How many polypeptide chains are present in a single molecule of globin protein?

Find the correct option for the sequence given below:
$(1)$ Alanine $\rightarrow$ Valine
$(2)$ Asparagine $\rightarrow$ . . . . . .
$(3)$ Aspartic acid $\rightarrow$ Glutamic acid
$(4)$ Arginine $\rightarrow$ Histidine

Which is the most abundant protein in the animal world?

In the structure of a protein,which groups are responsible for the $N$-terminal and $C$-terminal,respectively?

Which of the following is an amino acid?

Which of the following proteins helps in the transport of nutrients across the cell membrane?