermal insulation, defence mechanisms, and/or the catching of prey. The mechanical attributes of each of these appendages originate from its fibril-matrix texture, where the two filamentous structures present, i.e., the corneous ß-proteins (CBP or ß-keratins) that form 3.4 nm diameter filaments and the α-fibrous molecules that form the 7-10 nm diameter keratin intermediate filaments (KIF), provide much of the required tensile properties. The matrix, which is composed of the terminal domains of the KIF molecules and the proteins of the epidermal differentiation complex (EDC) (and which include the terminal domains of the CBP), provides the appendages, with their ability to resist compression and torsion. Only by knowing the detailed structures of the individual components and the manner in which they interact with one another will a full understanding be gained of the physical properties of the tissues as a whole. Towards that end, newly-derived aspects of the detailed conformations of the two filamentous structures will be discussed and then placed in the context of former knowledge.