Myoglobin is a protein found primarily in muscle tissue, where it plays a role in oxygen storage and supply. This molecule helps ensure muscles receive the oxygen needed for activity. Its unique structure allows it to efficiently bind and store oxygen, making it available when muscle cells require it.
The Myoglobin Molecule
Myoglobin is a compact, globular protein located predominantly in the striated muscles of vertebrates. It is abundant in muscles requiring sustained activity, such as those in diving mammals like whales and seals, which need significant oxygen reserves for prolonged underwater excursions. Unlike more complex proteins, myoglobin consists of a single polypeptide chain. This single chain has a molecular weight of approximately 16,700 Daltons and overall dimensions of roughly 45 by 35 by 25 angstroms (Å).
The Alpha-Helical Architecture
The single polypeptide chain of myoglobin folds into a specific three-dimensional shape, primarily composed of alpha helices. It contains eight alpha-helical segments, labeled A through H, which make up about 70-75% of its structure. These helices are connected by shorter, non-helical loops and turns. Human myoglobin consists of 154 amino acids, with approximately 121 residues located within the helical regions.
This folding results in a compact structure where most hydrophobic (water-avoiding) amino acid residues are tucked away in the interior. This creates a non-polar environment within the protein, important for its function. Conversely, many polar (water-attracting) residues are on the protein’s surface, making the molecule relatively soluble in the cell’s watery environment.
The Heme Group: Oxygen’s Anchor
Myoglobin’s ability to bind oxygen depends on the heme group, a non-protein component. This flat, ring-like molecule, a protoporphyrin, has a single iron atom at its center. The iron atom, in its ferrous (Fe²⁺) oxidation state, is the direct site where oxygen reversibly binds. This iron atom is coordinated by six ligands; four come from nitrogen atoms within the porphyrin ring.
One remaining coordination site is occupied by a nitrogen atom from a histidine residue (His-93) within the polypeptide chain, known as the proximal histidine. The sixth position is where the oxygen molecule binds. Another histidine residue (His-64), the distal histidine, is positioned near the oxygen-binding site. It helps stabilize the bound oxygen molecule and prevents other molecules, such as carbon monoxide, from binding too strongly.
Myoglobin’s Role in Muscle
Myoglobin serves as an intracellular oxygen storage site within muscle cells. Its high affinity for oxygen allows it to bind and “hoard” oxygen that diffuses into muscle cells from the bloodstream when oxygen levels are ample. This stored oxygen acts as a reserve, particularly during periods of increased muscle activity or when oxygen supply is limited, such as during intense exercise.
When muscle activity intensifies and oxygen demand rises, myoglobin releases its stored oxygen to the surrounding muscle cells. This release ensures a continuous supply of oxygen to the mitochondria, the cellular powerhouses responsible for producing energy (ATP) through aerobic respiration. Myoglobin also facilitates oxygen diffusion from the cell membrane to the mitochondria within muscle fibers, supporting sustained muscle contraction and preventing fatigue during strenuous conditions.