SEQUENCE AND HOMOLOGY MODEL OF 3-ISOPROPYLMALATE DEHYDROGENASE FROM THE PSYCHROTROPHIC BACTERIUM VIBRIO SP I5 SUGGEST REASONS FOR THERMAL-INSTABILITY

The leuB gene from the psychrotrophic strain Vibrio sp. 15 has been cl oned and sequenced, The gene codes for 3-isopropylmalate dehydrogenase , a 360-residue, dimeric enzyme involved in the biosynthesis of leucin e. Three recently solved homologous isopropylmalate dehydrogenase (IPM DH) crystal structures from thermophilic and mesophilic organisms have been used to build a homology model for the psychrotrophic IPMDH and to deduce the possible structural reasons for its decreased thermostab ility, According to our model the psychrotrophic IPMDH contains fewer stabilizing interactions than its mesophilic and thermophilic counterp arts, Elements that have been identified as destabilizing in the compa rison of the psychrotrophic, mesophilic and thermophilic IPMDHs are a smaller number of salt-bridges, a reduction in aromatic-aromatic inter actions, fewer proline residues and longer surface loops. In addition, there are a number of substitutions of otherwise strictly conserved r esidues that can be linked to thermostability.