Which of the following is not a classification of proteins?

Identify the correct statement among the following $:$

The correct structure of product $P$ in the following reaction is
$Asn-Ser + (excess) (CH_3CO)_2O \xrightarrow{NEt_3} P$

In an amino acid,the carboxylic group ionises at $pH = 2.56$ $(pK_{a_1})$ and the ammonium ion ionises at $pH = 9.38$ $(pK_{a_2})$. The isoelectric point of the amino acid is at:

Protein found in a biological system with a unique three-dimensional structure and biological activity is called a native protein. When a protein in its native form is subjected to a physical change like a change in temperature or a chemical change like a change in $pH$,denaturation of proteins takes place. Explain the cause.

Denaturation of proteins leads to the loss of their biological activity due to: