HOMOLOGY MODELING OF A NEWLY DISCOVERED THIOREDOXIN PROTEIN AND ANALYSIS OF THE FORCE-FIELD AND ELECTROSTATIC PROPERTIES

Thioredoxin is a small protein (Mr approximately 12,000) found in all living cells from archaebacteria to humans. The active site is highly conserved and has two redox-active cysteine residues in the sequence: -Trp-Cys-Gly-Pro-Cys-. Besides the function of the reduced form as a p owerful protein disulfide oxidoreductase, thioredoxin is known to regu late and activate different target enzymes, i.e. ribonucleotide reduct ase and the mitochondrial 2-oxoacid dehydrogenase multienzyme complexe s. Despite the high degree of homology between thioredoxin proteins fr om different species, there exists a strong variation in the capabilit y of activating target enzymes. This is yet unexplainable since there still exists no model of a thioredoxin/receptor complex. On the basis of the recently determined amino acid sequence of the thioredoxin Trx2 from rat mitochondria, which is known to be highly efficient in activ ating mitochondrial 2-oxoacid dehydrogenase multienzyme complexes, we construct the 3-D structure of this protein by homology modelling meth ods, using the X-ray structures of thioredoxin from E. coli and human as background information. We analyze the differences in the electrost atic properties of the different protein structures and show, that des pite the observed homology between the primary sequences, the dipole m oment of the protein structures shows significant variations, which mi ght lead to deviations with respect to the binding to the target prote in. Using the AMBER 4.0 program package we further investigate and com pare the force field energies of the different thioredoxin structures.