However, some proteins are considered to have minimal tertiary structure, meaning they don't form complex, well-defined 3D shapes. These proteins often have primarily fibrous roles, where their elongated structures provide structural support or contribute to the cytoskeleton. Examples include:
* Collagen: Found in connective tissues, tendons, and skin. Its structure is dominated by a triple helix, which is a type of secondary structure.
* Keratin: Found in hair, nails, and skin. It has an alpha-helical structure that can form coiled coils.
* Elastin: Found in connective tissues, allowing for stretching and elasticity. Its structure is dominated by random coils.
These proteins are still classified as having tertiary structure, though it's very limited and often lacks a well-defined shape.
It's important to note that:
* While these proteins have minimal tertiary structure, they still have functional domains and interactions that are dependent on their amino acid sequence and secondary structure.
* There is likely some level of tertiary structure present, even in these fibrous proteins, but it's not as defined or complex as in globular proteins.
Therefore, while it's technically inaccurate to say a protein has *only* primary and secondary structure, proteins like collagen, keratin, and elastin are often described as having minimal tertiary structure due to their mostly fibrous nature.
