A comparison of binding surfaces for SPR biosensing using an antibody-antigen system and affinity distribution analysis.

The application of optical biosensors in the study of macromolecular interactions requires immobilization of one binding partner to the surface. It is often highly desirable that the immobilization is uniform and does not affect the thermodynamic and kinetic binding parameters to soluble ligands. To achieve this goal, a variety of sensor surfaces, coupling strategies and surface chemistries are available. Previously, we have introduced a technique for determining the distribution of affinities and kinetic rate constants from families of binding and dissociation traces acquired at different concentrations of soluble ligand. In the present work, we explore how this affinity distribution analysis can be useful in the assessment and optimization of surface immobilization. With this goal, using an antibody-antigen interaction as a model system, we study the activity, thermodynamic and kinetic binding parameters, and heterogeneity of surface sites produced with different commonly used sensor surfaces, at different total surface densities and with direct immobilization or affinity capture.

Bayesian analysis of heterogeneity in the distribution of binding properties of immobilized surface sites.

Once a homogeneous ensemble of a protein ligand is taken from solution and immobilized to a surface, for many reasons the resulting ensemble of surface binding sites to soluble analytes may be heterogeneous. For example, this can be due to the intrinsic surface roughness causing variations in the local microenvironment, nonuniform density distribution of polymeric linkers, or nonuniform chemical attachment producing different protein orientations and conformations. We previously described a computational method for determining the distribution of affinity and rate constants of surface sites from analysis of experimental surface binding data. It fully exploits the high signal/noise ratio and reproducibility provided by optical biosensor technology, such as surface plasmon resonance. Since the computational analysis is ill conditioned, the previous approach used a regularization strategy assuming a priori all binding parameters to be equally likely, resulting in the broadest possible parameter distribution consistent with the experimental data. We now extended this method in a Bayesian approach to incorporate the opposite assumption, i.e., that the surface sites a priori are expected to be uniform (as one would expect in free solution). This results in a distribution of binding parameters as close to monodispersity as possible given the experimental data. Using several model protein systems immobilized on a carboxymethyl dextran surface and probed with surface plasmon resonance, we show microheterogeneity of the surface sites in addition to broad populations of significantly altered affinity. The distributions obtained are highly reproducible. Immobilization conditions and the total surface density of immobilized sites can have a substantial impact on the functional distribution of the binding sites.

Eukaryotic RNases H1 act processively by interactions through the duplex RNA-binding domain.

Ribonucleases H have mostly been implicated in eliminating short RNA primers used for initiation of lagging strand DNA synthesis. Escherichia coli RNase HI cleaves these RNA-DNA hybrids in a distributive manner. We report here that eukaryotic RNases H1 have evolved to be processive enzymes by attaching a duplex RNA-binding domain to the RNase H region. Highly conserved amino acids of the duplex RNA-binding domain are required for processivity and nucleic acid binding, which leads to dimerization of the protein. The need for a processive enzyme underscores the importance in eukaryotic cells of processing long hybrids, most of which remain to be identified. However, long RNA-DNA hybrids formed during immunoglobulin class-switch recombination are potential targets for RNase H1 in the nucleus. In mitochondria, where RNase H1 is essential for DNA formation during embryogenesis, long hybrids may be involved in DNA replication.

Comparative analyses of a small molecule/enzyme interaction by multiple users of Biacore technology.

To gauge the experimental variability associated with Biacore analysis, 36 different investigators analyzed a small molecule/enzyme interaction under similar conditions. Acetazolamide (222 g/mol) binding to carbonic anhydrase II (CAII; 30000 Da) was chosen as a model system. Both reagents were stable and their interaction posed a challenge to measure because of the low molecular weight of the analyte and the fast association rate constant. Each investigator created three different density surfaces of CAII and analyzed an identical dilution series of acetazolamide (ranging from 4.1 to 1000 nM). The greatest variability in the results was observed during the enzyme immobilization step since each investigator provided their own surface activating reagents. Variability in the quality of the acetazolamide binding responses was likely a product of how well the investigators' instruments had been maintained. To determine the reaction kinetics, the responses from the different density surfaces were fit globally to a 1:1 interaction model that included a term for mass transport. The averaged association and dissociation rate constants were 3.1+/-1.6 x 10(6)M(-1)s(-1) and 6.7+/-2.5 x 10(-2)s(-1), respectively, which corresponded to an average equilibrium dissociation constant (K(D) of 2.6+/-1.4 x 10(-8)M. The results provide a benchmark of variability in interpreting binding constants from the biosensor and highlight keys areas that should be considered when analyzing small molecule interactions.

Humanized docking system for assembly of targeting drug delivery complexes.

Targeted drug delivery requires 'loading' drugs onto targeting proteins. Traditional technologies for loading drugs rely on chemical conjugation of drugs or drug carriers to targeting proteins. An alternative approach might rely on assembly of targeting complexes using a docking system that includes two components: a 'docking' tag fused to a targeting protein, and a 'payload' module containing an adapter protein for non-covalent binding to the docking tag. We describe here a fully humanized adapter/docking tag system based on non-covalent interaction between two fragments of human pancreatic RNase I. A 15 amino acid long N-terminal fragment of RNase I designed to serve as a docking tag, was fused to the N-terminus of human vascular endothelial growth factor that served as a targeting protein. An 18-125 and an 18-127 amino acid long fragments of RNase I were engineered, expressed and refolded into active conformations to serve as adapter proteins. Interactions between the targeting and adapter proteins were characterized using enzymatic analysis and surface plasmon resonance. Targeting DNA delivery complexes were assembled, characterized by dynamic light scattering, and found to be very effective in receptor-mediated DNA delivery.