Regulation of glycolysis
The glycolytic pathway of the bloodstream trypanosome is thought to be essentially unregulated. Hexokinase is not subject to any significant feedback inhibition by glucose 6-phosphate (Nwagwu and Opperdoes 1982; Opperdoes and Van Roy, unpublished), as are the hexokinases from most other eukaryotes. Phosphofructokinase (PFK), is also essentially unregulated. Although it is an allosteric enzyme with regard to the binding of its substrate fructose 6-phosphate (Fru6P), that is activated by AMP and ADP, it does not respond to fructose 1,6-bisphosphate or fructose 2,6-bisphosphate (Fru-2,6-P2) or other compounds known to regulate PFKs (Cronin and Tipton, 1985; 1987).
Van Schaftingen et al (1985) discovered that in trypanosomes Fru(2,6)P2 modulates the activity of the cytosolic enzyme pyruvate kinase (PYK) rather than that of PFK. This peculiar way of regulating the glycolytic pathway is most likely related to the fact that PFK in the Kinetoplastida is present inside glycosomes, a sequestration that most likely renders it inaccessible to Fru(2,6)P2, which is synthesized from Fru6P by the cytosolic enzyme phosphofructo-2-kinase (PFK2, Van Schaftingen et al., 1987). The compartmentation of the first part of the glycolytic pathway has probably led to an adaptation of the original Fru(1,6)P2-binding site of cytosolic PYK to accommodate Fru(2,6)P2 as its most potent allosteric regulator (Callens et al., 1991a; Callens and Opperdoes, 1992). T. brucei PYK, together with the PYKs from the other glycosome-containing members of the Kinetoplastida, all differ from their homologous counterparts from other eukaryotes in that they constitute the only class of PYKs that are activated by Fru(2,6)P2 (Van Schaftingen et al., 1985; Van Schaftingen et al., 1987).
Van Schaftingen et al. (1987) have shown that in the vertebrate stage of T. brucei, both with aerobic and anaerobic glycolysis, the cellular concentrations of Fru(2,6)P2 and PEP were inversely related. Since the conversion of phosphoenolpyruvate (PEP) to pyruvate is the only known fate of this glycolytic metabolite in the vertebrate stage, it is not unexpected that a decrease in the concentration of Fru(2,6)P2 will be compensated for by an increase in PEP. This indicates that a regulatory mechanisms is operational at the level of PYK, but the advantage for bloodstream form glycolysis of being controlled at this level by Fru(2,6)P2 is at present not clear. It is probably only aimed at maintaining a maximal flux through the pathway at all times.