Theoretical study of the unusual protonation properties of the active site cysteines in thioredoxin

• Type: Article in International Scientific Journal
• Date: 2006

Authors

Carvalho, ATP (Author) Other; Fernandes, PA (Author) FCUP; Ramos, MJ (Author) FCUP

Journal

Title: Journal of Physical Chemistry B

Vol. 110

Pages: 5758-5761

ISSN: 1520-6106

Publisher: American Chemical Society

Indexing

ISI Web of Knowledge - 27 Citations

Scopus - 27 Citations

Scientific classification

FOS: Natural sciences > Chemical sciences

Other information

Authenticus ID: P-004-MPV

DOI: 10.1021/jp053275f

Abstract (EN): The thiol/disulfide oxidoreductases of the thioredoxin family have, in the active site, two cysteines that can be in a reduced or an oxidized form. One of the cysteines in the reduced state is deprotonated, and it is called nucleophilic cysteine. The pK(a) of this cysteine is different from that of a normal cysteine and varies widely among the different enzymes of this family. However, the factors responsible for the different degrees of stabilization of nucleophilic cysteine thiolate are not fully understood. Here, we have studied the well-known hypothesis of proton sharing between the active site thiols by performing a linear transit scan for the transfer of the proton between the active site cysteines. We used a two-layered (DFT/MM) ONIOM formalism, with the active site region treated at the B3LYP/6-31+G(d) level and the remains of the protein treated with the Amber Parm94 force field. The solvation free energy was accounted for with a continuum solvent model, by solving the Poisson-Boltzmann equation using the program Delphi. We have obtained excellent agreement with the experimental data available in the literature. Besides refuting the proton sharing hypothesis, our results include a value of 14.0 for the pK(a) of the buried cysteine, a quantity that has not been possible to obtain experimentally but which has been proven to be higher than 11. Additionally, this study also provides detailed information on the very interesting and so far unknown fact that the contribution of the enzymatic structure (8.3 kcal/mol) prevails in relation to that of the solvent (0.60 kcal/mol) concerning the differential stabilization of the active site thiolates.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 4