SOLUTION STRUCTURE OF THERMOSTABLE CYTOCHROME C-552 FROM HYDROGENOBACTER-THERMOPHILUS DETERMINED BY H-1-NMR SPECTROSCOPY

The solution structure of a thermostable cytochrome c-552 from a therm ophilic hydrogen oxidizing bacterium Hydrogenobacter thermophilus was determined by proton nuclear magnetic resonance spectroscopy. Twenty s tructures were calculated by the X-PLOR program on the basis of 902 in terproton distances, 21 hydrogen bonds, and 13 torsion angle constrain ts. The pairwise average root-mean-square deviation for the main chain heavy atoms was 0.91 +/- 0.11 Angstrom. The main chain folding of the cytochrome c-552 was almost the same as that of Pseudomonas aeruginos a cytochrome c-551 that has 59% sequence identity to the cytochrome c- 552 but is less thermostable. We found several differences in local st ructures between the cytochromes c-552 and c-551. In the cytochrome c- 552, aromatic-amino interactions were uniquely formed between Arg 35 a nd Tyr 32 and/or Tyr 41, the latter also having hydrophobic contacts w ith the side chains of Tyr 32, Ala 38, and Leu 42. Small hydrophobic c ores were more tightly packed in the cytochrome c-552 because of the o ccupancies of Ala 5, Met 11, and lie 76, each substituted by Phe 7, Va l 13, and Val 78, respectively, in the cytochrome c-551. Some of these structural differences may contribute to the higher thermostability o f the cytochrome c-552.