Binding selectivity

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Binding selectivity is determined in relation to the binding of the ligand to the substrate that forms the complex. The selectivity coefficient is the equilibrium constant for the displacement reaction by one ligand of the other ligand in the complex with the substrate. Binding selectivity is very important in biochemistry and in chemical separation processes.

Video Binding selectivity

Koefisien selektivitas

[..] mewakili sebuah konsentrasi. Koefisien selektivitas didefinisikan sebagai rasio dari dua konstanta kesetimbangan.

${\ displaystyle K _ {{\ ce {B, C}}} = {\ frac {K_ {{\ ce {AC}}}} {K _ {{\ ce { AB}}}}}}  ÂÂ$

It is easy to show that the same definition applies to different stoichiometric complexes, A _p B _q and A _p C _q . The greater the selectivity coefficient, the more the C ligand will replace the B ligand from the complex formed with substrate A. Alternative interpretation is the greater the selectivity coefficient, the lower the C concentration required to replace B from AB. The selectivity coefficient is determined experimentally by measuring two equilibrium constants, K _AB and K _AC .

Maps Binding selectivity

Apps

Biochemistry

In biochemistry, substrates are known as receptors. Receptors are protein molecules, which are embedded in either the plasma membrane or the cell cytoplasm, of which one or more specific types of signaling molecules can bind. Ligands may be peptides or other small molecules, such as neurotransmitters, hormones, pharmaceutical drugs, or toxins. The specificity of a receptor is determined by its spatial geometry and the way it binds the ligand through non-covalent interactions, such as hydrogen bonds or Van der Waals forces.

If the receptors can be isolated, synthetic drugs can be developed either to stimulate receptors, agonists or to block them, antagonists. Cimetidine stomach ulcers are developed as H ₂ antagonists by molecular chemical engineering for maximum specificity to isolated tissues containing receptors. Further use of quantitative activity-structure relations (QSAR) leads to the development of other agents such as ranitidine.

It is important to note that "selectivity" when referring to the drug is relative and not absolute. For example, in higher doses, certain drug molecules may also bind to other receptors than are said to be "selective".

Chelation therapy

Chelation therapy is a form of medical treatment in which chelating ligands are used to selectively remove metals from the body. When the metal exists as divalent ions, such as lead, Pb ² or mercury, Hg ² selectivity to calcium, Ca ² and magnesium, Mg < soup> 2 , is essential so that treatment does not remove essential metals.

Selectivity is determined by various factors. In the case of iron overload, which may occur in individuals with -essalemia who have received blood transfusions, the target metal ion is in a state of oxidation 3 and forms a stronger complex than the divalent ion. It also forms a stronger complex with oxygen-donor ligand compared to a donor-nitrogen ligand. deferoxamine, a natural siderophore produced by actinobacter Streptomyces pilosus and was originally used as a chelation therapy agent. Synthetic siderophores such as deferiprone and deferasirox have been developed, using a deferoxamine structure known as the starting point. Chelation occurs with two oxygen atoms.

Wilson's disease is caused by a defect in copper metabolism that produces copper metal accumulation in various organs of the body. The target ion in this case is divalent, Cu ² . These ions are classified as boundary lines in the Ahrland, Chatt and Davies schemes. This means that it forms around a complex just as strong as the ligand whose donor atoms are N, O or F as the ligand whose donor atoms P, S or Cl. Penicillamine, which contains nitrogen and sulfur donor atoms, is used because this type of ligand binds more strongly to copper ions than calcium and magnesium ions.

Treatment of poisoning by heavy metals such as lead and mercury is more problematic, since the ligands used do not have high specificity relative to calcium. For example, EDTA may be administered as a calcium salt to reduce calcium expenditure from bone along with heavy metals. Factors that determine the selectivity for lead to zinc, cadmium and calcium have been reviewed,

Chromatography

In column chromatography, the mixture of the substance is dissolved in the mobile phase and passes through the stationary phase in the column. The selectivity factor is defined as the ratio of the distribution coefficient, which describes the distribution of the analytical balance between the stationary phase and the mobile phase. The selectivity factor is equal to the selectivity coefficient with the additional assumption that the stationary phase activity, the substrate in this case, equals 1, the standard assumption for pure phase. The column chromatographic resolution, R _S is related to the selectivity factor by:

$Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â}$ _{         R            S                          =                        Â         <Â>     ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,       Â   Â 4                                    (                                                    ?              -     Â 1        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,                   ?      ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,                     )                           (                          Â      Â                                  B        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,       Â                   Â 1                  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ...                  k                                     B     ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,        Â        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,          ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,                     )                       {\ displaystyle R_ {S} = {\ frac {\ sqrt {N}} {4}} \ left ({\ frac {\ alpha -1} {\ alpha}} \ right) \ left ({\ frac {k_ {B}} {1 k_ {B}}} \ right)}  Â}

Where? is the selectivity factor, N is the number of theoretical plates k _A and k _B is the retention factor of the two analytes. The retention factor is proportional to the distribution coefficient. In practice, substances with selectivity factors very close to 1 can be separated. This is especially true in gas-liquid chromatography where column lengths up to 60 m are possible, providing a large number of theoretical plates.

In ion exchange chromatography, the selectivity coefficient is defined in slightly different ways

Solvent extraction

Solvent extraction is used to extract individual lanthanide elements from mixtures found in nature in ores such as monazite. In one process, metal ions in aqueous solutions are made to form complexes with tributylphosphate (TBP), which are extracted into organic solvents such as kerosene. Complete separation is done by using the counter-exchange method. A number of cells are arranged as cascades. After equilibration, the aqueous component of each cell is transferred to the previous cell and the organic component is transferred to the next cell, which initially contains only water. In this way the metal ion with the most stable complex passes through the cascade in the organic phase and the metal with the most stable complex passes through the cascade in the aqueous phase.

If solubility in the organic phase is not a problem, the selectivity coefficient equals the ratio of TBP complex stability constants of the two metal ions. For adjacent lanthanide elements in the periodic table, this ratio is not greater than 1, so many cells are required in the cascade.

Chemical sensors

The potentiometric selectivity coefficient defines the ability of the ion-selective electrode to distinguish one particular ion from another. The selectivity coefficient, K _{B, C} is evaluated by using the emf response of the ion selective electrode in the mixed solution of the primary ions, B, and the interfering ions, C (interference method fixed) or less desirable, in a separate solution B and C (separate solution method). For example, potassium-selective ion membrane electrodes using naturally occurring macrocyclic antibiotic valinomycin. In this case the cavity in the macrocyclic ring is the right size to wrap potassium ions, but is too large to bind sodium ions, the most likely disturbance, very strong.

Chemical sensors, are being developed for certain target molecules and ions in which the target (guest) forms a complex with a sensor (host). Sensors are designed to be an excellent pair in terms of size and shape of the target to provide maximum binding selectivity. The indicator is associated with a sensor that changes when the target forms a complex with the sensor. Indicator changes are usually a change of color (gray to yellow in the illustration) seen in absorbance or, with greater sensitivity, luminescence. The indicator can be attached to the sensor through the spacer, in the ISR setting, or it may be removed from the sensor, the IDA setting.

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See also

• Bind
• Affinity
• Functional selectivity

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Note

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References

Source of the article : Wikipedia