# Difference between revisions of "Cyspka"

Type Python Script cyspka.py Troels E. Linnet BSD This code has been put under version control in the project Pymol-script-repo

## Introduction

This script is an experimental surface cysteine pKa predictor.
The script is solely based on the work by:

Predicting Reactivities of Protein Surface Cysteines as Part of a Strategy for Selective Multiple Labeling.
Maik H. Jacob, Dan Amir, Vladimir Ratner, Eugene Gussakowsky, and Elisha Haas.
Biochemistry. Vol 44, p. 13664-13672, doi:10.1021/bi051205t

Questions to the article should be send to Maik Jacob.

### Overview

The authors Jacob et al. were able to describe a computational algorithm that could predict the reactivity of surface cysteines. The algorithm was based on reaction rates with Ellmans reagent, Riddles et al.], on 26 single cysteine mutants of adenylate kinase. The authors could predict the reactivity of the cysteines with a pearson correlation coe�cient of 0.92. The algorithm was based on predicting the pKa values of cysteines by a calculation of electrostatic interactions to the backbone and sidechains of the protein and a energetic solvation effect from the number of atom neighbours. The algorithm is diff�erent from other pKa algorithms, since it calculates a Boltzmann energy distribution for the rotational states of cysteine. The reaction rate with Ellman's reagent was set proportional to the fraction of negatively charged cysteines, Bulaj et al.

### Algorithm development

The algorithm is based on electrostatic calculations, where some parameters have been fine-tuned.
The distance from the sulphur atom (SG) of the cysteine to the nearest backbone amide groups and residues with a partial charge, is considered in the electrostatic model.
The model is including a evalution of Boltzman distribution of the rotation of the SG atom around the CA->CB bond.

Twenty-six mutants of Escherichia coli adenylate kinase (4AKE) were produced, each containing a single cysteine at the protein surface, and the rates of the reaction with Ellman's reagent were measured. The reaction rate was set proportional to the pKa, to fine-tune the parameters in the electro static model.

### Correction to article

There is a type error in equation 6. There is missing a minus "-". The equations should read:
${\displaystyle W_{MC,SC(i)}=-\left(\sum W_{MC(i)}+\sum W_{SC(i)}\right)}$

## Example of use

reinitialize
import cyspka

fetch 4AKE, async=0
create 4AKE-A, /4AKE//A and not resn HOH
delete 4AKE
hide everything
show cartoon, 4AKE-A
cyspka 4AKE-A, A, 18

### You can loop over several residues.
loopcyspka 4AKE-A, A, residue=18.25.41-42

### OR for the original 26 residues. Takes a long time, so not to many at the time.
#loopcyspka 4AKE-A, A, residue=18.25.41-42.55.73.90.113.162.188-189.203.28.58.75.102.138.142.148.154.169.214.3.24.86.109


## References

• Ellman's Reagent: 5,5'-Dithiobis(2-nitrobenzoic Acid) a Reexamination. Peter W. Riddles, Robert L. Blakeley, and Burt Zerner

. Analytical Biochemistry. Vol 94, p. 75-81, 1979