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How do myoglobin and cytochromes contribute to the red color of some muscle fibers?

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Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

Myoglobin and cytochromes contribute to the red color of certain muscle fibers primarily through their structure as hemoproteins, which contain iron-bearing heme groups.123

Myoglobin: This protein acts as an oxygen-storage unit within muscle cells, facilitating oxidative metabolism. It contains a heme group—a porphyrin molecule bound to an iron atom—which imparts a red-brown color to the protein. Muscle fibers with a high concentration of myoglobin are known as red fibers; these fibers have a greater capacity for oxidative metabolism compared to white fibers, which have low myoglobin content.124
Cytochromes: These are a group of hemoprotein cell components that function as electron carriers within the mitochondria. Like myoglobin, they contain iron-bearing heme groups that can undergo reversible oxidation and reduction. Because these proteins are essential for cellular respiration and energy transfer, they are abundant in tissues with high metabolic activity, further contributing to the characteristic pigmentation of oxidative muscle fibers.35

Together, the presence of these iron-rich, heme-containing proteins is a defining feature of red muscle fibers, reflecting their specialized role in aerobic energy production.413