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Purification and physicochemical, kinetic and immunological properties of allosteric serine hydroxymethyltransferase from monkey liver.

ABSTRACT: The homogeneous serine hydroxymethyltransferase purified from monkey liver, by the use of Blue Sepharose affinity chromatography, exhibited positive homotropic co-operative interactions (h = 2.5) with tetrahydrofolate and heterotropic interactions with L-serine and nicotinamide nucleotides. The enzyme had an unusually high temperature optimum of 60 degrees C and was protected against thermal inactivation by L-serine. The allosteric effects were abolished when the monkey liver enzyme was purified by using a heat-denaturation step in the presence of L-serine, a procedure adopted by earlier workers for the purification of this enzyme from mammalian and bacterial sources. The enzyme activity was inhibited completely by N5-methyltetrahydrofolate, N5-formyltetrahydrofolate, dichloromethotrexate, aminopterin and D-cycloserine, whereas methotrexate and dihydrofolate were partial inhibitors. The insoluble monkey liver enzyme-antibody complex was catalytically active and failed to show positive homotropic co-operative interactions with tetrahydrofolate (h = 1) and heterotropic interactions with NAD+. The enzyme showed a higher heat-stability in a complex with its antibody than as the free enzyme. These results highlight the pitfalls in using a heat-denaturation step in the purification of allosteric enzymes.

SUBMITTER: Ramesh KS

PROVIDER: S-EPMC1162445 | biostudies-other | 1980 Jun

REPOSITORIES: biostudies-other

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Project description:Serine hydroxymethyltransferase (SHMT), encoded by the glyA gene, is a ubiquitous pyridoxal 5'-phosphate (PLP)-dependent enzyme that catalyzes the formation of glycine from serine. The thereby generated 5,10-methylene tetrahydrofolate (MTHF) is a major source of cellular one-carbon units and a key intermediate in thymidylate biosynthesis. While in virtually all eukaryotic and many bacterial systems thymidylate synthase ThyA, SHMT and dihydrofolate reductase (DHFR) are part of the thymidylate/folate cycle, the situation is different in organisms using flavin-dependent thymidylate synthase ThyX. Here the distinct catalytic reaction directly produces tetrahydrofolate (THF) and consequently in most ThyX-containing organisms, DHFR is absent. While the resulting influence on the folate metabolism of ThyX-containing bacteria is not fully understood, the presence of ThyX may provide growth benefits under conditions where the level of reduced folate derivatives is compromised. Interestingly, the third key enzyme implicated in generation of MTHF, serine hydroxymethyltransferase (SHMT), has a universal phylogenetic distribution, but remains understudied in ThyX-containg bacteria. To obtain functional insight into these ThyX-dependent thymidylate/folate cycles, we characterized the predicted SHMT from the ThyX-containing bacterium Helicobacter pylori. Serine hydroxymethyltransferase activity was confirmed by functional genetic complementation of a glyA-inactivated E. coli strain. A H. pylori ?glyA strain was obtained, but exhibited markedly slowed growth and had lost the virulence factor CagA. Biochemical and spectroscopic evidence indicated formation of a characteristic enzyme-PLP-glycine-folate complex and revealed unexpectedly weak binding affinity of PLP. The three-dimensional structure of the H. pylori SHMT apoprotein was determined at 2.8? resolution, suggesting a structural basis for the low affinity of the enzyme for its cofactor. Stabilization of the proposed inactive configuration using small molecules has potential to provide a specific way for inhibiting HpSHMT.

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Purification and physicochemical, kinetic and immunological properties of allosteric serine hydroxymethyltransferase from monkey liver.

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