Interaction of thioredoxins with target proteins: Role of particular structural elements and electrostatic properties of thioredoxins in their interplay with 2‐oxoacid dehydrogenase complexes

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Abstract
The thioredoxin action upon the 2‐oxoacid dehydrogenase complexes is investigated by using different thioredoxins, both wild‐type and mutated. The attacking cysteine residue of thioredoxin is established to be essential for the thioredoxin‐dependent activation of the complexes. Mutation of the buried cysteine residue to serine is not crucial for the activation, but prevents inhibition of the complexes, exhibited by the Clamydomonas reinhardtii thioredoxin m disulfide. Site‐directed mutagenesis of D26, W31, F/W12, and Y/A70 (the Escherichia coli thioredoxin numbering is employed for all the thioredoxins studied) indicates that both the active site and remote residues of thioredoxin are involved in its interplay with the 2‐oxoacid dehydrogenase complexes. Sequences of 11 thioredoxin species tested biochemically are aligned. The thioredoxin residues at the contact between the α3/310 and a1 helices, the length of the α1 helix and the charges in the α2‐β3 and β4‐β5 linkers are found to correlate with the protein influence on the 2‐oxoacid dehydrogenase complexes (the secondary structural elements of thioredoxin are defined according to Eklund H et al., 1991, Proteins 11:13–28). The distribution of the charges on the surface of the thioredoxin molecules is analyzed. The analysis reveals the species specific polarization of the thioredoxin active site surroundings, which corresponds to the efficiency of the thioredoxin interplay with the 2‐oxoacid dehydrogenase systems. The most effective mitochondrial thioredoxin is characterized by the strongest polarization of this area and the highest value of the electrostatic dipole vector of the molecule. Not only the magnitude, but also the orientation of the dipole vector show correlation with the thioredoxin action. The dipole direction is found to be significantly influenced by the charges of the residues 13/14, 51, and 83/85, which distinguish the activating and inhibiting thioredoxin disulfides.