Long-range structural defects by pathogenic mutations in most severe glucose-6-phosphate dehydrogenase deficiency.

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common blood disorder, presenting multiple symptoms, including hemolytic anemia. It affects 400 million people worldwide, with more than 160 single mutations reported in G6PD. The most severe mutations (about 70) are classified as class I, leading to more than 90% loss of activity of the wild-type G6PD. The crystal structure of G6PD reveals these mutations are located away from the active site, concentrating around the noncatalytic NADP+-binding site and the dimer interface. However, the molecular mechanisms of class I mutant dysfunction have remained elusive, hindering the development of efficient therapies. To resolve this, we performed integral structural characterization of five G6PD mutants, including four class I mutants, associated with the noncatalytic NADP+ and dimerization, using crystallography, small-angle X-ray scattering (SAXS), cryogenic electron microscopy (cryo-EM), and biophysical analyses. Comparisons with the structure and properties of the wild-type enzyme, together with molecular dynamics simulations, bring forward a universal mechanism for this severe G6PD deficiency due to the class I mutations. We highlight the role of the noncatalytic NADP+-binding site that is crucial for stabilization and ordering two ß-strands in the dimer interface, which together communicate these distant structural aberrations to the active site through a network of additional interactions. This understanding elucidates potential paths for drug development targeting G6PD deficiency.

Lipid loss and compositional change during preparation of liposomes by common biophysical methods.

Liposomes are widely used as model lipid membrane platforms in many fields, ranging from basic biophysical studies to drug delivery and biotechnology applications. Various methods exist to prepare liposomes, but common procedures include thin-film hydration followed by extrusion, freeze-thaw, and/or sonication. These procedures have the potential to produce liposomes at specific concentrations and membrane compositions, and researchers often assume that the concentration and composition of their liposomes are similar to, if not identical, to what would be expected if no lipid loss occurred during preparation. However, lipid loss and concomitant biasing of lipid composition can in principle occur at any preparation step due to nonideal mixing, lipid-surface interactions, etc. Here, we report a straightforward method using HPLC-ELSD to quantify the lipid concentration and membrane composition of liposomes, and apply that method to study the preparation of simple POPC/cholesterol liposomes. We examine many common steps in liposome formation, including vortexing during re-suspension, hydration of the lipid film, extrusion, freeze-thaw, sonication, and the percentage of cholesterol in the starting mixture. We found that the resuspension step can play an outsized role in determining the overall lipid loss (up to ~50% under seemingly rigorous procedures). The extrusion step yielded smaller lipid losses (~10-20%). Freeze-thaw and sonication could both be employed to improve lipid yields. Hydration times up to 60 minutes and increasing cholesterol concentrations up to 50 mole% had little influence on lipid recovery. Fortunately, even conditions with large lipid loss did not substantially influence the target membrane composition more than ~5% under the conditions we tested. From our results, we identify best practices for producing maximum levels of lipid recovery and minimal changes to lipid composition during liposome preparation protocols. We expect our results can be leveraged for improved preparation of model membranes by researchers in many fields.

Choice of buffer in mobile phase can substantially alter peak areas in quantification of lipids by HPLC-ELSD.

Evaporative light scattering detectors (ELSD) are commonly used with high-performance liquid chromatography (HPLC) to separate and quantify lipids, which are typically not easily detectable by more conventional methods such as UV-visible detectors. In many HPLC-ELSD methods to analyze lipids, a volatile buffer is included in the mobile phase to control the pH and facilitate separation between lipid species. Here, we report an unintended effect that buffer choice can have in HPLC-ELSD analysis of lipids - the identity and concentration of the buffer can substantially influence the resulting ELSD peak areas. To isolate this effect, we use a simple isocratic methanol mobile phase supplemented with different concentrations of commonly used buffers for ELSD analysis, and quantify the effect on peak width, peak shape, and peak area for seven different lipids (POPC, DOPE, cholesterol, sphingomyelin, DOTAP, DOPS, and lactose ceramide). We find that the ELSD peak areas for different lipids can change substantially depending on the mobile phase buffer composition, even in cases where the peak width and shape are unchanged. For a subset of analytes which are UV-active, we also demonstrate that the peak area quantified by UV remains unchanged under different buffer conditions, indicating that this effect is particular to ELSD quantification. We speculate that this ELSD-buffer effect may be the result of a variety of physical phenomenon, including: modification of aerosol droplet size, alteration of clustering of analytes during evaporation of the mobile phase, and mass-amplification or ion-pair effects, all of which could lead to differences in observed peak areas. Such effects would be expected to be molecule-specific, consistent with our data. We anticipate that this report will be useful for researchers designing and implementing HPLC-ELSD methods, especially of lipids.