SMALL STRUCTURAL-CHANGES ACCOUNT FOR THE HIGH THERMOSTABILITY OF 1[4FE-4S] FERREDOXIN FROM THE HYPERTHERMOPHILIC BACTERIUM THERMOTOGA-MARITIMA

Background: The characterization of the structural features that accou nt for the high thermostability of some proteins is of great scientifi c and biotechnological interest. Proteins from hyperthermophilic organ isms with optimum growth temperatures of 80 degrees C and higher gener ally show high intrinsic stabilities, The comparison of high resolutio n X-ray structures of these proteins with their counterparts from meso philic organisms has therefore helped to identify potentially stabiliz ing forces in a number of cases. Small monomeric proteins which compri se only a single domain, such as ferredoxins, are especially suitable for such comparisons since the search for determinants of protein stab ility is considerably simplified. Results: The 1.75 Angstrom crystal s tructure of the extremely thermostable 1[4Fe-4S] ferredoxin from Therm otoga maritima (Fd(Tm)) was determined and compared with other monoclu ster-containing ferredoxins with different degrees of thermostability. Conclusions: A comparison of the three-dimensional structure of Fd(Tm ) with that of ferredoxins from mesophilic organisms suggests that the very high thermostability of Fd(Tm) is unexpectedly achieved without large changes of the overall protein structure, Instead, an increased number of potentially stabilizing features is observed in Fd(Tm) compa red with mesophilic ferredoxins. These include stabilization of alpha helices, replacement of residues in strained conformation by glycines, strong docking of the N-terminal methionine and an overall increase i n the number of hydrogen bonds, Most of these features stabilize sever al secondary structure elements and improve the overall rigidity of th e polypeptide backbone, The decreased flexibility will certainly play a relevant role in shielding the iron-sulfur cluster against physiolog ically high temperatures and further improve the functional integrity of Fd(Tm). (C) Current Biology Ltd