DIFFERENCES IN THE INTERSUBUNIT CONTACTS IN TRIOSEPHOSPHATE ISOMERASEFROM 2 CLOSELY-RELATED PATHOGENIC TRYPANOSOMES

The aligned amino acid sequences of TIM from Trypanosoma cruzi (TcTIM) and Trypanosoma brucei (TbTIM) have a positional identity of 68%. The two enzymes have markedly similar catalytic properties. Agents that i nteract with their interface Cys inhibit TcTIM and TbTIM; and those TI Ms that lack this Cys (such as human TIM) are largely or completely in sensitive to these agents. The susceptibility of TcTIM to the agents i s approximately 100 times higher than that of TbTIM. To ascertain the cause of this large difference, the crystal structure of TcTIM was sol ved at 1.83 Angstrom resolution. The two enzymes are very similar homo dimers. In TcTIM and TbTIM their respective Cys, 15 or 14, forms part of the dimer interface. In both, the contacts of the Cys with residues of the other subunit are almost identical. Nevertheless, there are no teworthy differences between the two; the existence of glutamine 18 in TbTIM instead of glutamic acid in TcTIM at the beginning of helix 1 d ecreases the contacts between this portion of the protein and helix 3 of the other subunit. In addition, TcTIM has proline at position 24 in the first helix of the TIM barrel; this is absent in the other TIM. P ro24 disrupts the regular helix arrangement, making the pitch of this helix 1.2 Angstrom longer than in TbTIM. When Pro24 of TcTIM was subst ituted for Glu, the sensitivity of TcTIM to sulfhydryl reagents increa sed about fivefold, possibly as a consequence of an increase in the sp ace between the first portion of helix 1 and helix 3 of the other subu nit. Therefore, it may be concluded that the geometry of the latter re gion is central in the accessibility to agents that perturb the interf ace Cys. In human TIM this region is more compact. (C) 1998 Academic P ress.