Oxygen Binding by Myoglobin

Physiological Usefulness of Myoglobin - Myoglobin in tissues (especially in muscle) accepts oxygen from hemoglobin in the circulating arterial blood, then delivers the oxygen to the mitochondria when oxygen needs are sufficiently great (Figure 7.1).

From Henry's law, the concentration of any gas dissolved in a fluid is proportional to the partial pressure of that gas above the fluid. For O 2, this is expressed as P O 2.

Measuring Myoglobin Binding of Oxygen - Spectrophotometry can measure the fraction of myoglobin molecules that are oxygenated. The O 2 binding curve of myoglobin in solution at neutral pH, shown in Figure 7.6, illustrates how the fraction of the myoglobin sites that have oxygen bound to them () depends on the concentration (partial pressure) of free oxygen.

Derivation of the Hyperbolic Shape of the Myoglobin Binding Curve - here.

Useful Properties of Myglobin's Binding of Oxygen - The P 50 (oxygen partial pressure required for half saturation) for myoglobin is very low, signifying that myoglobin has a high affinity for oxygen-an important characteristic for a protein that must extract oxygen from the small amounts present in blood. At the oxygen concentration existing in the capillaries, the myoglobin in adjacent tissues is nearly saturated. When cells are metabolically active, their internal P O 2 falls to levels where myoglobin will lose (deliver) its oxygen.

Binding and Release of Oxygen - The affinity with which myoglobin binds oxygen can be expressed with an affinity constant, K. K is an equilibrium constant that is the ratio of two rate constants.

K = k 1/k-1,

where k 1 is the rate constant for the binding reaction and k-1 is the rate constant for the release reaction.

Computer simulations of the behavior of oxymyoglobin suggest that the rate-limiting process in oxygen release is the opening of a pathway for the O 2 molecule to escape from the heme pocket. Oxygen may spend time "rattling in its cage" - and perhaps being recaptured - before the tertiary structure of the myoglobin shifts enough to let it escape (Figure 7.7). This process is an explicit example of a principle set forth elsewhere (here) - the dynamic internal motions of globular protein molecules play important roles in regulating the processes proteins mediate. The flexibility of myoglobin may thus determine how strongly myoglobin binds oxygen.

See also: Oxygen Binding by Heme Proteins, Oxygen Binding by Hemoglobin, Myoglobin