Molecular oxygen is indispensable for survival of aerobic organisms, but the use of oxygen is also associated with the generation of reactive oxygen species (ROS).[1] ROS may damage various biomolecules such as DNA, proteins, and lipids, and may contribute to the development of cancer, neurodegenerative diseases and other maladies. This work is devoted to investigation of mammalian methionine sulfoxide reductase B1 (MsrB1) and thioredoxin (Trx) proteins constituting a part of defense system that protect cells against oxidative stress. MsrB1 protein utilizes catalytic redox-active cysteine/selenocystein residues to reverse oxidized methionine of damaged proteins back to reduced one.[2] Protein Trx is used to regenerate the active catalytic cystein/selenocystein of MsrB1. In this work we investigate mutant forms of proteins U95C MsrB1 and C35S Trx, which can organize stable but inactive protein complex. The aim of the work is to determine a possible spatial structure of these mutant proteins in complex.