Which of the following are protein fibers?

The correct statements are:
$A$. Activation energy for enzyme catalysed hydrolysis of sucrose is lower than that of acid catalysed hydrolysis.
$B$. During denaturation, secondary and tertiary structures of a protein are destroyed but primary structure remains intact.
$C$. Nucleotides are joined together by phosphodiester linkage between $C_{3}$ and $C_{5}$ carbons of the pentose sugar.
$D$. Quaternary structure of proteins represents the spatial arrangement of two or more polypeptide chains.
Choose the correct answer from the options given below:

Identify the pair of reactants that upon reaction,with elimination of $HCl$,will give rise to the dipeptide $Gly-Ala$.

The number of chiral centres present in threonine is..........

$\alpha$-$helix$ 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?

Find the isoelectric point $(pI)$ of the given amino acid.
$CH_3-CH(NH_3^+)-CO_2H, \ pK_a = 2.2, \ pK_b = 4.4$