Which of the following is not an amino acid?

$\alpha$-$helix$ is a secondary structure of proteins formed by twisting of polypeptide chain into right-handed screw-like structures. Which type of interactions are responsible for making the $\alpha$-$helix$ structure stable?

Enzymes belong to which class of compounds?

Insulin is a .........

Which of the following are protein fibers?

Proteins do not respond to