Myoglobin is a small, single-chain protein found primarily within the cardiac and skeletal muscle tissue of mammals. Its function is to store oxygen, acting as a reserve that muscles can tap into when their oxygen supply temporarily drops, such as during intense physical exertion. This reserve ensures a steady supply of oxygen for the muscle cell’s energy production, helping to sustain output and delay fatigue. The effectiveness of myoglobin in binding oxygen depends entirely on its unique and compact physical structure.
The Polypeptide Chain: Primary and Secondary Structure
The foundation of the myoglobin molecule is its primary structure: the precise, linear sequence of approximately 153 amino acids linked together by peptide bonds. This specific sequence dictates the entire three-dimensional shape and function of the final protein.
The secondary structure refers to the local, repeating shapes adopted by sections of the polypeptide chain. Myoglobin has an unusually high content of alpha-helices, with about 70 to 75 percent of its amino acids participating in this structure. These rigid, rod-like structures are stabilized by hydrogen bonds between the backbone atoms.
The polypeptide chain is organized into eight distinct alpha-helical segments, conventionally labeled A through H. These segments are connected by short, non-helical loops or turns. The entire chain folds into a stable, compact unit, creating the internal architecture necessary for oxygen storage.
The Functional Core: The Heme Group
Myoglobin’s ability to bind oxygen is due to the heme group, a prosthetic group. This group is a large, ring-like organic structure known as protoporphyrin, which contains a single ferrous iron atom (\(\text{Fe}^{2+}\)) positioned at its center. The iron atom is the actual site where the oxygen molecule attaches.
The iron atom must maintain six coordination sites to function. Four sites are occupied by nitrogen atoms from the four pyrrole rings of the porphyrin structure. The fifth site is occupied by a nitrogen atom from a specific amino acid residue on the polypeptide chain, known as the Proximal Histidine.
The Proximal Histidine forms a direct covalent bond with the iron atom, securely anchoring the heme group to the protein. The sixth coordination site remains vacant in the deoxygenated state, projecting into the open pocket. This position is reserved for the reversible binding of an oxygen molecule.
The Globular Shape: Tertiary Structure and Oxygen Binding
The tertiary structure describes the three-dimensional arrangement of the myoglobin molecule, which is a compact, roughly spherical shape. This structure forms when the eight alpha-helices fold together, creating a hydrophobic pocket where the heme group is tightly encapsulated. The protein’s internal environment is maintained by interactions between hydrophobic amino acids, which are tucked away from the surrounding water.
Conversely, hydrophilic amino acid residues are located on the exterior surface of the protein, allowing myoglobin to remain soluble within the muscle cell’s interior. This folded structure protects the ferrous iron atom in the heme group from being oxidized to its ferric form (\(\text{Fe}^{3+}\)), a state that cannot bind oxygen.
The structure facilitates oxygen binding by positioning a second amino acid, the Distal Histidine, near the iron atom’s sixth coordination site. This residue does not bond directly to the iron but hovers in the binding pocket, stabilizing the bound oxygen molecule through a hydrogen bond. The Distal Histidine also sterically hinders the binding of smaller molecules like carbon monoxide, ensuring the molecule prioritizes oxygen storage.