NMR solution structure of a novel thioredoxin from Bacillus acidocaldarius- Possible determinants of protein stability

The thioredoxin (Trx) from Bacillus acidocaldarius (BacTrx), an eubacterium growing optimally at 333 K, is the first Trx described to date from a mode rate thermophilic source. To understand the molecular basis of its thermost ability, the three-dimensional structure in the oxidized form was determine d by NMR methods. A total of 2276 H-1-NMR derived distance constraints alon g with 23 hydrogen-bonds, 72 phi and 27 chi(1) torsion angle restraints, we re used in a protocol employing simulated annealing followed by restrained molecular dynamics and restrained energy minimization. BacTrx consists of a well-defined core region of five strands of beta-sheet , surrounded by four exposed alpha-helices, features shared by other member s of the thioredoxin family. The BacTrx 3D structure was compared with the Escherichia coli Trx (EcTrx) determined by X-ray crystallographic diffracti on, and a number of structural differences were observed that may contribut e to its thermostability. The results of structural analysis indicated that protein stability is due to cumulative effects, the main factor being an i ncreased number of ionic interactions cross-linking different secondary str uctural elements and clamping the C-terminal alpha-helix to the core of the protein.