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In biochemistry, P50 indicates the partial pressure of a gas required to achieve 50% enzyme saturation. Values of P50 are negatively correlated with substrate affinity, with lower values of P50 corresponding to high affinity and vice versa. The term is analogous to the Michaelis-Menten constant (Km), which identifies the concentration of substrate required for an enzyme to achieve 50% of its maximum reaction velocity.

For example, the P50 of myoglobin for O2 is 1 mm Hg, while the P50 for adult hemoglobin is 26 mm Hg. Thus, when O2 partial pressure is low, hemoglobin-bound O2 is more readily transferred to myoglobin. Myoglobin, found in high concentrations in muscle tissue, can then transfer the oxygen to muscle fibers, where it will be used in the generation of energy to fuel muscle contraction.

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