Distinct substrate specificity and physicochemical characterization of native human hepatic thymidine phosphorylase

Engineering of the Recombinant Expression and PEGylation Efficiency of the Therapeutic Enzyme Human Thymidine Phosphorylase

Human thymidine phosphorylase (HsTP) is an enzyme with important implications in the field of rare metabolic diseases. Defective mutations of HsTP lead to mitochondrial neurogastrointestinal encephalomyopathy (MNGIE), a disease with a high unmet medical need that is associated with severe neurological and gastrointestinal complications. Current efforts focus on the development of an enzyme replacement therapy (ERT) using the Escherichia coli ortholog (EcTP). However, bacterial enzymes are counter-indicated for human therapeutic applications because they are recognized as foreign by the human immune system, thereby eliciting adverse immune responses and raising significant safety and efficacy risks. Thus, it is critical to utilize the HsTP enzyme as starting scaffold for pre-clinical drug development, thus de-risking the safety concerns associated with the use of bacterial enzymes. However, HsTP expresses very poorly in E. coli, whereas its PEGylation, a crucial chemical modification for achieving long serum persistence of therapeutic enzymes, is highly inefficient and negatively affects its catalytic activity. Here we focused on the engineering of the recombinant expression profile of HsTP in E. coli cells, as well as on the optimization of its PEGylation efficiency aiming at the development of an alternative therapeutic approach for MNGIE. We show that phylogenetic and structural analysis of proteins can provide important insights for the rational design of N’-terminus-truncation constructs which exhibit significantly improved recombinant expression levels. In addition, we developed and implemented a criteria-driven rational surface engineering strategy for the substitution of arginine-to-lysine and lysine-to-arginine residues to achieve more efficient, homogeneous and reproducible PEGylation without negatively affecting the enzymatic catalytic activity upon PEGylation. Collectively, our proposed strategies provide an effective way to optimize enzyme PEGylation and E. coli recombinant expression and are likely applicable for other proteins and enzymes.

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.