$A$ tetrapeptide $x$ on complete hydrolysis produced glycine $(Gly)$,alanine $(Ala)$,valine $(Val)$,and leucine $(Leu)$ in equimolar proportions. The number of possible tetrapeptide sequences involving each of these amino acids exactly once is:

The prosthetic group of haemoglobin is

Proteins can be used:

Read the following statements carefully:
$(A)$ Albumin is a simple protein.
$(B)$ The amino acid alanine contains an acidic side chain.
$(C)$ Insulin is a hormone.
$(D)$ Muscles contain the protein keratin.
Point out the wrong statements in the above set of statements.

Enzymes belong to which class of compounds?

Which is the $C$-terminal residue in glycyl alanine?