Kinetic mechanism of Toxoplasma gondii adenosine kinase and the highly efficient utilization of adenosine

S-Adenosyl-l-Methionine Salvage Impacts Psilocybin Formation in "Magic" Mushrooms

Psychotropic Psilocybe mushrooms biosynthesize their principal natural product psilocybin in five steps, among them a phosphotransfer and two methyltransfer reactions, which consume one equivalent of 5'-adenosine triphosphate (ATP) and two equivalents of S-adenosyl-l-methionine (SAM). This short but co-substrate-intensive pathway requires nucleoside cofactor salvage to maintain high psilocybin production rates. We characterized the adenosine kinase (AdoK) and S-adenosyl-l-homocysteine (SAH) hydrolase (SahH) of Psilocybe cubensis. Both enzymes are directly or indirectly involved in regenerating SAM. qRT-PCR expression analysis revealed an induced expression of the genes in the fungal primordia and carpophores. A one-pot in vitro reaction with the N-methyltransferase PsiM of the psilocybin pathway demonstrates a concerted action with SahH to facilitate biosynthesis by removal of accumulating SAH.

Kinetics mechanism and regulation of native human hepatic thymidine phosphorylase

Thymidine phosphorylase (TP; EC 2.4.2.4) catalyzes the reversible phosphorolysis of thymidine, deoxyuridine, and their analogues to their respective nucleobases and 2-deoxy-α-d-ribose-1-phosphate (dRib-1-P). TP is a key enzyme in the pyrimidine salvage pathways. Activity of the enzyme is crucial in angiogenesis, cancer chemotherapy, radiotherapy, and tumor imaging, Nevertheless, a complete set of kinetic parameters has never been reported for any human TP. This study describes the kinetic mechanism and regulation of native human hepatic TP. The liver is a main site of pyrimidine metabolism and contains high levels of TP. Initial velocity and product inhibition studies demonstrated that the basic mechanism of this enzyme is a sequential random bi-bi mechanism. Initial velocity studies showed an intersecting pattern, consistent with substrate-enzyme-co-substrate complex formation, and a binding pattern indicating that the binding of the substrate interferes with the binding of the co-substrate and vice versa. Estimated kinetic parameters were K_Thymidine = 284 ± 55, K_Pi = 5.8 ± 1.9, K_Thymine = 244 ± 69, and K_dRib-1-P = 90 ± 33 μM. Thymine was a product activator, but becomes a substrate inhibitor at concentrations eight times higher than its K_m. dRib-1-P was a non-competitive product inhibitor of the forward reaction. It bounded better to the Enzyme●P_i complex than the free enzyme, but had better affinity to the free enzyme than the Enzyme●Thymidine complex. In the reverse reaction, dRib-1-P enhanced the binding of thymine. The enhancement of the thymine binding along with the fact that dRib-1-P was a non-competitive product inhibitor suggests the presence of another binding site for dRib-1-P on the enzyme.

Enzymes of pyrimidine salvage pathways in intraerythrocytic Plasmodium falciparum

Malaria remains a significant public health problem worldwide with an estimated annual global incidence of 200 million and an estimated 450,000 annual deaths. Among the five known human malarial species, Plasmodium falciparum is the deadliest and most resistant to antimalarials. Hence, there is a need for new antimalarial targets. The rational design of a drug is usually based on biochemical and physiological differences between pathogens and their hosts. In view of their high rate of replication, parasites require very active nucleic acid synthesis which necessitates large supplies of the indispensable pyrimidine nucleotides. Consequently, delineation of P. falciparum pyrimidine metabolic pathways may reveal potential targets for the chemotherapy of malaria. Previous studies reported the existence of pyrimidine de novo pathways in this organism. The present results demonstrate the presence of enzymes of the pyrimidine salvage pathways in P. falciparum and indicate that this parasite is capable of pyrimidine salvage. Furthermore, some of the pyrimidine salvage enzymes, e.g., dTMP kinase, phosphoribosyltransferase, and uridine phosphorylase could be excellent targets for chemotherapeutic intervention against this parasite.

Focused screening to identify new adenosine kinase inhibitors

Adenosine kinase (AdK) is a key player in controlling intra- and extracellular concentrations of the signaling molecule adenosine. Extensive evidence points to an important role of AdK in several diseases, and suggests that AdK inhibition might be a promising therapeutic strategy. The development of a new AdK assay and subsequent screening of part of our focused compound library led to the identification of 12 hit compounds (hit rate of 6%) representing six new classes of non-nucleoside human AdK inhibitors. The most potent inhibitor 1 displayed a K_i value of 184nM. Compound screening with a newly developed assay was useful and efficient for discovering novel AdK inhibitors which may serve as lead structures for developing drugs for adenosine augmentation therapy.