Myoglobin holds an O2 reserve

Muscle cells have their own O2-binding molecule, myoglobin. Myoglobin consists of just one polypeptide chain associated with an iron-containing ring structure that can bind one O2 molecule. *Myoglobin has a higher affinity for O2 than hemoglobin does, so it picks up and holds O2 at PO2 values at which hemoglobin is releasing its bound O2 (Figure 48.13).

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Figure 48.13 Oxygen-Binding Adaptations Myoglobin and the different hemoglobins have different O2-binding properties adapted for different circumstances. Fetal hemoglobin, for example, has a higher affinity for O2 than does adult hemoglobin, facilitating O2 transfer in the placenta. When high metabolism lowers the pH of the blood, or low O2 increases bisphosphoglyceric acid (BPG), hemoglobin releases more of its O2.

Question

Q: How would the breathing of some carbon monoxide affect the hemoglobin–oxygen binding curve?

The hemoglobin would not fully saturate with O2 at 100 mm Hg PO2.

Activity 48.3 Oxygen-Binding Curves

www.life11e.com/ac48.3

*connect the concepts The evolutionary relationships between myoglobin and the various hemoglobin subunits is shown in Figure 23.10.

Myoglobin facilitates the diffusion of O2 in muscle cells and provides an O2 reserve for times when metabolic demands are high and blood flow is interrupted. Interruption of blood flow in muscles is common because contracting muscles squeeze blood vessels. When tissue PO2 values are low and hemoglobin can no longer supply more O2, myoglobin releases its bound O2. Diving mammals such as seals have high concentrations of myoglobin in their muscles, which is one reason they can stay under water for so long (Investigating Life: Seals Are Champion Breath-Hold Divers). Even in non-diving animals, muscles called on for extended periods of work frequently have more myoglobin than do muscles that are used for short, intermittent periods, as noted in Key Concept 47.2.