The spiral configuration of $\alpha$-helix and $\beta$-pleated sheet of polypeptide chains are held together by . . . . . . bonds to form the secondary structure of protein.

Given below are two statements:
Statement $I:$ Amino acids have a property of ionizable nature of $-NH_2$ and $-COOH$ groups,hence have different structures at different $pH$.
Statement $II:$ Amino acids can exist as Zwitterionic form at acidic and basic $pH$.
In the light of the above statements,choose the most appropriate answer from the options given below:

Based on which group do the properties and types of amino acids differ?

$A$: The polypeptide coil of collagen helix is strengthened by the establishment of hydrogen bonds between the $>NH$ group of the glycine residue of each strand and the $-CO$ group of the other two strands.
$R$: In the collagen helix,a locking effect also occurs with the help of proline and hydroxyproline amino acids.

What is the function of the protein $GLUT-4$?

What is the basic unit of protein structure?