Ruan van Deventer

The interplay between the cholesterol metabolism and assembly of Aβ42 (the 42‐residue form of the amyloid‐β peptide) peptides in pathological aggregates is considered one of the major molecular mechanisms in the development of Alzheimer's disease (AD). Numerous in vitro studies led to the finding that high cholesterol levels in membranes accelerate the production of Aβ aggregates. The molecular mechanisms explaining how cholesterol localized inside the membrane bilayer catalyzes the assembly of Aβ aggregates above the membrane remain unknown. We addressed this problem by combining different AFM modalities, including imaging and force spectroscopy, with fluorescence spectroscopy. Our combined studies revealed that Aβ42 was capable of removing cholesterol from the membrane. Importantly, physiologically low concentrations of Aβ42 demonstrate such ability. Extracted cholesterol interacts with Aβ42 and accelerates its on‐membrane aggregation, which is a molecular mechanism explaining how cholesterol embedded in the membrane accelerates Aβ42 aggregation. The discovered ability of Aβ42 to remove cholesterol from membranes resulted in three major AD‐related events. First, free cholesterol catalyzes the assembly of Aβ42 in aggregates, which is the mechanism by which physiologically important Aβ42 monomers are converted into their pathological form. Second, the release of cholesterol from membranes leads to its accumulation in the brain, which is one of the risk factors associated with disease development and progression. Third, cholesterol depletion decreases membrane stiffness, which can result in deterioration of the function of membrane‐bound proteins, such as dendritic spine degeneration and, ultimately, synapse loss, a common pathological feature of AD. image

The continuous threat of drug-resistant Klebsiella pneumoniae justifies identifying novel targets and developing effective antibacterial agents. A potential target is nicotinate nucleotide adenylyltransferase (NNAT), an indispensable enzyme in the biosynthesis of the cell-dependent metabolite, NAD⁺. NNAT catalyses the adenylation of nicotinamide/nicotinate mononucleotide (NMN/NaMN), using ATP to form nicotinamide/nicotinate adenine dinucleotide (NAD⁺/NaAD). In addition, it employs divalent cations for co-substrate binding and catalysis and has a preference for different divalent cations. Here, the biophysical structure of NNAT from K. pneumoniae (KpNNAT) and the impact of divalent cations on its activity, conformational stability and substrate-binding are described using experimental and computational approaches. The experimental study was executed using an enzyme-coupled assay, far-UV circular dichroism, extrinsic fluorescence spectroscopy, and thermal shift assays, alongside homology modelling, molecular docking, and molecular dynamic simulation. The structure of KpNNAT revealed a predominately α-helical secondary structure content and a binding site that is partially hydrophobic. Its substrates ATP and NMN share the same binding pocket with similar affinity and exhibit an energetically favourable binding. KpNNAT showed maximum activity and minimal conformational changes with Mg²⁺ as a cofactor compared to Zn²⁺, Cu²⁺ and Ni²⁺. Overall, ATP binding affects KpNNAT dynamics, and the dynamics of ATP binding depend on the presence and type of divalent cation. The data obtained from this study would serve as a basis for further evaluation towards designing structure-based inhibitors with therapeutic potential.