Myoglobin

Myoglobin is abundant in muscles involved in laborious activity. Why ?

To know the answer, let's begin with some basics. 

Myoglobin contains only one polypeptide chain which contains only one Haem molecule. One Haem molecule can bind to only one O2 molecule.

Quaternary structure is possible only in the presence of >1 subunits in a protein. As Myoglobin is made up of a single polypeptide chain, Quaternary structure is NOT possible. As there is no Quaternary structure, Bohr effect, 2,3 BPG effect and co-operative effect are not possible. After all there is only one O2 in haem. So there is no role for co-operative effect. (As there is no co-operative effect, Oxygen dissociation curve of myoglobin is hyperbolic in shape)

Myoglobin has very high affinity for oxygen. Why ?

Myoglobin is NOT an Oxygen carrier. It is just an oxygen reservoir. Whenever the oxygen tension is  critically low in the myocyte it releases its Oxygen.

This is why Myoglobin is abundant in muscles with high activity.