Triose-phosphate isomerase (TIM)
This enzyme catalyses the reversible conversion of the triosephosphates DHAP and GA3P. In the bloodstream form the enzyme is mainly found in the glycosome with 155 subunits per organelle, where it represents 1% of the protein, or less than 0.04% of the total trypanosome protein (Misset et al., 1986). TIM is a dimer of 52 000 Mr of two identical subunits of 250 amino acids that have about 50% positional identity with the TIMs from other organisms (Swinkels et al., 1986). Only the dimer is active since each subunit contributes to the formation of the enzyme's active site. Its measured pI is 9.8, indicating that the enzyme carries many positively charged amino acids on its surface. The kinetic properties of the enzyme have been studied but these are not too dissimilar from other TIMs that have been described (Lambeir et al., 1987). The trypanosome enzyme is inhibited by sulphate, phosphate, arsenate, and by 2-phosphoglycolate. The trypanosome TIM was the first parasite enzyme to be crystallised and its three dimensional structure was subsequently solved (Wierenga et al., 1991). The TIM dimer of the trypanosome is less stable than its homologues from other organisms and easily dissociates and loses its activity upon dilution (Borchert et al., 1993). Kuntz et al. (1992) have tried to exploit this reduced affinity of the subunits for each other by synthesizing cyclic peptide inhibitors mimicking part of a subunit interface loop that either would interfere specifically with the formation of the active dimer, or would promote its dissociation. Although this specific approach has failed because the enzyme precipitated in the presence of such peptides (Callens et al., 1993), Gao et al. (1998), using the trypanocidal drug suramin as a tool, have shown that this is indeed a valid approach. Although suramin inhibits the active enzyme to the same extent as its rabbit-muscle homologue (Lambeir et al., 1987), it binds tightly to the monomers of T. brucei TIM and so interferes with the formation of an active dimer. Suramin that cannot be used for the treatment of Chagas' disease or leishmaniasis, binds less tightly to the subunits of the homologous enzymes from T. cruzi and Leishmania, and human TIM and thus hardly interferes with the formation of these active dimers. Fairlamb and Bowman (1980) found that trypanosomes that were exposed to suramin for prolonged times exhibited a significant reduction of their glycolytic rate. It may be that this specific effect of suramin, resulting on the one hand from the high positive charge of the trypanosome TIM and on the other hand from the fact that the enzyme has to operate at relatively high protein concentrations, is the explanation for the mode of action of this important trypanocide.