EFFECTS OF TETRAHYDROFOLATE POLYGLUTAMATES ON THE KINETIC-PARAMETERS OF SERINE HYDROXYMETHYLTRANSFERASE AND GLYCINE DECARBOXYLASE FROM PEA LEAF MITOCHONDRIA

Plant tissues contain highly conjugated forms of folate. Despite this, the ability of plant folate-dependent enzymes to utilize tetrahydrofo late polyglutamates has not been examined in detail. in leaf mitochond ria, the glycine-cleavage system and serine hydroxymethyltransferase, present in large amounts in the matrix space and involved in the photo respiratory cycle, necessitate the presence of tetrahydrofolate as a c ofactor. The aim of the present work was to determine whether glutamat e chain length (one to six glutamate residues) influenced the affinity constant for tetrahydrofolate and the maximal velocities displayed by these two enzymes. The results show that the affinity constant decrea sed by at least one order of magnitude when the tetrahydrofolate subst rate contained three or more glutamate residues. In contrast, maximal velocities were not altered in the presence of these substrates. These results are consistent with analyses of mitochondrial folates which r evealed a pool of polyglutamates dominated by tetra and pentaglutamate s. The equilibrium constant of the serine hydroxymethyltransferase sug gests that, during photorespiration, the reaction must be permanently pushed toward the formation of serine (the unfavourable direction) to allow the recycling of tetrahydrofolate necessary for the operation of the glycine decarboxylase T-protein.