title: Three new crystal structures of point mutation variants of monoTIM: conformational flexibility of loop-1, loop-4 and loop-8.
abstract: BACKGROUND
  Wild-type triosephosphate isomerase (TIM) is a very stable dimeric enzyme. This dimer can be converted into a stable monomeric protein (monoTIM) by replacing the 15-residue interface loop (loop-3) by a shorter, 8-residue, loop. The crystal structure of monoTIM shows that two active-site loops (loop-1 and loop-4), which are at the dimer interface in wild-type TIM, have acquired rather different structural properties. Nevertheless, monoTIM has residual catalytic activity.
  
  
  RESULTS
  Three new structures of variants of monoTIM are presented, a double-point mutant crystallized in the presence and absence of bound inhibitor, and a single-point mutant in the presence of a different inhibitor. These new structures show large structural variability for the active-site loops, loop-1, loop-4 and loop-8. In the structures with inhibitor bound, the catalytic lysine (Lys13 in loop-1) and the catalytic histidine (His95 in loop-4) adopt conformations similar to those observed in wild-type TIM, but very different from the monoTIM structure.
  
  
  CONCLUSIONS
  The residual catalytic activity of monoTIM can now be rationalized. In the presence of substrate analogues the active-site loops, loop-1, loop-4 and loop-8, as well as the catalytic residues, adopt conformations similar to those seen in the wild-type protein. These loops lack conformational flexibility in wild-type TIM. The data suggest that the rigidity of these loops in wild-type TIM, resulting from subunit-subunit contacts at the dimer interface, is important for optimal catalysis.
authors.affiliation: EMBL, Heidelberg, Germany.
authors.forename: T V
authors.initials: TV
authors.lastname: Borchert
authors.forename: K V
authors.initials: KV
authors.lastname: Kishan
authors.forename: J P
authors.initials: JP
authors.lastname: Zeelen
authors.forename: W
authors.initials: W
authors.lastname: Schliebs
authors.forename: N
authors.initials: N
authors.lastname: Thanki
authors.forename: R
authors.initials: R
authors.lastname: Abagyan
authors.forename: R
authors.initials: R
authors.lastname: Jaenicke
authors.forename: R K
authors.initials: RK
authors.lastname: Wierenga
mesh_terms: D000595:Amino Acid Sequence; D003001:Cloning, Molecular; D018360:Crystallography, X-Ray / Q000379:methods; D004926:Escherichia coli; D014644:Genetic Variation; D006639:Histidine; D046911:Macromolecular Substances; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017354:Point Mutation; D017433:Protein Structure, Secondary; D014305:Triose-Phosphate Isomerase / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics*
chemical_list: D046911:Macromolecular Substances; D006639:Histidine; D014305:Triose-Phosphate Isomerase