Final answer:
Vertebrate myoglobin is a monomer, which means it is composed of a single polypeptide chain, as opposed to a tetramer like hemoglobin, which has four subunits.
Step-by-step explanation:
Vertebrate myoglobin is a type of protein that is found in the muscle tissues of most mammals, where it functions by binding with oxygen to supply cells with this essential element. Regarding its structure, vertebrate myoglobin is a monomer. This means it is a single polypeptide chain, and unlike hemoglobin, which is a tetramer consisting of four polypeptide subunits (two α-subunits and two β-subunits), myoglobin does not have a quaternary structure composed of multiple polypeptide chains.