Pyruvate kinase (PYK)
This enzyme catalyses the synthesis of ATP and pyruvate from PEP and ADP. The enzyme is cytosolic, it is a homotetramer of subunits of a mass of 54 378 and the overall identity of the trypanosome PYK with other PYKs ranges from 41-51% (Allert et al., 1991). The allosteric enzyme appears to be a crucial regulator of the glycolytic flux and its most striking regulator is Fru(2,6)P2 (see above and Van Schaftingen et al., 1985;1987; Callens et al., 1991a; Callens and Opperdoes, 1992). The enzymes from T. brucei and Leishmania mexicana have been cloned and sequenced and overexpressed in E. coli (Allert et al., 1991, Ernest et al., 1994). In addition to Fru(2,6)P2 the enzyme is activated by Fru(1,6)P2, glucose(1,6)P2 and ribulose bisphosphate (Callens et al., 1991a) and its activity is modulated by adenine nucleotides and inorganic phosphate, while it is insensitive to regulation by most of the metabolites that modulate PYK activity from other sources. A model of the structure of T. brucei PYK, based on the atomic coordinates of the cat-muscle enzyme has revealed a potential binding site for Fru(2,6)P2. Many residues in this site are conserved within the trypanosomatids, but not in the other PYKs and thus this site would form an interesting target for drug design.