What protein carries oxygen in muscles?

The protein that carries oxygen in muscles is myoglobin.

Myoglobin is a compact, oxygen-binding protein found mainly inside skeletal and cardiac muscle cells. It contains a heme group with an iron atom that can bind oxygen reversibly. Myoglobin helps move oxygen through muscle cells and provides a temporary oxygen reserve for mitochondria, where much of the cell's energy is produced.

Unlike hemoglobin, which transports oxygen through the bloodstream and is made of four protein subunits, myoglobin consists of a single polypeptide chain. Its simpler structure gives it a hyperbolic oxygen-binding curve rather than hemoglobin's cooperative, S-shaped curve.

Myoglobin also helps explain why some muscles and meats appear red or dark: higher concentrations of the pigment deepen the color. Diving mammals such as seals and whales have unusually high muscle-myoglobin levels, allowing them to store more oxygen during long dives. John Kendrew's work on myoglobin helped reveal the first three-dimensional protein structure at atomic resolution, earning him the 1962 Nobel Prize in Chemistry.