Hill equation (biochemistry)

In biochemistry and pharmacology, the Hill equation refers to two closely related equations that reflect the binding of ligands to macromolecules, as a function of the ligand concentration. A ligand is "a substance that forms a complex with a biomolecule to serve a biological purpose" (ligand definition), and a macromolecule is a very large molecule, such as a protein, with a complex structure of components (macromolecule definition). Protein-ligand binding typically changes the structure of the target protein, thereby changing its function in a cell.
The distinction between the two Hill equations is whether they measure "occupancy" or "response". The Hill–Langmuir equation reflects the occupancy of macromolecules: the fraction that is saturated or bound by the ligand. This equation is formally equivalent to the Langmuir isotherm. Conversely, the Hill equation proper reflects the cellular or tissue response to the ligand: the physiological output of the system, such as muscle contraction. 
The Hill–Langmuir equation was originally formulated by Archibald Hill in 1910 to describe the sigmoidal O2 binding curve of haemoglobin. 
The binding of a ligand to a macromolecule is often enhanced if there are already other ligands present on the same macromolecule (this is known as cooperative binding). The Hill–Langmuir equation is useful for determining the degree of cooperativity of the ligand(s) binding to the enzyme or receptor. The Hill coefficient provides a way to quantify the degree of interaction between ligand binding sites.
The Hill equation (for response) is important in the construction of dose-response curves.
Proportion of ligand-bound receptors.
The Hill–Langmuir equation is a special case of a rectangular hyperbola and is commonly expressed in the following ways.
where:
Constants.
In pharmacology, formula_6 is often written as formula_7, where formula_8 is the ligand, equivalent to L, and formula_9 is the receptor. formula_6 can be expressed in terms of the total amount of receptor and ligand-bound receptor concentrations: formula_11. formula_3 is equal to the ratio of the dissociation rate of the ligand-receptor complex to its association rate (formula_13). Kd is the equilibrium constant for dissociation. formula_14 is defined so that formula_15, this is also known as the microscopic dissociation constant and is the ligand concentration occupying half of the binding sites. In recent literature, this constant is sometimes referred to as formula_16.
Gaddum equation.
The Gaddum equation is a further generalisation of the Hill-equation, incorporating the presence of a reversible competitive antagonist. The Gaddum equation is derived similarly to the Hill-equation but with 2 equilibria: both the ligand with the receptor and the antagonist with the receptor. Hence, the Gaddum equation has 2 constants: the equilibrium constants of the ligand and that of the antagonist
Hill plot.
The Hill plot is the rearrangement of the Hill–Langmuir Equation into a straight line.
Taking the reciprocal of both sides of the Hill–Langmuir equation, rearranging, and inverting again yields: formula_17. Taking the logarithm of both sides of the equation leads to an alternative formulation of the Hill-Langmuir equation:
This last form of the Hill–Langmuir equation is advantageous because a plot of formula_19 versus formula_20 yields a linear plot, which is called a Hill plot. Because the slope of a Hill plot is equal to the Hill coefficient for the biochemical interaction, the slope is denoted by formula_21. A slope greater than one thus indicates positively cooperative binding between the receptor and the ligand, while a slope less than one indicates negatively cooperative binding.
Transformations of equations into linear forms such as this were very useful before the widespread use of computers, as they allowed researchers to determine parameters by fitting lines to data. However, these transformations affect error propagation, and this may result in undue weight to error in data points near 0 or 1. This impacts the parameters of linear regression lines fitted to the data. Furthermore, the use of computers enables more robust analysis involving nonlinear regression.
Tissue response.
A distinction should be made between quantification of drugs binding to receptors and drugs producing responses. There may not necessarily be a linear relationship between the two values. In contrast to this article's previous definition of the Hill-Langmuir equation, the IUPHAR defines the Hill equation in terms of the tissue response formula_22, as
formula_23
where <chem>[A]</chem> is the drug concentration and formula_24 is the drug concentration that produces a 50% maximal response. Dissociation constants (in the previous section) relate to ligand binding, while formula_24 reflects tissue response.
This form of the equation can reflect tissue/cell/population responses to drugs and can be used to generate dose response curves. The relationship between formula_3 and EC50 may be quite complex as a biological response will be the sum of myriad factors; a drug will have a different biological effect if more receptors are present, regardless of its affinity.
The Del-Castillo Katz model is used to relate the Hill–Langmuir equation to receptor activation by including a second equilibrium of the ligand-bound receptor to an "activated" form of the ligand-bound receptor.
Statistical analysis of response as a function of stimulus may be performed by regression methods such as the probit model or logit model, or other methods such as the Spearman–Karber method. Empirical models based on nonlinear regression are usually preferred over the use of some transformation of the data that linearizes the dose-response relationship.
Hill coefficient.
The Hill coefficient is a measure of ultrasensitivity (i.e. how steep is the response curve).
The Hill coefficient, formula_5 or formula_21, may describe cooperativity (or possibly other biochemical properties, depending on the context in which the Hill–Langmuir equation is being used). When appropriate, the value of the Hill coefficient describes the cooperativity of ligand binding in the following way:
The Hill coefficient can be calculated in terms of potency as:
where <chem>EC90</chem> and <chem>EC10</chem> are the input values needed to produce the 10% and 90% of the maximal response, respectively.
Derivation from mass action kinetics.
The Hill-Langmuir equation is derived similarly to the Michaelis Menten equation but incorporates the Hill coefficient. Consider a protein (formula_34), such as haemoglobin or a protein receptor, with formula_35 binding sites for ligands (formula_36). The binding of the ligands to the protein can be represented by the chemical equilibrium expression:
</chem>,
where formula_37 (forward rate, or the rate of association of the protein-ligand complex) and formula_38 (reverse rate, or the complex's rate of dissociation) are the reaction rate constants for the association of the ligands to the protein and their dissociation from the protein, respectively. From the law of mass action, which in turn may be derived from the principles of collision theory, the apparent dissociation constant formula_3, an equilibrium constant, is given by:
At the same time, formula_2, the ratio of the concentration of occupied receptor to total receptor concentration, is given by:
By using the expression obtained earlier for the dissociation constant, we can replace formula_43 with formula_44 to yield a simplified expression for formula_45:
which is a common formulation of the Hill equation.
Assuming that the protein receptor was initially completely free (unbound) at a concentration formula_47, then at any time, formula_48 and formula_49. Consequently, the Hill–Langmuir Equation is also commonly written as an expression for the concentration formula_50of bound protein:
All of these formulations assume that the protein has formula_35 sites to which ligands can bind. In practice, however, the Hill Coefficient formula_35 rarely provides an accurate approximation of the number of ligand binding sites on a protein. Consequently, it has been observed that the Hill coefficient should instead be interpreted as an "interaction coefficient" describing the cooperativity among ligand binding sites.
Applications.
The Hill and Hill–Langmuir equations are used extensively in pharmacology to quantify the functional parameters of a drug and are also used in other areas of biochemistry.
The Hill equation can be used to describe dose-response relationships, for example ion channel open-probability (P-open) vs. ligand concentration.
Regulation of gene transcription.
The Hill–Langmuir equation can be applied in modelling the rate at which a gene product is produced when its parent gene is being regulated by transcription factors (e.g., activators and/or repressors). Doing so is appropriate when a gene is regulated by multiple binding sites for transcription factors, in which case the transcription factors may bind the DNA in a cooperative fashion.
If the production of protein from gene is up-regulated (activated) by a transcription factor , then the rate of production of protein can be modeled as a differential equation in terms of the concentration of activated protein:
where is the maximal transcription rate of gene .
Likewise, if the production of protein from gene is down-regulated (repressed) by a transcription factor , then the rate of production of protein can be modeled as a differential equation in terms of the concentration of activated protein:
where is the maximal transcription rate of gene .
Limitations.
Because of its assumption that ligand molecules bind to a receptor simultaneously, the Hill–Langmuir equation has been criticized as a physically unrealistic model. Moreover, the Hill coefficient should not be considered a reliable approximation of the number of cooperative ligand binding sites on a receptor except when the binding of the first and subsequent ligands results in extreme positive cooperativity.
Unlike more complex models, the relatively simple Hill–Langmuir equation provides little insight into underlying physiological mechanisms of protein-ligand interactions. This simplicity, however, is what makes the Hill–Langmuir equation a useful empirical model, since its use requires little "a priori" knowledge about the properties of either the protein or ligand being studied. Nevertheless, other, more complex models of cooperative binding have been proposed. For more information and examples of such models, see Cooperative binding.
Global sensitivity measure such as Hill coefficient do not characterise the local behaviours of the s-shaped curves. Instead, these features are well captured by the response coefficient measure.
There is a link between Hill Coefficient and Response coefficient, as follows. Altszyler et al. (2017) have shown that these ultrasensitivity measures can be linked.