How many polypeptide chains are present in the structure of hemoglobin?

In protein synthesis,the first amino acid is known as $.....P.....$ and the last amino acid is known as $.....Q.....$.

$A$: Tertiary structure of protein molecules makes them biologically active.
$R$: It is the native configuration of protein molecules maintained by multiple covalent bonds only.

Identify $X$ in the given chemical structure: $H_3N^+ - CH(R_1) - C(=O) - X - CH(R_2) - COO^-$.

Identify the type of bond formed between the carboxyl group of one amino acid residue and the amino group of another amino acid residue.

Select the incorrect pair from the following.