Biocatalyst as a core component of an enzyme-catalyzed redox system for the biocatalytic reduction of cystine

An enzyme for reducing cystine to cysteine, wherein the enzyme is a fusion protein that comprises the protein activities of

• i) thioredoxin (protein i) having the protein activity of thioredoxin 1 and
• ii) thioredoxin reductase (protein ii) having the protein activity of thioredoxin reductase,
• wherein the activity of the fusion protein is at least 100% of the activity of a mixture of the same but unfused individual proteins i and ii,
• wherein the fusion protein has the enzyme activity to reduce cystine to cysteine and
• wherein the coding sequence of protein i is located N-terminally or C-terminally with respect to the coding sequence of protein ii in the fusion protein, and wherein the fusion protein is an amino acid sequence selected from the group consisting of SEQ ID No. 9, SEQ ID No. 10, and SEQ ID No. 28.