Distinct biochemical properties of human serine hydroxymethyltransferase compared with the Plasmodium enzyme: implications for selective inhibition.

Serine hydroxymethyltransferase ( SHMT) catalyzes the transfer of a hydroxymethyl group from l-serine to tetrahydrofolate to yield glycine and 5,10-methylenetetrahydrofolate. Our previous investigations have shown that SHMTs from Plasmodium spp. ( P. falciparum, Pf; P. vivax, Pv) are different from the enzyme from rabbit liver in that Plasmodium SHMT can use d-serine as a substrate. In this report, the biochemical and biophysical properties of the Plasmodium and the human cytosolic form (hc SHMT) enzymes including ligand binding and kinetics were investigated. The data indicate that, similar to Plasmodium enzymes, hc SHMT can use d-serine as a substrate. However, hc SHMT displays many properties that are different from those of the Plasmodium enzymes. The molar absorption coefficient of hc SHMT-bound pyridoxal-5′-phosphate ( PLP) is much greater than Pv SHMT-bound or Pf SHMT-bound PLP. The binding interactions of hc SHMT and Plasmodium SHMT with d-serine are different, as only the Plasmodium enzyme undergoes formation of a quinonoid-like species upon binding to d-serine. Furthermore, it has been noted that hc SHMT displays strong substrate inhibition by tetrahydrofolate ( THF) (at THF > 40 μ m), compared with SHMTs from Plasmodium and other species. The pH-activity profile of hc SHMT shows higher activities at lower pH values corresponding to a p K_a value of 7.8 ± 0.1. Thiosemicarbazide reacts with hc SHMT following a one-step model [ k₁ of 12 ± 0.6 m⁻¹·s⁻¹ and k₋₁ of (1.0 ± 0.6) × 10⁻³ s⁻¹], while the same reaction with Pf SHMT involves at least three steps. All data indicated that the ligand binding environment of SHMT from human and Plasmodium are different, indicating that it should be possible to develop species-selective inhibitors in future studies. Database serine hydroxymethyltransferase, ; 5,10-methylenetetrahydrofolate dehydrogenase [ABSTRACT FROM AUTHOR]