General Characteristics and Structure
Myoglobin is classified as a conjugated protein consisting of two key components: a protein moiety called apomyoglobin (apoMb), and a specific non-protein prosthetic group known as heme.
A fundamental feature of myoglobin is its monomeric structure. The protein consists of a single polypeptide chain. The primary structure of apomyoglobin plays a critical role, as it dictates the formation of a compact globular conformation (tertiary structure) and the precise organization of the active site. The secondary structure is highly ordered, consisting of eight $\alpha$-helices. In biochemistry, these helices are conventionally designated by capital letters from A to H.
Active Site Architecture
The core of the myoglobin molecule is its active site, which is formed predominantly by hydrophobic amino acid residues. Interestingly, the amino acids that create the active site may be located far apart in the linear primary structure. A classic example is tryptophan-39 (Trp_{39}) and phenylalanine-138 (Phe_{138}), which are brought into close spatial proximity only during the folding process when the protein adopts its final globular shape.
Water-insoluble ligands—specifically heme and the oxygen molecule ($O_2$)—bind to this active site. In addition to numerous hydrophobic residues, the active site contains two critically important hydrophilic amino acids:
- Distal histidine (His $E_7$ or His$_{64}$);
- Proximal histidine (His $F_8$ or His$_{93}$), which is directly coordinated to the heme iron atom.
Heme Structure and Oxygen Binding Mechanism
Heme acts as a specific prosthetic ligand for apomyoglobin. Its structural backbone consists of four pyrrole rings linked together by methenyl bridges. At the center of this ring system lies a ferrous iron ion ($Fe^{2+}$). The iron atom is firmly held in the center of the molecule by four coordination bonds with the nitrogen atoms of the pyrrole rings.
The functional mechanism is as follows: oxygen delivered by the bloodstream diffuses into muscle tissue and binds to the $Fe^{2+}$ atom of the heme group. Due to its monomeric structure and the specialized microenvironment of the active site, myoglobin exhibits an extremely high affinity for oxygen. This means it readily binds and tightly holds $O_2$ molecules. Myoglobin releases its stored oxygen only under specific conditions—namely during intense muscular activity, when the intracellular partial pressure of oxygen drops significantly.
Differences Between Myoglobin and Hemoglobin A
Hemoglobin A (Hb A) is a related oxygen-transport protein. The structure of individual hemoglobin subunits (protomers) is strikingly similar to that of the myoglobin molecule.
A detailed comparison reveals the following patterns:
- Primary Structure: There is remarkably low sequence homology; only 24 amino acid residues are identical between the two proteins.
- Secondary and Tertiary Structures: Despite significant differences in amino acid sequence, their 3D spatial organization is remarkably similar. Both proteins contain eight $\alpha$-helices and fold into a compact globular conformation.
- Quaternary Structure: This represents the major difference. Myoglobin functions exclusively as a monomer. In contrast, hemoglobin is a complex oligomer (specifically a tetramer) composed of four polypeptide chains held together by non-covalent interactions.