biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...

, ''p''₅₀ represents the

partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...

of a gas required to achieve 50% saturation of a particular protein's binding sites. Values of ''p''₅₀ are negatively correlated with substrate affinity; lower values correspond to higher affinity and ''vice versa''. The term is analogous to the Michaelis–Menten constant (''K_M''), which identifies the

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', ''number concentration'', ...

of substrate required for an enzyme to achieve 50% of its maximum reaction velocity. The concept of ''p''₅₀ is derived from considering the fractional saturation of a protein by a gas. Imagine

myoglobin Myoglobin (symbol Mb or MB) is an iron- and oxygen-binding protein found in the cardiac and skeletal muscle tissue of vertebrates in general and in almost all mammals. Myoglobin is distantly related to hemoglobin. Compared to hemoglobin, myoglob ...

, a protein which is able to bind a single molecule of

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

, as per the reversible reaction below, whose

equilibrium constant The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium, a state approached by a dynamic chemical system after sufficient time has elapsed at which its composition has no measurable tendency ...

''K'' (which is also a dissociation constant, since it describes a reversible association-dissociation event) is equal to the product of the concentrations (at equilibrium) of free myoglobin and free oxygen, divided by the concentration of myoglobin-oxygen complex. : Mb + O_2 <=> Mb \cdot O_2 : \it=\rm\frac The fractional saturation ''Y''_''O''2 of the myoglobin is what proportion of the total myoglobin concentration is made up of oxygen-bound myoglobin, which can be rearranged as the concentration of free oxygen over the sum of that concentration and the dissociation constant ''K''. Since diatomic oxygen is a gas, its concentration in solution can be thought of as a partial pressure. :

Y_=\rm\frac
\Rightarrow\rm\frac
\Rightarrow\rm\frac

From defining the ''p''₅₀ as the partial pressure at which the fractional saturation is 50%, we can deduce that it is in fact equal to the dissociation constant ''K''. :

\frac=0.5
\Rightarrow p_=K

For example, myoglobin's ''p''₅₀ for O₂ is 130

pascal Pascal, Pascal's or PASCAL may refer to: People and fictional characters * Pascal (given name), including a list of people with the name * Pascal (surname), including a list of people and fictional characters with the name ** Blaise Pascal, Frenc ...

s while the ''P''₅₀ for adult hemoglobin is 3.5 kPa. Thus, when O₂ partial pressure is low, hemoglobin-bound O₂ is more readily transferred to myoglobin. Myoglobin, found in high concentrations in

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...

tissue, can then transfer the oxygen to muscle tissue

muscle fiber A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscl ...

s, where it will be used in the generation of energy to fuel muscle contraction. Another example is that of human fetal hemoglobin, which has a higher affinity (lower ''P''₅₀) than adult hemoglobin, and therefore allows uptake of oxygen across the placental diffusion barrier.

References

Enzyme kinetics Units of pressure {{biochem-stub