Serine hydroxymethyltransferase from Plasmodium vivax is different in substrate specificity from its homologues.
The putative gene of Plasmodium vivax serine hydroxymethyltransferase (PvSHMT; EC 2.1.2.1) was cloned and expressed in Escherichia coli. The purified enzyme was shown to be a dimeric protein with a monomeric molecular mass of 49 kDa. PvSHMT has a maximum absorption peak at 422 nm with a molar absorption coefficient of 6370 M(-1) x cm(-1). The K(d) for binding of the enzyme and pyridoxal-5-phosphate was 0.14 +/- 0.01 microM. An alternative assay for measuring the tetrahydrofolate-dependent SHMT activity based on the coupled reaction with 5,10-methylenetetrahydrofolate reductase (EC 1.5.1.20) from E. coli was developed. PvSHMT uses a ternary-complex mechanism with a k(cat) value of 0.98 +/- 0.06 s(-1) and K(m) values of 0.18 +/- 0.03 and 0.14 +/- 0.02 mM for L-serine and tetrahydrofolate, respectively. The optimum pH of the SHMT reaction was 8.0 and an Arrhenius's plot showed a transition temperature of 19 degrees C. Besides L-serine, PvSHMT forms an external aldimine complex with D-serine, L-alanine, L-threonine and glycine. PvSHMT also catalyzes the tetrahydrofolate-independent retro-aldol cleavage of 3-hydroxy amino acids. Although L-serine is a physiological substrate for SHMT in the tetrahydrofolate-dependent reaction, PvSHMT can also use D-serine. In the absence of tetrahydrofolate at high pH, PvSHMT forms an enzyme-quinonoid complex with D-serine, but not with L-serine, whereas SHMT from rabbit liver was reported to form an enzyme-quinonoid complex with L-serine. The substrate specificity difference between PvSHMT and the mammalian enzyme indicates the dissimilarity between their active sites, which could be exploited for the development of specific inhibitors against PvSHMT.
Sopitthummakhun
Kittipat
K
Department of Biochemistry and Center for Excellence in Protein Structure and Function, Faculty of Science, Mahidol University, Bangkok, Thailand.
Maenpuen
Somchart
S
Yuthavong
Yongyuth
Y
Leartsakulpanich
Ubolsree
U
Chaiyen
Pimchai
P
D000818:Animals; D003001:Cloning, Molecular; D003600:Cytosol / Q000201:enzymology; D012696:Glycine Hydroxymethyltransferase / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D008970:Molecular Weight; D010966:Plasmodium vivax / Q000201:enzymology*; D015800:Protozoan Proteins / Q000378:metabolism*; D011817:Rabbits; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D012756:Sheep; D013053:Spectrophotometry; D013379:Substrate Specificity; D013816:Thermodynamics
D015800:Protozoan Proteins; D011994:Recombinant Proteins; D012696:Glycine Hydroxymethyltransferase