Conclusion
The complete dependence of the bloodstream form trypanosome on glucose as a source of carbon and energy has made glycolysis an interesting target for drug intervention. No other pathway in the trypanosome has been studied in such detail as the glycolytic pathway. Its peculiar characteristics such as its compartmentation inside glycosomes and the invariably high positive charge of the enzymes in it (Wierenga et al., 1987), the fact that pyruvate rather than lactate is excreted as the end-product of the pathway and the reoxidation of glycosomal NADH via the trypanosome alternative oxidase, all have attracted considerable attention. Most of its enzymes have been overexpressed and studied in great detail and their properties have been compared with their mammalian homologues, in order to identify differences that could be exploited for the design of new and better drugs. One of the drawbacks of the glycolytic pathway as target for drug design is the fact that it is probably one of the oldest pathways in Nature. It is present in almost every cell and in general its enzymes are very well conserved. Despite the great evolutionary distance between the trypanosome and its mammalian host, most of the glycolytic enzymes still share some 50% positional identity at the amino-acid level and thus share the same three-dimensional structure. Moreover, active-site residues of the glycolytic enzymes are even better conserved and this does certainly not facilitate the use of substrate analogues in the development of specific inhibitors. Moreover, most substrate analogues would act as competitive inhibitors, which would require them to be of very high affinity in order to exert any significant effect on the functioning of the overall pathway. Therefore, the development of irreversible inhibitors has received a lot of attention (Willson et al., 1994) and also much attention has been given to differences found on the surface of these enzymes, such as their high overall positively charge (Wierenga et al., 1987; Willson et al., 1993), modified interface loops and unique amino-acid insertions. The obvious differences with the host, such as the trypanosome GAPDH binding pocket for NAD (Verlinde et al., 1994), the peculiar nature of trypanosome PFK (Michels et al., 1997), the regulation of PYK by Fru(2,6)P2, the pyruvate transporter (Wiemer et al., 1995) and the trypanosome alternative oxidase, all constitute the more attractive targets for the development of a new and specific chemotherapy. On the other hand the recent mathematical modelling of the pathway (Bakker et al., 1997; 1998) has identified the steps that exert the highest control in the pathway, such as the glucose transporter, aldolase, GAPDH and G3PDH and PGK. These steps, at least from theoretical considerations, would best translate an inhibitory effect on an individual enzyme into an inhibition of the overall flux of the pathway. An entirely different type of approach, not discussed in this review, would be the interference with the compartmentation of glycolysis inside the glycosome itself. Any inhibition of, or interference with, receptors for peroxisome targeting signals would directly affect the importation of several glycolytic enzymes at the same time and eventually slow down or even interrupt the glycolytic flux.
Many different approaches have been taken so far. It can only be hoped that one of them, either alone or in combination, will lead to the development of a more effective and safer treatment of human African trypanosomiasis.