Name: measuring protein-ligand binding strength
Uploaded: 2020-11-23T11:47:34.000Z
Duration: 2 min 18 s
Channel: JoVE

K12 Standards

Tennessee Academic Standards

High School

Biology

from-molecules-to-organisms-structures-and-processes

From Molecules to Organisms: Structures and Processes

BIO1.LS1.3

the-equilibrium-binding-constant-and-binding-strength

The Equilibrium Binding Constant and Binding Strength

In the cell, proteins randomly collide with other molecules, such as other proteins, nucleic acids, and small-molecule ligands.
If the binding is non-specific, few non-covalent interactions between the molecules result in a brief association.
If a specific ligand binds to the protein, it forms extensive non-covalent interactions along complementary surfaces. Such complexes are stable, staying bound for a long time, before dissociating.
The strength of a binding interaction is reported in terms of its equilibrium constant, Kb, also called the binding or association constant. 
Kb can be calculated from the ratio of the concentration of protein-ligand complex over the concentrations of unbound protein and ligand found at equilibrium. 
Given its relation with the free energy change due to binding, a large Kb means a large decrease in delta G, indicating strong affinity between protein and ligand.
Two competing processes are important for protein-ligand binding: the association of a protein and a ligand to form a complex and the dissociation of the complex into the reactants.
The association constant, kon, is a measure of the number of binding events per second between a protein and its ligand; it can be used to calculate the rate of ligand binding to the protein at a given concentration. 
Conversely, koff, is a measure of the number of dissociation events; it can be used to calculate how quickly the complex comes apart.
When the rate of association equals the rate of dissociation, an equilibrium is reached, where the net concentrations of products and reactants remain constant.
Thus, at equilibrium, kon times the product of the equilibrium concentrations of the protein and the ligand equals koff times the equilibrium concentration of the protein-ligand complex.
Rearranging this expression also shows that the ratio of kon to koff equals Kb at equilibrium.

the equilibrium binding constant and binding strength

The equilibrium binding constant (Kb) quantifies the strength of a protein-ligand interaction. Kb can be calculated as follows when the reaction is at equilibrium:
<img alt="Static equilibrium physics diagram; includes &#931;Fx=0, &#931;Fy=0, rigid bodies, torque analysis symbols." src="/files/ftp_upload/11513/Equation%201.png" style="text-align: center;" data-altupdatedat="1761830439000" data-alt="Eq1" />
where P and L are the unbound protein and ligand, respectively, and PL is the protein-ligand complex. 
As the amount of bound ligand is also related to the rate of ligand binding, experiments can also determine Kb by examining the rates of protein-ligand association (kon) and dissociation (koff) using the following ratio:
<img alt="Fluid dynamics principles, Navier-Stokes equations, vector notation diagram, velocity field analysis." src="/files/ftp_upload/11513/Equation%202.png" style="text-align: center;" data-altupdatedat="1761830439000" data-alt="Eq1" />
Thus, two categories of binding assays are used to determine the equilibrium binding constant &#8211; those that measure equilibrium concentrations and those that measure a reaction&#8217;s kinetics. In case, the reaction must be at equilibrium at the time of measurement.
The method of determining the equilibrium concentrations depends on the desired sensitivity and ease of signal detection. For these reasons, spectroscopic assays are most widely used. In these experiments, the reaction produces an absorbance change of a reactant or a product at a given wavelength, detectable by a UV-Vis spectrophotometer. Alternatively, the reactant or product can be tagged with a fluorescent probe or may contain an intrinsic fluorophore. Then, the reaction progress can be measured from the change in fluorescence. These assays are performed by varying one reactant&#8217;s concentrations while the rest of the experiment is held constant. The results can then be graphed and analyzed with various curve fitting methods.
Interactions between proteins and ligands are also studied using a variety of biochemical and spectroscopic techniques. Structural analysis, using X-ray crystallography and NMR spectroscopy, aids in predicting protein-ligand interactions through molecular simulations. Theoretical and computational approaches, such as protein-ligand docking studies, are used extensively to characterize the position and interactions of small molecule ligands, including drug candidates. Computer-aided drug design is a fast and low-cost alternative to accelerate the pace of conventional trial and error drug testing.

The equilibrium binding constant (Kb) quantifies the strength of a protein-ligand interaction. Kb can be calculated as follows when the reaction is at ...

Education

JoVE Core

Molecular Biology

Unit 1

Protein Function

KEY TERMS AND CONCEPTS

measuring protein-ligand binding strength

Protein-ligand binding strength can be measured with the equilibrium binding constant, Kb. This value describes how strongly a protein binds a ligand when the reaction has reached equilibrium. In a bound complex, the protein and ligand are joined as PL, while P and L are the unbound forms.

Kb can be determined in two main ways. One approach uses equilibrium concentrations. The other uses reaction kinetics, which are the rates of binding and unbinding. Kb can also be related to the binding rate constant, kon, and the dissociation rate constant, koff, by taking their ratio.

When scientists measure equilibrium concentrations, the reaction must already be at equilibrium. The best method depends on how sensitive the assay needs to be and how easy it is to detect the signal. For this reason, spectroscopic assays are used most often. These assays track absorbance changes in a reactant or product with a UV-Vis spectrophotometer.

Fluorescence can also be used to follow binding. A reactant or product may be tagged with a fluorescent probe, or it may already have an intrinsic fluorophore. In that case, the progress of the reaction is measured by changes in fluorescence. These experiments usually change the concentration of one reactant while the other parts of the setup stay constant, and the results are then graphed and fit to curves.

Protein-ligand interactions are also studied with other biochemical and spectroscopic methods. X-ray crystallography and NMR spectroscopy can provide structural information that helps predict how a protein and ligand may interact through molecular simulations. Protein-ligand docking studies are also used to place small molecule ligands, including drug candidates, and to examine their interactions. These computer-aided methods offer a fast and low-cost way to support drug design.

Measuring Protein-Ligand Binding Strength

measuring-protein-ligand-binding-strength

ligand binding sites

How Ligands Attach to Proteins

Ligands attach to proteins at specific binding sites on the protein surface. These sites let proteins interact with other molecules or ions and help them carry out important cell functions. Although many possible ligands may be nearby, only a matching ligand can bind to a given protein. A ligand-binding site has a shape and chemical pattern that fit its ligand. This fit comes from the protein’s amino acid chain, which gives the site its structure and reactivity. The ligand stays in place...

protein-protein interfaces

How Proteins Connect to Form Complexes

Proteins connect with other proteins to form complexes that carry out cell functions. These protein-protein interactions, or PPIs, are important for an organism's survival. Most PPIs are held together by many weak, noncovalent chemical forces rather than one strong bond. The shape of the interaction surface helps determine how two proteins fit together. Many globular proteins have surface shapes that match closely and can form many weak bonds. Other PPIs happen between two helices or between a...

conserved binding sites

Predicting Protein Ligand-Binding Pockets

Protein ligand-binding pockets help explain how proteins work. A ligand is a small molecule that binds to a specific spot on a protein. These sites are often conserved in related proteins because they are important for function. When the exact site on a protein surface is not known, it can be predicted. One approach is the energetic method. It examines how amino acid residues interact with the ligand and looks for positions where the binding energy is lowest. Those low-energy regions are...

Protein-ligand binding strength can be measured with the equilibrium binding constant, Kb. This value describes how strongly a protein binds a ligand when the reaction has reached equilibrium. In a bound complex, the protein and ligand are joined as PL, while P and L are the unbound forms. Kb can be determined in two main ways. One approach uses equilibrium concentrations. The other uses reaction kinetics, which are the rates of binding and unbinding. Kb can also be related to the binding rate...

cofactors and coenzymes

Protein Helpers: Cofactors and Coenzymes

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allosteric regulation

How Allosteric Control Changes Enzymes

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ligand binding and linkage

Allosteric Proteins: Positive and Negative Effects

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cooperative allosteric transitions

Allosteric Switching in Multimeric Proteins

Allosteric switching in multimeric proteins helps explain how ligand binding at one site can change the behavior of the whole protein. In these proteins, each subunit has its own ligand-binding site. When a ligand binds to one subunit, it can cause a shape change that affects the other subunits and their binding sites. This change in shape comes from the protein’s structure, which includes both flexible and stable segments. A molecule that causes the shift is called a modulator. Because the...

phosphorylation

Phosphorylation in Cell Signaling

Phosphorylation is a key step in cell signaling. It changes how a molecule behaves by adding a phosphate group. In many cells, this modification helps control activity, shape, and interactions with other molecules. Phosphorylation is carried out by enzymes called kinases. These enzymes transfer a phosphate group to a target molecule. A phosphatase can remove that phosphate group and reverse the change. Together, kinases and phosphatases help cells turn signals on and off in a controlled way. <br...

protein kinases and phosphatases

Phosphorylation Switches in Protein Control

Protein control often depends on chemical changes that alter a protein’s charge, shape, and conformation. These changes can include phosphorylation, acetylation, glycosylation, nitrosylation, ubiquitination, lipidation, methylation, and proteolysis. In cells, these modifications are usually driven by enzymes. Phosphorylation is one of the most important protein modifications. It adds a phosphate group to a protein and often changes how that protein works. A family of enzymes called kinases...

gtpases and their regulation

GTPase Signaling Switches in Cells

GTPases are cell proteins that act as signaling switches. They help control cell signaling, vesicle transport, cell shape, and cell movement. When these proteins change or stop working properly, disease can result. There are about 40,000 known G-proteins. They are usually grouped into two main types. Small G-proteins have one domain, while large G-proteins have multiple domains. Large G-proteins are also called heterotrimeric G-proteins. They have three subunits: alpha, beta, and gamma. The...

covalently linked protein regulators

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Proteins can be changed after they are made by adding small chemical tags or small proteins. These post-translational modifications help control protein function and stability when the cell environment changes. Common covalent regulators include methyl, acetyl, and phosphate groups, as well as small proteins such as ubiquitin. About 200 types of covalent regulators have been identified. These tags attach to specific amino acids in a protein. Phosphate groups can attach only to serine,...

protein complexes with interchangeable parts

protein-complexes-with-interchangeable-parts

SCF Ubiquitin Ligase and F-Box Variants

The SCF ubiquitin ligase is a protein complex that uses an F-box protein to help choose which target proteins get marked for destruction. This complex is made of five separate proteins, and each one has a different job. Together, they attach ubiquitin to target proteins so the cell can send those proteins to be degraded. The F-box protein is the part that binds the substrate, or target protein. That binding step lets the ubiquitin-conjugating enzyme reach the target and attach ubiquitin. In...

mechanical protein functions

Protein Force: Cell Movement and Support

Proteins can create and respond to mechanical force in cells. Some proteins generate force, while others are pulled or stretched by it. Proteins such as keratin provide structural support and are subjected to mechanical force. Proteins such as ion pumps help move molecules across cell membranes and can generate mechanical force. Cell movement and muscle contraction depend on turning chemical energy into motion. This often happens through conformational change, which is a shift in a protein’s...

structural protein function

Collagen and the Cell Skeleton

Structural proteins give cells and tissues shape, support, and strength. They help build bones, cartilage, hair, and muscles. Important examples include collagen, actin, myosin, and keratin. Collagen is the most abundant protein in mammals. It is found throughout the body. In skin, ligaments, and tendons, collagen adds tensile strength and elasticity. In bones and teeth, it helps form hard, load-bearing tissue through mineralization. Collagen also does more than provide support. It can...

protein networks

Mapping Protein Interactions

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allosteric proteins-atcase

ATCase Feedback Inhibition in Pyrimidine Synthesis

ATCase is a key enzyme in pyrimidine synthesis. It is a cytosolic enzyme that catalyzes the condensation of L-aspartate and carbamoyl phosphate to form N-carbamoyl-L-aspartate. This reaction starts the pathway that makes pyrimidines. ATCase is regulated by feedback inhibition. The end products UTP and CTP bind to the enzyme and reduce its activity when pyrimidine levels are high compared with purines in the cell. This binding at one site affects the linked catalytic site, which helps keep...

key terms and concepts

Learn how the equilibrium binding constant, Kb, is measured using equilibrium concentrations, kinetics, absorbance, fluorescence, and docking methods.

Measuring Protein-Ligand Binding Strength | JoVE K12

molecular biology

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Life Science

HS+B.L1

HS+B.L1U1.5

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BIO1.LS1.6

Measuring Protein-Ligand Binding Strength

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