Keratin is a protein, and therefore contains multiple functional groups, most notably amide groups (peptide bonds) which link the amino acids together in the polypeptide chain. It also contains other functional groups contributed by the amino acid side chains, including disulfide bonds, amino groups, and carboxylic acid groups.
Functional Groups Present in Keratin
Keratin, being a protein, doesn't have just one single defining functional group. Instead, it possesses a variety of functional groups that contribute to its overall structure and properties. These include:
- Amide (Peptide Bond): The most prevalent functional group. This is the linkage between amino acids forming the polypeptide chain. (-CO-NH-)
- Disulfide Bonds (-S-S-): Formed between cysteine amino acid residues. These bonds are crucial for cross-linking keratin chains, providing strength and stability, especially in hard keratins like hair and nails.
- Amino Groups (-NH2): Present at the N-terminus of the polypeptide chain and on the side chains of amino acids like lysine.
- Carboxylic Acid Groups (-COOH): Present at the C-terminus of the polypeptide chain and on the side chains of amino acids like aspartic acid and glutamic acid.
- Hydroxyl Groups (-OH): Present on the side chains of amino acids like serine, threonine, and tyrosine. These can participate in hydrogen bonding.
- Thiol Groups (-SH): Found in the side chain of cysteine. These groups can form disulfide bonds or exist as free thiols.
Importance of Functional Groups in Keratin
The functional groups in keratin play a critical role in determining its physical and chemical properties:
- Strength and Insolubility: Disulfide bonds contribute significantly to the strength, rigidity, and insolubility of keratin, particularly in hair, nails, and feathers.
- Chemical Reactivity: Amino and carboxylic acid groups make keratin chemically reactive, allowing it to participate in various chemical reactions, though under normal conditions, keratin exhibits low reactivity.
- Hydrogen Bonding: Amide, hydroxyl, amino, and carboxylic acid groups can participate in hydrogen bonding, which influences the secondary and tertiary structure of the protein.
In summary, while keratin doesn't have one singular defining functional group, the amide (peptide) bond is the most fundamental, linking amino acids together. Furthermore, the presence of disulfide bonds, along with amino, carboxylic acid, and hydroxyl groups, determine the protein's ultimate structure and function.
