E. Maldonado et al., DIFFERENCES IN THE INTERSUBUNIT CONTACTS IN TRIOSEPHOSPHATE ISOMERASEFROM 2 CLOSELY-RELATED PATHOGENIC TRYPANOSOMES, Journal of Molecular Biology, 283(1), 1998, pp. 193-203
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.