DFT-computation of 13Cα chemical shifts of cystine residues.

MARTIN, O.A.; VILLEGAS M.E.; VILA, J.A.; SCHERAGA H.A.

Salta

Congreso; 3rd Latin American Protein Society Meeting and XXXIX Annual Meeting of the Argentinean Biophysical Society; 2010

Cysteine residues have especial structural and functional properties due to their ability to be present in proteins in either the reduced or oxidized form. CheShift (1), our web-server based on quantum mechanics database of 13Cα chemical shifts is able to make predictions only for reduced cysteine, and not for cysteine residues in cystine. In this work we explore an approximation to circumvent this limitation.13Cα chemical shifts values depend mainly on the backbone torsional angles (2), although the influence of the side-chain torsional angles, cannot be disregarded (3-9). This property enables us to treat each residue X of a protein as a terminally-blocked tripeptide with the sequence Ac-GXG-NMe. To study cysteine residues in cystine we use a couple of tetrapeptides linked by the cysteines sidechains forming anhexaptide (Fig 1). Using this model we have shown (10) that computed 13Cα chemical shifts from oxidized cysteines cannot be inferred straightforwardly from the values computed for the reduced state.This means that explicit consideration of the disulphide bonds is necessary for an accurate prediction of 13Cα chemical shifts of cysteines in cystines. Although, the hexapeptide model is good to accuratelycompute the 13Cα chemical shifts for cysteines in cystine (10). This is not the best model to perform a large number of systematic calculations to obtain a database of 13Cα chemical shifts of cysteines in cystines.In the present work, we introduce 4-thia-methionine (Fig 1) as an adequate model with which to compute the 13Cα and 13Cβ chemical shifts of cysteines in cystines. A test on 7 proteins (Table 1), determined by X-ray crystallography and NMR spectroscopy, containing 837 cysteines in disulfide-bonded cystine reveals the accuracy of the calculated 13Cα and 13Cβ chemical shifts.