Protein pKa calculations


Protein pKa calculations

In computational biology, protein pKa calculations are used to estimate the pKa values of amino acids as they exist within proteins. These calculations complement the pKa values reported for amino acids in their free state, and are used frequently within the fields of molecular modeling, structural bioinformatics, and computational biology.

Amino acid pKa values

pKa values of amino acid side chains play an important role in defining the pH-dependent characteristics of a protein. The pH-dependence of the activity displayed by enzymes and the pH-dependence of protein stability, for example, are properties that are determined by the pKa values of amino acid side chains.

The pKa values of an amino acid side chain in solution is typically inferred from the pKa values of model compounds (compounds that are similar to the side chains of amino acids). ("See Amino acid for the pKa values of all amino acid side chains inferred in such a way.) The table below lists the model pKa values that are normally used in a protein pKa calculation.

The effect of the protein environment

When a protein folds, the titratable amino acids in the protein are transferred from a solution-like environment to an environment determined by the 3-dimensional structure of the protein. For example, in an unfolded protein an Aspartic acid typically is in an environment which exposes the titratable side chain to water. When the protein folds the Aspartic acid could find itself buried deep in the protein interior with no exposure to solvent.Furthermore, in the folded protein the Aspartic acid will be closer to other titratable groups in the protein and will also interact with permanent charges (e.g. ions) and dipoles in the protein.All of these effects alter the pKa value of the amino acid side chain, and pKa calculation methods generally calculate the effect of the protein environment on the model pKa value of an amino acid side chain.

Typically the effects of the protein environment on the amino acid pKa value are divided into pH-independent effects and pH-dependent effects. The pH-independent effects (desolvation, interactions with permanent charges and dipoles) are added to the model pKa value to give the intrinsic pKa value. The pH-dependent effects cannot be added in the same straight-forward way and have to be accounted for using Boltzmann summation, Tanford-Roxby iterations or other methods.

The interplay of the intrinsic pKa values of a system with the electrostatic interaction energies between titratable groups can produce quite spectacular effects such as non-Henderson-Hasselbalch titration curves and even back-titration effects A. Onufriev, D.A. Case and G. M. Ullmann (2001). "Biochemistry" 40: 3413-3419. ] . [http://enzyme.ucd.ie/Science/pKa/pKaTool pKaTool] provides an easy interactive and instructive way of playing around with these effects.

The image below shows a theoretical system consisting of three acidic residues. One group is displaying a back-titration event (blue group).

pKa calculation methods

Several software packages and webserver are available for the calculation of protein pKa values. See links below or [http://enzyme.ucd.ie/Science/pKa/Software#other this table]

Using the Poisson-Boltzmann equation

Some methods are based on solutions to the Poisson-Boltzmann equation (PBE), often referred to as FDPB-based methods ("FDPB" is for "finite difference Poisson-Boltzmann"). The PBE is a modification of Poisson's equation that incorporates a description of the effect of solvent ions on the electrostatic field around a molecule.

The [http://biophysics.cs.vt.edu/H++/ H++ web server] , the [http://enzyme.ucd.ie/pKD pKD webserver] , [http://www.sci.ccny.cuny.edu/~mcce/ MCCE] and [http://agknapp.chemie.fu-berlin.de/karlsberg Karlsberg+] use the FDPB method to compute pKa values of amino acid side chains.

FDPB-based methods calculate the change in the pKa value of an amino acid side chain when that side chain is moved from a hypothetical fully solvated state to its position in the protein. To perform such a calculation, one needs theoretical methods that can calculate the effect of the protein interior on a pKa value, and knowledge of the pKa values of amino acid side chains in their fully solvated state.

Empirical methods

A set of empirical rules relating the protein structure to the pKa values of ionizable residues have been developed by [http://dx.doi.org/10.1002/prot.20660 Li, Robertson, and Jensen] . These rules form the basis for the [http://propka.ki.ku.dk web-accessible] program called PROPKA for rapid predictions of pKa values.

Molecular dynamics (MD)-based methods

Molecular dynamics methods of calculating pKa values involve computationally measuring the free energy difference between the protonated and deprotonated forms of the molecule. This free energy differences is measured using methods such as free-energy perturbation, thermodynamic integration and the Bennett acceptance ratio.

Molecular dynamics is typically a much more computationally expensive way to predict pKa's than using the Poisson-Boltzmann equation. Currently used molecular force fields do not take polarizability into account, which could be an important property for protonation energies.

Using titration curves to determine pKa values

The pH value where the titratable group is half-protonated is equal to the pKa if the titration curve follows the Henderson-Hasselbalch equation. Most pKa calculation methods silently assume that all titration curves are Henderson-Hasselbalch shaped, and pKa values in pKa calculation programs are therefore often determined in this way.

References

oftware For Protein pKa Calculations

* [http://www.accelrys.com/products/dstudio/ AccelrysPKA] Accelrys CHARMm based pKa calculation
* [http://biophysics.cs.vt.edu/H++/ H++] Poisson-Boltzmann based pKa calculations
* [http://www.sci.ccny.cuny.edu/~mcce/ MCCE] Multi-Conformation Continuum Electrostatics
* [http://agknapp.chemie.fu-berlin.de/karlsberg Karlsberg+] pKa computation with multiple pH adapted conformations.
* [http://enzyme.ucd.ie/pKD pKD server] pKa calculations and pKa value re-design
* [http://propka.ki.ku.dk/ PROPKA] Empirical calculation of pKa values
* [http://enzyme.ucd.ie/Science/pKa/Software WHAT IF pKa calculation package]

Analysis and teaching software

* [http://enzyme.ucd.ie/Science/pKa/pKaTool pKaTool]
* [http://biophysics.cs.vt.edu/H++ H++ webserver]


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Enzyme catalysis — is the catalysis of chemical reactions by specialized proteins known as enzymes. Catalysis of biochemical reactions in the cell is vital due to the very low reaction rates of the uncatalysed reactions. The mechanism of enzyme catalysis is similar …   Wikipedia

  • Proteinogenic amino acid — Proteinogenic amino acids are those amino acids that can be found in proteins and require cellular machinery coded for in the genetic code [1] of any organism for their isolated production. There are 22 standard amino acids, but only 21 are found …   Wikipedia

  • Acid dissociation constant — Acetic acid, a weak acid, donates a proton (hydrogen ion, high …   Wikipedia

  • WHAT IF software — WHAT IF is a computer program used in a wide variety of in silico macromolecular structure research fields such as: * Homology models of protein tertiary structures as well as quaternary structures, * Validation of protein structures, noticeably… …   Wikipedia

  • Partition coefficient — In chemistry and the pharmaceutical sciences, a partition (P) or distribution coefficient (D) is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium.[1] The terms gas/liquid partition …   Wikipedia

  • Isoelectric point — The isoelectric point (pI) is the pH at which a particular molecule or surface carries no net electrical charge. Amphoteric molecules called zwitterions contain both positive and negative charges depending on the functional groups present in the… …   Wikipedia

  • Arieh Warshel — is a professor of Chemistry and Biochemistry at the University of Southern California. Known for his work on Computational biochemistry and biophysics, he is the originator of modern force field approaches where the use of Cartesian coordinates… …   Wikipedia

  • Quantitative structure-activity relationship — (QSAR) is the process by which chemical structure is quantitatively correlated with a well defined process, such as biological activity or chemical reactivity.For example, biological activity can be expressed quantitatively as in the… …   Wikipedia

  • Formaldehyde — For other uses, see Formaldehyde (disambiguation). Formaldehyde …   Wikipedia

  • Squaric acid — chembox new Reference= [ [http://www.sigmaaldrich.com/catalog/search/ProductDetail/ALDRICH/123447?cm mmc=PubChem public%20database products products 3,4 Dihydroxy 3 cyclobutene 1,2 dione] at Sigma Aldrich] ImageFile=Squaric acid.svg… …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.