Characterization and primary structure of a second thioredoxin from the green alga, Chlamydomonas reinhardtii

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Abstract
A second thioredoxin, Ch1, distinct from the one recently reported [Decottignies, P., Schmitter, J. M., Jacquot, J. P., Dutka, S., Picaud, A. & Gadal, P. (1990) Arch. Biochem. Biophys. 280, 112–121] has been purified from the green alga, Chlamydomonas reinhardtii, and its functional and structural properties investigated. Its activity in various enzymatic assays has been compared with the activities of different plant thioredoxins (Ch2 from C. reinhardtii and spinach m and f). Ch1 cannot serve as a substrate for Escherichia coli thioredoxin reductase, but can be reduced by spinach ferredoxin‐thioredoxin reductase. It is less efficient than its spinach counterpart in the activation of corn leaf NADP‐dependent malate dehydrogenase by light or dithiothreitol, and it only activates spinach fructose‐1,6‐bisphosphatase at very high concentrations. The complete primary structure of C. reinhardtii thioredoxin Ch1 was determined by automated Edman degradation of the intact protein and of peptides derived from trypsin, chymotrypsin and Staphylococcus aureus V8 protease digestions. When needed, peptide masses were verified by plasma desorption mass spectrometry. Ch1 consists of a polypeptide of 111 amino acids (11 634 Da) and contains the well‐conserved active site sequence Trp‐Cys‐Gly‐Pro‐Cys. Compared to thioredoxins from other sources, the algal thioredoxin Ch1 displays few sequence similarities with all the thioredoxins sequenced so far. Preliminary evidence indicates that Ch1 may be an h‐type thioredoxin.