Which is the most common source of protein for vegetarians in our country?

An example of a protein with a quaternary structure is

In the given figure,what does $A$ indicate?

What is the $R$ group present in glycine?

Which one of the following amino acids is not found in proteins?

$A$ - In the solution of different $pH$,the structure of amino acids changes.
$R$ - It is because of the ionisable nature of $-NH_2$ and $-COOH$ groups.