Myoglobin is an oxygen-binding protein found primarily within the muscle tissue of vertebrates. It functions as an intracellular reservoir, holding onto oxygen until it is needed to support the high metabolic demands of working muscles, such as during intense exercise or periods of low oxygen availability. The ability to store and release oxygen is determined by its unique structure.
The Molecular Architecture of Myoglobin
Myoglobin is classified as a monomeric protein, consisting of a single polypeptide chain of amino acids. This chain folds into a compact, globular shape largely composed of eight alpha-helices. Tucked into a hydrophobic pocket is the non-protein component known as the heme prosthetic group.
The heme group is the site where oxygen binding occurs, making it the functional center of the molecule. At the core of this ring-like structure is a single iron atom in the ferrous (Fe2+) oxidation state. This iron atom has six coordination sites.
Four sites are bound to the nitrogen atoms of the heme ring, and one is bound to a histidine residue from the polypeptide chain. The sixth coordination site on the ferrous iron atom remains available for the reversible binding of an oxygen molecule (O2). The protein folds around the heme to protect the iron from oxidation, ensuring it remains in the Fe2+ state.
Maximum Oxygen Binding Capacity
Myoglobin binds only one molecule of oxygen (O2) per protein molecule. This single-binding capacity is a consequence of its molecular architecture. Since myoglobin is a monomer, it contains only one polypeptide chain and one heme prosthetic group.
The single heme group means there is only one iron atom available as the oxygen binding site. This structure is suited to myoglobin’s role as an oxygen storage unit. It maintains a high affinity for oxygen, binding the molecule tightly and releasing it only under conditions of extremely low oxygen concentration, such as during intense muscle activity.
This high affinity is reflected in myoglobin’s hyperbolic oxygen saturation curve, indicating a straightforward binding relationship. The protein can become fully saturated even at the low oxygen partial pressures typical of resting muscle tissue. Myoglobin releases the stored O2 molecule to the mitochondria to support energy production when the muscle cell is deprived.
Functional Differences: Myoglobin Versus Hemoglobin
The difference in oxygen binding capacity is best understood by contrasting myoglobin with hemoglobin. Myoglobin is a monomer that binds one O2 molecule, while hemoglobin is a tetramer composed of four polypeptide subunits. Since each of hemoglobin’s four subunits contains a heme group, a single hemoglobin molecule can bind four oxygen molecules.
This structural difference dictates their roles: myoglobin is optimized for oxygen storage in muscle cells, and hemoglobin is optimized for oxygen transport throughout the body. Hemoglobin’s four binding sites exhibit cooperative binding, where the attachment of one oxygen molecule increases the affinity of the remaining sites. This results in a sigmoidal, or S-shaped, oxygen dissociation curve.
Myoglobin lacks this cooperative mechanism and has a significantly higher oxygen affinity than hemoglobin. Hemoglobin picks up oxygen efficiently in the lungs and releases it incrementally in peripheral tissues. Myoglobin, with its hyperbolic curve and strong affinity, strips oxygen from hemoglobin in the muscle and holds it in reserve, releasing its single molecule only when the oxygen pressure drops to very low levels.