CRYSTAL-STRUCTURE OF ARABIDOPSIS-THALIANA NADPH DEPENDENT THIOREDOXINREDUCTASE AT 2.5 ANGSTROM RESOLUTION

Thioredoxin exists in all organisms and is responsible for the hydroge n transfer to important enzymes for ribonucleotide reduction and the r eduction of methionine sulphoxide and sulphate. Thioredoxins have also been shown to regulate enzyme activity in plants and are also involve d in the regulation of transcription factors and several other regulat ory activities. Thioredoxin is reduced by the flavoenzyme thioredoxin reductase using NADPH. We have now determined the first structure of a eukaryotic thioredoxin reductase, from the plant Arabidopsis thaliana , at 2.5 Angstrom resolution. The dimeric A. thaliana thioredoxin redu ctase is structurally similar to that of the Escherichia coli enzyme, and most differences occur in the loops. Because the plant and E. coli enzymes have the same architecture, with the same dimeric structure a nd the same position of the redox active disulphide bond, a similar me chanism that involves very large domain rotations is likely for the tw o enzymes. The subunit is divided into two domains, one that binds FAD and one that binds NADPH. The relative positions of the domains in A. thaliana thioredoxin reductase differ from those of the E. coli reduc tase. When the FAD domains are superimposed, the NADPH domain of A. th aliana thioredoxin reductase must be rotated by 8 degrees to superimpo se on the corresponding domain of the E. coli enzyme. The domain rotat ion we now observe is much smaller than necessary for the thioredoxin reduction cycle. (C) 1996 Academic Press Limited