The α-helix is a regularly repeated polypeptide backbone structural motif that can be identified to varying degrees in the folded 3-D conformations of most proteins. Myoglobin (Mb), and its evolutionary cousins, the α- and β-polypeptide chains of hemoglobin (Hb), exhibit unusually high percentages of α-helical structure (more than 70%).

The 3-D image on the right highlights (with colored ribbons) the 8 α-helical segments of sperm whale Mb and their relative organization in the folded protein. These helical segments are labeled A-H and span the following residues of this 153 amino acid residue polypeptide chain of Mb. Segments (residues): A (3-18), B (20-35), C (36-42), D (51-57), E (58-76), F (83-95), G (100-118), & H (124-149).

Note that the nonhelcal polypeptide backbone segments connecting one helix to the next, as well as those segments extending to N-terminal and C-terminal ends of the polypeptide chain, are referred to aperiodic backbone structures. They are not "random" structures as sometimes referred to incorrectly.

Aside from lending overall stability to the folded structure of Mb, an important function of two of these helical segments (labled E and F) is to create a stable pocket for the noncovlaent binding of a heme "prosthetic" group, which is an organic protoporphyrin ring chelated to a central FeII iron atom that serves as the strong molecular binding site for one oxygen molecule.

In muscle tissue, where Mb is found in abundance, it functions as an O₂ "buffer" by reversibly binding or releasing O₂ on demand. When O₂ tension is high in inactive muscle, Mb is nearly saturated with O₂ and effectively acts as an O₂ storage protein. However, when O₂ levels drop in active muscle, Mb releases O₂ accordingly to help replenish muscle O₂ availability for the ATP-producing Krebs cycle. Thus, Mb helps sustain muscle output for longer periods of time by delaying muscle fatigue.