Myoglobin

General characteristics and structure

Myoglobin is classified as a complex protein (chromoprotein) consisting of two key components: the protein part, called apomyoglobin (apoMb), and a specific non-protein group — heme.

A fundamental feature of myoglobin is its monomeric structure. The protein is represented by just a single polypeptide chain. It is the primary structure of apomyoglobin that plays a decisive role: it rigidly determines the formation of a compact globular conformation in space (tertiary structure) and the proper organization of the active center. Regarding the secondary structure of the molecule, it is characterized by high organization and includes eight $\alpha$-helices. For convenience in biochemistry, these helices are usually designated by capital Latin letters from A to H.

Active center architecture

The heart of the myoglobin molecule is its active center. This zone is formed predominantly by hydrophobic amino acid residues. An interesting fact is that the amino acids forming the active center can be located at a significant distance from each other in the linear primary structure of the molecule. As a classic example, we can cite the tryptophan-39 (Trp_{39}) and phenylalanine-138 (Phe_{138}) residues. Spatially, they approach each other only during the folding process, when the protein coils into its final globular shape.

Ligands that are poorly soluble in water bind to the active center — these are heme and the oxygen molecule ($O_2$). In addition to numerous hydrophobic residues, the active center contains two critically important amino acids with hydrophilic properties:

• Distal histidine (His $E_7$ or His$_{64}$);
• Proximal histidine (His $F_8$ or His$_{93}$) — it is directly bound to the iron atom of the heme.

Heme structure and oxygen binding mechanism

Heme acts as a specific ligand for apomyoglobin. Its structural basis consists of 4 pyrrole rings securely connected to each other by methenyl bridges. In the very center of this complex ring system lies a ferrous iron ion ($Fe^{+2}$). Iron is firmly held in the center of the molecule due to four coordination bonds with nitrogen atoms belonging to the pyrrole rings.

The mechanism of protein action is as follows: oxygen from the bloodstream enters the muscle tissue and binds to the $Fe^{+2}$ atom of the heme. Due to its monomeric structure and the specific arrangement of the active center, myoglobin has a very high affinity for oxygen. This means it easily captures and tightly holds the $O_2$ molecule. The release of accumulated oxygen to the tissues occurs only under specific conditions — during intense muscular work, when the partial pressure of oxygen in the cells experiences a sharp decrease.

Differences between myoglobin and hemoglobin A

Hemoglobin A (Hb A) is a protein related to myoglobin. The structure of individual hemoglobin protomers is strikingly similar to the arrangement of the myoglobin molecule.

If a detailed comparison is made, the following patterns can be identified:

1. Primary structure: There is extremely low similarity. Only 24 amino acid residues are identical between these proteins.
2. Secondary and tertiary structures: Despite radical differences in the amino acid sequence, the spatial organization demonstrates very high similarity. Both proteins have 8 $\alpha$-helices and fold into a compact globule.
3. Quaternary structure: This is where the main difference lies. Myoglobin functions exclusively as a monomer. Hemoglobin, on the other hand, is a complex oligomer (specifically a tetramer) consisting of four polypeptide chains. These chains are connected inside the hemoglobin molecule by non-covalent bonds.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Spatial arrangement of the 8 alpha-helices (from A to H) in the globule
• The role of tryptophan-39 and phenylalanine-138 in the formation of the hydrophobic core
• Interaction of the proximal and distal histidines with heme
• Formation of coordination bonds of iron (Fe⁺²) with pyrrole rings

Other topics in the section "Structure, Properties, and Functions of Proteins"

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