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, a ...

, ''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 Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

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, skeletal Muscle, muscle tissue of vertebrates in general and in almost all mammals. Myoglobin is distantly related to hemoglobin. Compar ...

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

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

, 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 pascals while the ''P''₅₀ for

adult hemoglobin Hemoglobin A (HbA), also known as adult hemoglobin, hemoglobin A1 or α2β2, is the most common human hemoglobin tetramer, accounting for over 97% of the total red blood cell hemoglobin. Hemoglobin is an oxygen-binding protein, found in erythrocyt ...

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 Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

tissue, can then transfer the oxygen to muscle tissue

muscle fiber A muscle cell, also known as a myocyte, is a mature contractile cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal, smooth, and cardiac (cardiomyocytes). A skeletal muscle cell is long and threadl ...

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