Effects of Buried Charged Groups on Cysteine Thiol Ionization and Reactivity in Escherichia coli Thioredoxin: Structural and Functional Characterization of Mutants of Asp 26 and Lys 57

Abstract

To investigate the role of Asp 26 and Lys 57, two conserved, buried residues, in the redox mechanism of Escherichia coli thioredoxin (Trx), three mutant proteins, Asp 26 → Ala (D26A), Lys 57 → Met (K57M), and the double mutant D26A/K57M, were prepared, replacing the charged amino acids with hydrophobic residues with similar sizes. Both the oxidized (Trx-S₂) and reduced [Trx-(SH)₂] forms of the mutant thioredoxins are fully folded and similar in overall structure to the wild-type protein (wt). The structure of the active site hydrophobic surface is unchanged by the mutation of Asp 26 and Lys 57, since DNA polymerase activity in the 1:1 complex of the T7 gene 5 protein and mutant Trx-(SH)₂ shows similar K_d values (∼5 nM) for both mutants and wt. In contrast, redox reactions involving thioredoxin as a catalyst of the reduction of disulfides or oxidation of dithiols are strongly affected by the mutations. In the reaction of Trx-S₂ with thioredoxin reductase at pH 8.0, the k_cat/K_m value for the D26A mutant is decreased by a factor of 10 from that of wt, while the value for the D26A/K57M mutant is reduced 40-fold. The activity of Trx-(SH)₂ as a protein disulfide reductase was measured with insulin, using fluorescence to detect oxidation of thioredoxin. At 15 °C and pH 8.0, both the D26A and K57M mutants showed 5−10-fold decreases in rates of reaction compared to those of the wild type, and the pH−rate profiles for the mutants were shifted 1 (K57M) and 2 (D26A) units to higher pH compared with the wt curve. NMR measurements for the three mutant proteins indicate that the proteins have the same global fold as that of the wild type, although changes in the chemical shifts of a number of resonances indicate local structural changes in the active site region. The resonances of oxidized D26A and D26A/K57M are pH-independent between pH 6.0 and 10.0, confirming the identification of the active site group titrating with a pK_a of 7.5 in wt Trx-S₂ as Asp 26. A profound change in the pK_a of Asp 26, from 7.5 in the wild type to 9.4 in the mutant, is observed for K57M Trx-S₂. The pH-dependent behavior of the resonances is affected in all mutant Trx-(SH)₂ proteins. A single pK_a shifted to higher values is observed on both the Cys 32 and Cys 35 C^β resonances. Ultraviolet absorbance measurements (A₂₄₀) as a function of pH for wt Trx-(SH)₂ demonstrate that the cysteine thiols titrate with apparent pK_as of about 7.1 and 9.9. The mutant proteins each show a single transition in the A₂₄₀ measurements, with a midpoint at pH 7.8−8.0, consistent with the NMR results. The change in absorbance at 240 nm with increasing pH indicates that the number of thiols titrating in each mutant is greater than one but less than two. It is clear that both thiol pK_as have been significantly shifted by the mutations. The Cys 32 pK_a is moved from 7.1 in wt to 7.8−8.0 in the mutants. The value of the Cys 35 pK_a either is indistinguishable from that of Cys 32, thus accounting for more than one thiol titrating in the UV absorbance measurements or else is shifted to much higher pHs (>10) where its transition is masked in both UV and NMR measurements by the effects of ionization of the tyrosine residues and unfolding of the protein. Our results strongly suggest that the buried Asp 26 carboxyl and Lys 57 ε-amino groups significantly affect the pK_as of the active site thiols, particularly that of the exposed low-pK_a thiol Cys 32, thereby enhancing the rates of thiol−disulfide reactions at physiological pH.