Species-Specific Inactivation of Triosephosphate Isomerase from Trypanosoma brucei: Kinetic and Molecular Dynamics Studies

Human African Trypanosomiasis (HAT), a disease that provokes 2184 new cases a year in Sub-Saharan Africa, is caused by Trypanosoma brucei. Current treatments are limited, highly toxic, and parasite strains resistant to them are emerging. Therefore, there is an urgency to find new drugs against HAT. In this context, T. brucei depends on glycolysis as the unique source for ATP supply; therefore, the enzyme triosephosphate isomerase (TIM) is an attractive target for drug design. In the present work, three new benzimidazole derivatives were found as TbTIM inactivators (compounds 1, 2 and 3) with an I₅₀ value of 84, 82 and 73 µM, respectively. Kinetic analyses indicated that the three molecules were selective when tested against human TIM (HsTIM) activity. Additionally, to study their binding mode in TbTIM, we performed a 100 ns molecular dynamics simulation of TbTIM-inactivator complexes. Simulations showed that the binding of compounds disturbs the structure of the protein, affecting the conformations of important domains such as loop 6 and loop 8. In addition, the physicochemical and drug-like parameters showed by the three compounds suggest a good oral absorption. In conclusion, these molecules will serve as a guide to design more potent inactivators that could be used to obtain new drugs against HAT.

Engineered Soluble Monomeric IgG1 Fc with Significantly Decreased Non-Specific Binding

Due to the long serum half-life provided by the neonatal Fc receptor (FcRn) recycling, the IgG1 Fc has been pursued as the fusion partner to develop therapeutic Fc-fusion proteins, or as the antibody-derived scaffold that could be engineered with antigen-binding capabilities. In previous studies, we engineered the monomeric Fc by mutating critical residues located on the IgG1 Fc dimerization interface. Comparing with the wild-type dimeric Fc, monomeric Fc might possess substantial advantages conferred by its smaller size, but also suffers the disadvantage of non-specific binding to some unrelated antigens, raising considerable concerns over its potential clinical development. Here, we describe a phage display-based strategy to examine the effects of multiple mutations of IgG1 monomeric Fc and, simultaneously, to identify new Fc monomers with desired properties. Consequently, we identified a novel monomeric Fc that displayed significantly decreased non-specificity. In addition, it exhibited higher thermal stability and comparable pH-dependent FcRn binding to the previous reported monomeric Fc. These results provide baseline to understand the mechanism underlying the generation of soluble IgG1 Fc monomers and warrant the further clinical development of monomeric Fc-based fusion proteins as well as antigen binders.

Triosephosphate isomerase: a highly evolved biocatalyst

Triosephosphate isomerase (TIM) is a perfectly evolved enzyme which very fast interconverts dihydroxyacetone phosphate and D: -glyceraldehyde-3-phosphate. Its catalytic site is at the dimer interface, but the four catalytic residues, Asn11, Lys13, His95 and Glu167, are from the same subunit. Glu167 is the catalytic base. An important feature of the TIM active site is the concerted closure of loop-6 and loop-7 on ligand binding, shielding the catalytic site from bulk solvent. The buried active site stabilises the enediolate intermediate. The catalytic residue Glu167 is at the beginning of loop-6. On closure of loop-6, the Glu167 carboxylate moiety moves approximately 2 Å to the substrate. The dynamic properties of the Glu167 side chain in the enzyme substrate complex are a key feature of the proton shuttling mechanism. Two proton shuttling mechanisms, the classical and the criss-cross mechanism, are responsible for the interconversion of the substrates of this enolising enzyme.