SECONDARY STRUCTURE AND PROTEIN-FOLDING OF RECOMBINANT CHLOROPLASTIC THIOREDOXIN CH2 FROM THE GREEN-ALGA CHLAMYDOMONAS-REINHARDTII AS DETERMINED BY H-1-NMR

The recombinant form of the chloroplastic thioredoxin Ch2 from the gre en alga Chlamydomonas reinhardtii [Jacquot et al. (1992) Nucleic Acids Res. 20, 617] that preferentially activates the NADP dependent malate dehydrogenase [EC 1.1.1.82] (m-type thioredoxin) through a light prom oted reductive system, has been subjected to an extensive two-dimensio nal H-1 NMR analysis. A complete H-1 NMR assignment of the resonance l ines in both the oxidized and the reduced states at pH 5.8 has been ob tained allowing the recognition of the secondary structure patterns an d the global protein folding. The single polypeptide chain, made of 10 6 residues plus one additional Met located at the N-terminal position (11.6 kDa) due to the protein expression system, folds into a pattern characteristic of the open twisted alpha/beta structures already found for Escherichia coli and human thioredoxins for which the protein sha res 46 and 20% of sequence identity, respectively. The open alpha/beta structure is made of 5 beta-sheets associated in a parallel (beta1 to beta3) and anti parallel manner (beta3 to beta5) and surrounded by 4 helices. This represents the first structural exploratory study of the ubiquitous oxido-reductase thioredoxins in a photosynthetic living sy stem.