Human wild-type alanine: glyoxylate aminotransferase and its naturally occurring G82E variant: functional properties and physiological implications

Barbara Cellini , Mariarita Bertoldi , Riccardo Montioli , Alessandro Paiardini , Carla Borri Voltattorni (1)

1 Scienze Morfologico-Biomediche-Sezione: Chimica Biologica

Human hepatic peroxisomal alanine:glyoxylate aminotransferase (AGT) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme whose deficiency causes primary hyperoxaluria type I, a rare autosomal recessive disorder. To acquire experimental evidence for the physiological function of AGT, the K eq,overall} of the reaction, the steady-state kinetic parameters of the forward and reverse reactions, and the pre-steady-state kinetics of the half-reactions of the PLP form of AGT with L-alanine or glycine and the pyridoxamine 5'-phosphate (PMP) form with pyruvate or glyoxylate have been measured. The results indicate that the enzyme is highly specific for catalyzing glyoxylate to glycine processing, thereby playing a key role in glyoxylate detoxification. Analysis of the reaction course also reveals that PMP remains bound to the enzyme during the catalytic cycle and that the AGT-PMP complex displays a reactivity towards keto acids higher than that of apoAGT in the presence of PMP. These findings are tentatively related to possible subtle rearrangements at the active site also indicated by the putative binding mode of catalytic intermediates. Additionally, the catalytic and spectroscopic features of the naturally occurring G82E variant have been analyzed. Although, like the wild-type, the G82E variant is able to bind 2 mol PLP/dimer, it exhibits a significant reduced affinity for PLP and even more for PMP compared with wild-type, and an altered conformational state of the bound PLP. The striking molecular defect of the mutant, consisting in the dramatic decrease of the overall catalytic activity (~ 0.1% of that of normal AGT), appears to be related to the inability to undergo an efficient transaldimination of the PLP form of the enzyme with amino acids as well as an efficient conversion of AGT-PMP into AGT-PLP. Overall, careful biochemical analyses have allowed to elucidate the mechanism of action of AGT and the way in which the disease causing G82E mutation affects it.

Mots clés: Life Sciences

Barbara Cellini, Mariarita Bertoldi, Riccardo Montioli, Alessandro Paiardini, Carla Borri Voltattorni. Human wild-type alanine: glyoxylate aminotransferase and its naturally occurring G82E variant: functional properties and physiological implications. Biochemical Journal, 2007, 408 (1), pp.39-50. ⟨10.1042/BJ20070637⟩. ⟨hal-00478801⟩