$A$ lubricant,mucin in saliva,is made up of:

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

No cell could live without

$A$ - 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.

What are the number and types of polypeptide chains in human hemoglobin, respectively?

In the given figure,what does the part labeled $M$ indicate?