The most abundant protein in the whole biosphere is:

Which of the following amino acids is involved in the formation of $Heme$?

The diversity observed in protein molecules is primarily due to the diversity of which of the following?

Statement $P$: The quaternary structure of proteins is either globular or fibrous.
Statement $Q$: The three-dimensional arrangement of molecules consisting of a single polypeptide chain is the quaternary structure of proteins.

$A$: Each protein molecule is a polymer of amino acids.
$R$: There are $20$ types of amino acids in living cells.

What type of '$R$' group is present in Histidine?