Variable-Field Analytical Ultracentrifugation: I. Time-Optimized Sedimentation Equilibrium

The dimeric form of bacterial l-asparaginase YpAI is fully active

l-asparaginases from mesophilic bacteria (ASNases), including two enzymes very successfully used in the treatment of leukaemia, have been consistently described as homotetramers. On the contrary, structural studies show that homodimers of these enzymes should be sufficient to carry out the catalytic reaction. In this report, we investigated whether the type I Yersinia pestis asparaginase (YpAI) is active in a dimeric form or whether the tetrameric quaternary structure is critical for its activity. Using multiple biophysical techniques that investigate enzymatic properties and quaternary structure at either high or low protein concentration, we found that dimeric YpAI is fully active, suggesting that the tetrameric form of this subfamily of enzymes does not bear significant enzymatic relevance. In this process, we extensively characterized YpAI, showing that it is a cooperative enzyme, although the mechanism of allostery is still not definitely established. We showed that, like most type I ASNases, the substrate affinity of YpAI is low and this enzyme is very similar in terms of both the structure and enzymatic properties to homologous type I ASNase from Escherichia coli (EcAI). We extended these studies to more medically relevant type II ASNases, used as anti-leukaemia drugs. We confirmed that type II ASNases are not allosteric, and that they might also be functional in a dimeric form. However, the determination of the accurate tetramer⇆dimer dissociation constants of these enzymes that most likely lie in the picomolar range is not possible with currently available biophysical techniques.

Determining the Stoichiometry of a Protein-Polymer Conjugate Using Multisignal Sedimentation Velocity Analytical Ultracentrifugation

Advances in polymer science have broadened the applications of protein-polymer conjugates as biopharmaceuticals and biotechnology reagents. The complex nature of these conjugates makes characterization challenging. Here, we describe the use of multisignal sedimentation velocity analytical ultracentrifugation to measure the polymer-to-protein ratio. To demonstrate the principle, we applied this approach to a series of antibody-drug conjugates with different polymer-to-protein ratios and various degrees of heterogeneity, and validated results with orthogonal analytical techniques. We found that multisignal sedimentation velocity can provide accurate information on key attributes including polymer-to-protein ratio, which is important for maximizing the therapeutic potential of future protein-polymer conjugates.

A Reappraisal of Sedimentation Nonideality Coefficients for the Analysis of Weak Interactions of Therapeutic Proteins

The study of weak or colloidal interactions of therapeutic proteins in different formulations allows prediction and optimization of protein stability. Various biophysical techniques have been applied to determine the second osmotic virial coefficient B₂ as it reflects on the macromolecular distance distribution that governs solution behavior at high concentration. In the present work, we exploit a direct link predicted by hydrodynamic theory between B₂ and the nonideality of sedimentation, commonly measured in sedimentation velocity analytical ultracentrifugation through the nonideality coefficient of sedimentation, k_S. Using sedimentation equilibrium analytical ultracentrifugation for independent measurement of B₂, we have examined the dependence of k_S on B₂ for model proteins in different buffers. The data exhibit the expected linear relationship and highlight the impact of protein shape on the magnitude of the nonideality coefficient k_S. Recently, measurements of k_S have been considerably simplified allowing higher throughput and simultaneous polydispersity assessment at higher protein concentrations. Thus, sedimentation velocity may offer a useful approach to compare the impact of formulation conditions on weak interactions and simultaneously on higher-order structure of therapeutic proteins.

Measuring macromolecular size distributions and interactions at high concentrations by sedimentation velocity

In concentrated macromolecular solutions, weak physical interactions control the solution behavior including particle size distribution, aggregation, liquid-liquid phase separation, or crystallization. This is central to many fields ranging from colloid chemistry to cell biology and pharmaceutical protein engineering. Unfortunately, it is very difficult to determine macromolecular assembly states and polydispersity at high concentrations in solution, since all motion is coupled through long-range hydrodynamic, electrostatic, steric, and other interactions, and scattering techniques report on the solution structure when average interparticle distances are comparable to macromolecular dimensions. Here we present a sedimentation velocity technique that, for the first time, can resolve macromolecular size distributions at high concentrations, by simultaneously accounting for average mutual hydrodynamic and thermodynamic interactions. It offers high resolution and sensitivity of protein solutions up to 50 mg/ml, extending studies of macromolecular solution state closer to the concentration range of therapeutic formulations, serum, or intracellular conditions.

Many aspects of concentrated macromolecular solutions, such as encountered in cytosol or in pharmaceutical formulations, are dependent on particle size distributions and weak intermolecular interactions. Here, the authors exploit hydrodynamic separation in the centrifugal field to measure both.

Crystal Structure of Chicken γS-Crystallin Reveals Lattice Contacts with Implications for Function in the Lens and the Evolution of the βγ-Crystallins

Previous attempts to crystallize mammalian γS-crystallin were unsuccessful. Native L16 chicken γS crystallized avidly while the Q16 mutant did not. The x-ray structure for chicken γS at 2.3Å resolution shows the canonical structure of the superfamily plus a well-ordered N-arm aligned with a β-sheet of a neighboring N-domain. L16 is also in a lattice contact, partially shielded from solvent. Unexpectedly, the major lattice contact matches a conserved interface (QR) in the multimeric β-crystallins. QR shows little conservation of residue contacts, except for one between symmetry-related tyrosines, but molecular dipoles for the proteins with QR show striking similarities while other γ-crystallins differ. In γS, QR has few hydrophobic contacts and features a thin layer of tightly bound water. The free energy of QR is slightly repulsive and AUC confirms no dimerization in solution. The lattice contacts suggest how γcrystallins allow close packing without aggregation in the crowded environment of the lens.

Variable Field Analytical Ultracentrifugation: II. Gravitational Sweep Sedimentation Velocity

Sedimentation velocity (SV) analytical ultracentrifugation is a classical biophysical technique for the determination of the size-distribution of macromolecules, macromolecular complexes, and nanoparticles. SV has traditionally been carried out at a constant rotor speed, which limits the range of sedimentation coefficients that can be detected in a single experiment. Recently we have introduced methods to implement experiments with variable rotor speeds, in combination with variable field solutions to the Lamm equation, with the application to expedite the approach to sedimentation equilibrium. Here, we describe the use of variable-field sedimentation analysis to increase the size-range covered in SV experiments by ∼100-fold with a quasi-continuous increase of rotor speed during the experiment. Such a gravitational-sweep sedimentation approach has previously been shown to be very effective in the study of nanoparticles with large size ranges. In the past, diffusion processes were not accounted for, thereby posing a lower limit of particle sizes and limiting the accuracy of the size distribution. In this work, we combine variable field solutions to the Lamm equation with diffusion-deconvoluted sedimentation coefficient distributions c(s), which further extend the macromolecular size range that can be observed in a single SV experiment while maintaining accuracy and resolution. In this way, approximately five orders of magnitude of sedimentation coefficients, or eight orders of magnitude of particle mass, can be probed in a single experiment. This can be useful, for example, in the study of proteins forming large assemblies, as in fibrillation process or capsid self-assembly, in studies of the interaction between very dissimilar-sized macromolecular species, or in the study of broadly distributed nanoparticles.

3D-Printing for Analytical Ultracentrifugation

Analytical ultracentrifugation (AUC) is a classical technique of physical biochemistry providing information on size, shape, and interactions of macromolecules from the analysis of their migration in centrifugal fields while free in solution. A key mechanical element in AUC is the centerpiece, a component of the sample cell assembly that is mounted between the optical windows to allow imaging and to seal the sample solution column against high vacuum while exposed to gravitational forces in excess of 300,000 g. For sedimentation velocity it needs to be precisely sector-shaped to allow unimpeded radial macromolecular migration. During the history of AUC a great variety of centerpiece designs have been developed for different types of experiments. Here, we report that centerpieces can now be readily fabricated by 3D printing at low cost, from a variety of materials, and with customized designs. The new centerpieces can exhibit sufficient mechanical stability to withstand the gravitational forces at the highest rotor speeds and be sufficiently precise for sedimentation equilibrium and sedimentation velocity experiments. Sedimentation velocity experiments with bovine serum albumin as a reference molecule in 3D printed centerpieces with standard double-sector design result in sedimentation boundaries virtually indistinguishable from those in commercial double-sector epoxy centerpieces, with sedimentation coefficients well within the range of published values. The statistical error of the measurement is slightly above that obtained with commercial epoxy, but still below 1%. Facilitated by modern open-source design and fabrication paradigms, we believe 3D printed centerpieces and AUC accessories can spawn a variety of improvements in AUC experimental design, efficiency and resource allocation.