Crystallization of intact monoclonal antibodies

The growth of microcrystals for time resolved serial crystallography

The production of enzyme microcrystals for time resolved serial crystallography employing free electron laser or synchrotron radiation is a relatively new variation on traditional macromolecular crystallization for conventional single crystal X-ray analysis. While the fundamentals of macromolecular crystal growth are the same, some modifications and special considerations are in order if the objective is to produce uniform size, microcrystals in very large numbers for serial data collection. Presented here are the basic principles of protein crystal growth with particular attention to the approaches best employed to achieve the goal of microcrystals and some novel techniques, as well as old, that may be useful. Also discussed are the advantages of particular precipitants and certain methods of growing protein crystals that might be advantageous for serial data recording.

Mapping conformational changes on bispecific antigen-binding biotherapeutic by covalent labeling and mass spectrometry

Biotherapeutic's higher order structure (HOS) is a critical determinant of its functional properties and conformational relevance. Here, we evaluated two covalent labeling methods: diethylpyrocarbonate (DEPC)-labeling and fast photooxidation of proteins (FPOP), in conjunction with mass spectrometry (MS), to investigate structural modifications for the new class of immuno-oncological therapy known as bispecific antigen-binding biotherapeutics (BABB). The evaluated techniques unveiled subtle structural changes occurring at the amino acid residue level within the antigen-binding domain under both native and thermal stress conditions, which cannot be detected by conventional biophysical techniques, e.g., near-ultraviolet circular dichroism (NUV-CD). The determined variations in labeling uptake under native and stress conditions, corroborated by binding assays, shed light on the binding effect, and highlighted the potential of covalent-labeling methods to effectively monitor conformational changes that ultimately influence the product quality. Our study provides a foundation for implementing the developed techniques in elucidating the inherent structural characteristics of novel therapeutics and their conformational stability.

Solution structure of deglycosylated human IgG1 shows the role of CH2 glycans in its conformation

The human immunoglobulin G (IgG) class is the most prevalent antibody in serum, with the IgG1 subclass being the most abundant. IgG1 is composed of two Fab regions connected to a Fc region through a 15-residue hinge peptide. Two glycan chains are conserved in the Fc region in IgG; however, their importance for the structure of intact IgG1 has remained unclear. Here, we subjected glycosylated and deglycosylated monoclonal human IgG1 (designated as A33) to a comparative multidisciplinary structural study of both forms. After deglycosylation using peptide:N-glycosidase F, analytical ultracentrifugation showed that IgG1 remained monomeric and the sedimentation coefficients s⁰_20,w of IgG1 decreased from 6.45 S by 0.16-0.27 S. This change was attributed to the reduction in mass after glycan removal. X-ray and neutron scattering revealed changes in the Guinier structural parameters after deglycosylation. Although the radius of gyration (R_G) was unchanged, the cross-sectional radius of gyration (R_XS-1) increased by 0.1 nm, and the commonly occurring distance peak M2 of the distance distribution curve P(r) increased by 0.4 nm. These changes revealed that the Fab-Fc separation in IgG1 was perturbed after deglycosylation. To explain these changes, atomistic scattering modeling based on Monte Carlo simulations resulted in 123,284 and 119,191 trial structures for glycosylated and deglycosylated IgG1 respectively. From these, 100 x-ray and neutron best-fit models were determined. For these, principal component analyses identified five groups of structural conformations that were different for glycosylated and deglycosylated IgG1. The Fc region in glycosylated IgG1 showed a restricted range of conformations relative to the Fab regions, whereas the Fc region in deglycosylated IgG1 showed a broader conformational spectrum. These more variable Fc conformations account for the loss of binding to the Fcγ receptor in deglycosylated IgG1.

Matching pH values for antibody stabilization and crystallization suggest rationale for accelerated development of biotherapeutic drugs

Monoclonal antibodies (mAbs) are currently leading products in the global biopharmaceutical market. Multiple mAbs are in clinical development and novel biotherapeutic protein scaffolds, based on the canonical immunoglobulin G (IgG) fold, are emerging as treatment options for various medical conditions. However, fast approvals for biotherapeutics are challenging to achieve, because of difficult scientific development procedures and complex regulatory processes. Selecting molecular entities with superior physicochemical properties that proceed into clinical trials and the identification of stable formulations are crucial developability aspects. It is widely accepted that the solution pH has critical influences on both the protein's colloidal stability and its crystallization behavior. Furthermore, proteins usually crystallize best at solution conditions that enable high protein solubility, purity, stability, and monodispersity. Therefore, we hypothesize that the solution pH value is a central parameter that is linking together protein formulation, protein crystallization, and thermal protein stability. In order to experimentally test this hypothesis, we have investigated the effect of the solution pH on the thermal stabilities and crystallizabilities for three different mAbs. Combining biophysical measurements with high throughput protein (HTP) crystallization trials we observed a correlation in the buffer pH values for eminent mAb stability and successful crystallization. Specifically, differential scanning fluorimetry (DSF) was used to determine pH values that exert highest thermal mAb stabilities and additionally led to the identification of unfolding temperatures of individual mAb domains. Independently performed crystallization trials with the same mAbs resulted in their successful crystallization at pH values that displayed highest thermal stabilities. In summary, the presented results suggest a strategy how protein crystallization could be used as a screening method for the development of biotherapeutic protein formulations with improved in vitro stabilities.

Two-faced Fcab prevents polymerization with VEGF and reveals thermodynamics and the 2.15 Å crystal structure of the complex

Fcabs (Fc domain with antigen-binding sites) are promising novel therapeutics. By engineering of the C-terminal loops of the CH3 domains, 2 antigen binding sites can be inserted in close proximity. To elucidate the binding mode(s) between homodimeric Fcabs and small homodimeric antigens, the interaction between the Fcabs 448 and CT6 (having the AB, CD and EF loops and the C-termini engineered) with homodimeric VEGF was investigated. The crystal structures of these Fcabs, which form polymers with the antigen VEGF in solution, were determined. However, construction of heterodimeric Fcabs (JanusFcabs: one chain Fc-wt, one chain VEGF-binding) results in formation of distinct JanusFcab–VEGF complexes (2:1), which allowed elucidation of the crystal structure of the JanusCT6–VEGF complex at 2.15 Å resolution. VEGF binding to Janus448 and JanusCT6 is shown to be entropically unfavorable, but enthalpically favorable. Structure-function relationships are discussed with respect to Fcab design and engineering strategies.

Fcab-HER2 Interaction: a Ménage à Trois. Lessons from X-Ray and Solution Studies

The crystallizable fragment (Fc) of the immunoglobulin class G (IgG) is an attractive scaffold for the design of novel therapeutics. Upon engineering the C-terminal loops in the CH3 domains, Fcabs (Fc domain with antigen-binding sites) can be designed. We present the first crystal structures of Fcabs, i.e., of the HER2-binding clone H10-03-6 having the AB and EF loop engineered and the stabilized version STAB19 derived by directed evolution. Comparison with the crystal structure of the Fc wild-type protein reveals conservation of the overall domain structures but significant differences in EF-loop conformations. Furthermore, we present the first Fcab-antigen complex structures demonstrating the interaction between the engineered Fcab loops with domain IV of HER2. The crystal structures of the STAB19-HER2 and H10-03-6-HER2 complexes together with analyses by isothermal titration calorimetry, SEC-MALS, and fluorescence correlation spectroscopy show that one homodimeric Fcab binds two HER2 molecules following a negative cooperative binding behavior.

Structural analysis of the unmutated ancestor of the HIV-1 envelope V2 region antibody CH58 isolated from an RV144 vaccine efficacy trial vaccinee

Human monoclonal antibody CH58 isolated from an RV144 vaccinee binds at Lys169 of the HIV-1 Env gp120 V2 region, a site of vaccine-induced immune pressure. CH58 neutralizes HIV-1 CRF_01 AE strain 92TH023 and mediates ADCC against CD4 + T cell targets infected with CRF_01 AE tier 2 virus. CH58 and other antibodies that bind to a gp120 V2 epitope have a second light chain complementarity determining region (LCDR2) bearing a glutamic acid, aspartic acid (ED) motif involved in forming salt bridges with polar, basic side amino acid side chains in V2. In an effort to learn how V2 responses develop, we determined the crystal structures of the CH58-UA antibody unliganded and bound to V2 peptide. The structures showed an LCDR2 structurally pre-conformed from germline to interact with V2 residue Lys169. LCDR3 was subject to conformational selection through the affinity maturation process. Kinetic analyses demonstrate that only a few contacts were responsible for a 2000-fold increase in K_D through maturation, and this effect was predominantly due to an improvement in off-rate. This study shows that preconformation and preconfiguration can work in concert to produce antibodies with desired immunogenic properties.

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With only 2-3% mutation from germline, the HIV-1 antibody CH58 developed neutralizing and ADCC capabilities.

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The LCDR2 Glu–Asp motif of the RV144 antibody CH58 is pre-conformed from germline to interact with the gp120 V2 loop.

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Affinity and neutralization gains resulted from tuning local interactions rather than gross sequence or structure changes.

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Structural analyses show the second light chain complementarity determining region Glu–Asp motif of the CH58 antibody isolated from an RV144 vaccinee is optimally pre-conformed from germline to interact with the gp120 V2 loop. The increased binding affinity and neutralization capacity of the mature antibody compared to its germline precursor were achieved with only 2–3% mutation from germline, and the fact that these gains appeared to be a result of the tuning of local interactions rather than gross sequential or conformational changes provides hope that a rational immunogen design for HIV-1 treatment may become a reality.

Molecular basis for mid-region amyloid-β capture by leading Alzheimer's disease immunotherapies

Solanezumab (Eli Lilly) and crenezumab (Genentech) are the leading clinical antibodies targeting Amyloid-β (Aβ) to be tested in multiple Phase III clinical trials for the prevention of Alzheimer's disease in at-risk individuals. Aβ capture by these clinical antibodies is explained here with the first reported mid-region Aβ-anti-Aβ complex crystal structure. Solanezumab accommodates a large Aβ epitope (960 Å(2) buried interface over residues 16 to 26) that forms extensive contacts and hydrogen bonds to the antibody, largely via main-chain Aβ atoms and a deeply buried Phe19-Phe20 dipeptide core. The conformation of Aβ captured is an intermediate between observed sheet and helical forms with intramolecular hydrogen bonds stabilising residues 20-26 in a helical conformation. Remarkably, Aβ-binding residues are almost perfectly conserved in crenezumab. The structure explains the observed shared cross reactivity of solanezumab and crenezumab with proteins abundant in plasma that exhibit this Phe-Phe dipeptide.

From osmotic second virial coefficient (B22 ) to phase behavior of a monoclonal antibody

Antibodies are complex macromolecules and their phase behavior as well as interactions within different solvents and precipitants are still not understood. To shed some light into the processes on a molecular dimension, the occurring self-interactions between antibody molecules were analyzed by means of the osmotic second virial coefficient (B22 ). The determined B22 follows qualitatively the phenomenological Hofmeister series describing the aggregation probability of antibodies for the various solvent compositions. However, a direct correlation between crystallization probability and B22 in form of a crystallization slot does not seem to be feasible for antibodies since the phase behavior is strongly dependent on their anisotropy. Kinetic parameters have to be taken into account due to the molecular size and complexity of the molecules. This is confirmed by a comparison of experimental data with a theoretical phase diagram. On the other hand the solubility is thermodynamically driven and therefore the B22 could be used to establish a universal solubility line for the monoclonal antibody mAb04c and different solvent compositions by using thermodynamic models.

Large-scale crystallization of proteins for purification and formulation

Since about 170 years, salts were used to create supersaturated solutions and crystallize proteins. The dehydrating effect of salts as well as their kosmotropic or chaotropic character was revealed. Even the suitability of organic solvents for crystallization was already recognized. Interestingly, what was performed during the early times is still practiced today. A lot of effort was put into understanding the underlying physico-chemical interaction mechanisms leading to protein crystallization. However, it was understood that already the solvation of proteins is a highly complex process not to mention the intricate interrelation of electrostatic and hydrophobic interactions taking place. Although many basic questions are still unanswered, preparative protein crystallization was attempted as illustrated in the presented case studies. Due to the highly variable nature of crystallization, individual design of the crystallization process is needed in every single case. It was shown that preparative crystallization from impure protein solutions as a capture step is possible after applying adequate pre-treatment procedures like precipitation or extraction. Protein crystallization can replace one or more chromatography steps. It was further shown that crystallization can serve as an attractive alternative means for formulation of therapeutic proteins. Crystalline proteins can offer enhanced purity and enable highly concentrated doses of the active ingredient. Easy scalability of the proposed protein crystallization processes was shown using the maximum local energy dissipation as a suitable scale-up criterion. Molecular modeling and target-oriented protein engineering may allow protein crystallization to become part of a platform purification process in the near future.

Optimization of crystallization conditions for biological macromolecules

For the successful X-ray structure determination of macromolecules, it is first necessary to identify, usually by matrix screening, conditions that yield some sort of crystals. Initial crystals are frequently microcrystals or clusters, and often have unfavorable morphologies or yield poor diffraction intensities. It is therefore generally necessary to improve upon these initial conditions in order to obtain better crystals of sufficient quality for X-ray data collection. Even when the initial samples are suitable, often marginally, refinement of conditions is recommended in order to obtain the highest quality crystals that can be grown. The quality of an X-ray structure determination is directly correlated with the size and the perfection of the crystalline samples; thus, refinement of conditions should always be a primary component of crystal growth. The improvement process is referred to as optimization, and it entails sequential, incremental changes in the chemical parameters that influence crystallization, such as pH, ionic strength and precipitant concentration, as well as physical parameters such as temperature, sample volume and overall methodology. It also includes the application of some unique procedures and approaches, and the addition of novel components such as detergents, ligands or other small molecules that may enhance nucleation or crystal development. Here, an attempt is made to provide guidance on how optimization might best be applied to crystal-growth problems, and what parameters and factors might most profitably be explored to accelerate and achieve success.

Crystallization screening: the influence of history on current practice

While crystallization historically predates crystallography, it is a critical step for the crystallographic process. The rich history of crystallization and how that history influences current practices is described. The tremendous impact of crystallization screens on the field is discussed.

Conformational dynamics of individual antibodies using computational docking and AFM

Molecular recognition between a receptor and a ligand requires a certain level of flexibility in macromolecules. In this study, we aimed at analyzing the conformational variability of receptors portrayed by monoclonal antibodies that have been individually imaged using atomic force microscopy (AFM). Individual antibodies were chemically coupled to activated mica surface, and they have been imaged using AFM in ambient conditions. The resulting topographical surface of antibodies was used to assemble the three subunits constituting antibodies: two antigen-binding fragments and one crystallizable fragment using a surface-constrained computational docking approach. Reconstructed structures based on 10 individual topographical surfaces of antibodies are presented for which separation and relative orientation of the subunits were measured. When compared with three X-ray structures of antibodies present in the protein data bank database, results indicate that several arrangements of the reconstructed subunits are comparable with those of known structures. Nevertheless, no reconstructed structure superimposes adequately to any particular X-ray structure consequence of the antibody flexibility. We conclude that high-resolution AFM imaging with appropriate computational reconstruction tools is adapted to study the conformational dynamics of large individual macromolecules deposited on mica.

The structure of dual-variable-domain immunoglobulin molecules alone and bound to antigen

A dual-specific, tetravalent immunoglobulin G-like molecule, termed dual variable domain immunoglobulin (DVD-Ig™), is engineered to block two targets. Flexibility modulates Fc receptor and complement binding, but could result in undesirable cross-linking of surface antigens and downstream signaling. Understanding the flexibility of parental mAbs is important for designing and retaining functionality of DVD-Ig™ molecules. The architecture and dynamics of a DVD-Ig™ molecule and its parental mAbs was examined using single particle electron microscopy. Hinge angles measured for the DVD-Ig™ molecule were similar to the inner antigen parental mAb. The outer binding domain of the DVD-Ig™ molecule was highly mobile and three-dimensional (3D) analysis showed binding of inner antigen caused the outer domain to fold out of the plane with a major morphological change. Docking high-resolution X-ray structures into the 3D electron microscopy map supports the extraordinary domain flexibility observed in the DVD-Ig™ molecule allowing antigen binding with minimal steric hindrance.

Segregation of PIP2 and PIP3 into distinct nanoscale regions within the plasma membrane

PIP₂ and PIP₃ are implicated in a wide variety of cellular signaling pathways at the plasma membrane. We have used STORM imaging to localize clusters of PIP₂ and PIP₃ to distinct nanoscale regions within the plasma membrane of PC12 cells. With anti-phospholipid antibodies directly conjugated with AlexaFluor 647, we found that PIP₂ clusters in membrane domains of 64.5±27.558 nm, while PIP₃ clusters had a size of 125.6±22.408 nm. With two color direct STORM imaging we show that >99% of phospholipid clusters have only one or other phospholipid present. These results indicate that lipid nano-domains can be readily identified using super-resolution imaging techniques, and that the lipid composition and size of clusters is tightly regulated.

Molecular basis for passive immunotherapy of Alzheimer's disease

Amyloid aggregates of the amyloid-beta (Abeta) peptide are implicated in the pathology of Alzheimer's disease. Anti-Abeta monoclonal antibodies (mAbs) have been shown to reduce amyloid plaques in vitro and in animal studies. Consequently, passive immunization is being considered for treating Alzheimer's, and anti-Abeta mAbs are now in phase II trials. We report the isolation of two mAbs (PFA1 and PFA2) that recognize Abeta monomers, protofibrils, and fibrils and the structures of their antigen binding fragments (Fabs) in complex with the Abeta(1-8) peptide DAEFRHDS. The immunodominant EFRHD sequence forms salt bridges, hydrogen bonds, and hydrophobic contacts, including interactions with a striking WWDDD motif of the antigen binding fragments. We also show that a similar sequence (AKFRHD) derived from the human protein GRIP1 is able to cross-react with both PFA1 and PFA2 and, when cocrystallized with PFA1, binds in an identical conformation to Abeta(1-8). Because such cross-reactivity has implications for potential side effects of immunotherapy, our structures provide a template for designing derivative mAbs that target Abeta with improved specificity and higher affinity.

Surfactant-aided size-exclusion chromatography for the purification of immunoglobulin G

In the production of monoclonal antibodies, separate chains of the antibody are often present in the product mixture as well as other contaminating proteins. These fragments should be removed from the whole antibodies. This paper shows the purification of monoclonal immunoglobulin G (IgG) from its heavy chain contaminant. The heavy chain fragment is simulated experimentally using bovine serum albumin (BSA), which has approximately the same molecular weight. The purification is performed using traditional size-exclusion chromatography (SEC) and using surfactant-aided SEC (SASEC), testing two different surfactants (C(12)E(23) and Tween20) and two different gels (Sephacryl S200HR and Sephacryl S300 HR). Pulse experiments show that with SASEC both BSA and IgG are more distributed towards the solid phase than compared to using SEC. This effect is larger on IgG, the largest component than on BSA. As a consequence, azeotropes will be formed at a specific surfactant concentration. Above this concentration the selectivity is reversed and increased to values higher than obtained with conventional SEC. At 7.5% (w/w) of C(12)E(23), BSA actually elutes before IgG. These experiments further show that when using SASEC larger productivity, higher yields and lower solvent consumption can be achieved without loss of purity of IgG when compared to conventional SEC. Mathematical simulation of the separation of BSA and IgG using simulated moving bed (SMB) chromatography indicates a large increase in productivity when applying a surfactant gradient in SASEC SMB compared to conventional isocratic SEC-SMB. Furthermore, solvent consumption reductions with a factor 15 prove possible as well as concentrating the IgG by a factor 2.

Phase behavior of an intact monoclonal antibody

Understanding protein phase behavior is important for purification, storage, and stable formulation of protein drugs in the biopharmaceutical industry. Glycoproteins, such as monoclonal antibodies (MAbs) are the most abundant biopharmaceuticals and probably the most difficult to crystallize among water-soluble proteins. This study explores the possibility of correlating osmotic second virial coefficient (B(22)) with the phase behavior of an intact MAb, which has so far proved impossible to crystallize. The phase diagram of the MAb is presented as a function of the concentration of different classes of precipitants, i.e., NaCl, (NH4)2SO4, and polyethylene glycol. All these precipitants show a similar behavior of decreasing solubility with increasing precipitant concentration. B(22) values were also measured as a function of the concentration of the different precipitants by self-interaction chromatography and correlated with the phase diagrams. Correlating phase diagrams with B(22) data provides useful information not only for a fundamental understanding of the phase behavior of MAbs, but also for understanding the reason why certain proteins are extremely difficult to crystallize. The scaling of the phase diagram in B(22) units also supports the existence of a universal phase diagram of a complex glycoprotein when it is recast in a protein interaction parameter.

Challenges in the development of high protein concentration formulations

Development of formulations for protein drugs requiring high dosing (in the order of mg/kg) may become challenging for solubility limited proteins and for the subcutaneous (SC) route with <1.5 mL allowable administration volume that requires >100 mg/mL protein concentrations. Development of high protein concentration formulations also results in several manufacturing, stability, analytical, and delivery challenges. The high concentrations achieved by small scale approaches used in preformulation studies would have to be confirmed with manufacturing scale processes and with representative materials because of the lability of protein conformation and the propensity to interact with surfaces and solutes which render protein solubilities that are dependent on the process of concentrating. The concentration dependent degradation route of aggregation is the greatest challenge to developing protein formulations at these higher concentrations. In addition to the potential for nonnative protein aggregation and particulate formation, reversible self-association may occur, which contributes to properties such as viscosity that complicates delivery by injection. Higher viscosity also complicates manufacturing of high protein concentrations by filtration approaches. Chromatographic and electrophoretic assays may not accurately determine the non-covalent higher molecular weight forms because of the dilutions that are usually encountered with these techniques. Hence, techniques must be used that allow for direct measurement in the formulation without substantial dilution of the protein. These challenges are summarized in this review.

Freezing immunoglobulins to see them move

The issue of protein dynamics and its implications in the biological function of proteins are arousing greater and greater interest in different fields of molecular biology. In cryo-electron tomography experiments one may take several snapshots of a given biological macromolecule. In principle, a large enough collection of snapshots of the molecule may then be used to calculate its equilibrium configuration in terms of the experimentally accessible degrees of freedom and, hence, to estimate its potential energy. This information would be crucial in order to analyze the biological functions of biomolecules by directly accessing the relevant dynamical indicators. In this article, we analyze the results of cryo-electron tomography experiments performed on monoclonal murine IgG2a antibodies. We measure the equilibrium distribution of the molecule in terms of the relevant angular coordinates and build a mechanical model of the antibody dynamics. This approach enables us to derive an explicit expression of the IgG potential energy. Furthermore, we discuss the configuration space at equilibrium in relation to results from other techniques, and we set our discussion in the context of the current debate regarding conformation and flexibility of antibodies.

Contrasting IgG structures reveal extreme asymmetry and flexibility

The crystal structure of IgG1 b12 represents the first visualization of an intact human IgG with a full-length hinge that has all domains ordered and visible. In comparison to intact murine antibodies and hinge-deletant human antibodies, b12 reveals extreme asymmetry, indicative of the extraordinary interdomain flexibility within an antibody. In addition, the structure provides an illustration of the human IgG1 hinge in its entirety and of asymmetry in the composition of the carbohydrate attached to each C(H)2 domain of the Fc. The two separate hinges assume different conformations in order to accommodate the vastly different placements of the two Fab domains relative to the Fc domain. Interestingly, only one of two possible intra-hinge disulfides is formed.

Comparison of the conformations of two intact monoclonal antibodies with hinges

Structures of two intact monoclonal antibodies were solved by X-ray diffraction analysis revealing, in both cases, the dispositions of all segments, as well as the structures of the hinge polypeptides. An IgG1, whose antigen is the drug phenobarbital, assumed a completely different conformation when compared with an IgG2a specific for canine lymphoma cells. Though neither IgG displays global two-fold symmetry, both maintain two pseudo dyad axes, one relating Fab segments, and the other the halves of the Fc. In both IgGs, the Fc segment is obliquely disposed with respect to the plane of the Fabs, making an angle of 128 degrees in the IgG2a, and 107 degrees in the IgG1. Hinge angles of the IgG1 are notably different at 78 degrees and 123 degrees, and unique as well from IgG2a values of 66 degrees and 113 degrees. Elbow angles within the IgG1 Fabs are the same at 155 degrees, but non-identical in IgG2a where they took on values of 143 degrees and 159 degrees. The IgG2a has an angle of 172 degrees between Fabs so that it exhibits a "distorted T" shape, whereas that angle in the IgG1 is a much more acute 115 degrees producing a "distorted Y". CH2 domains appear, in both antibodies, to be the most independently mobile of the paired IgG domains. This perhaps reflects their importance in modulating effector functions through exposure of binding loci on the CH2, at the CH2-CH3 interface, and on lower hinge polypeptides. Hinges in both antibodies contain disulfide-linked cores bounded by fluid regions above and below, which provide mobility to the Fabs and Fc respectively. The conformations seen in these two structures are undoubtedly only two among many but they illustrate the modes of flexibility inherent to the IgG architecture.

Crystallographic structure of an intact IgG1 monoclonal antibody

The structure of an intact monoclonal antibody for phenobarbital, subclass IgG1, has been determined to 3.2 A resolution by X-ray crystallography. The molecule was visualized in a monoclinic unit cell having an entire immunoglobulin as the asymmetric unit. The two Fab segments, both with elbow angles of 155 degrees , were related by a rotation of 179.7 degrees plus a translation along the approximate dyad of 9 A. This is the first observation of such an Fab translation in a structurally defined antibody. The approximate 2-fold of the Fc was independent of that relating Fabs, making an angle of 107 degrees with the Fab dyad. The angle between long axes of the Fabs was 115 degrees, the most acute angle yet observed, yielding a distorted Y shaped molecule. This is in contrast to the distorted T shape of the only other intact IgG (2a) whose complete structure is known. Primary lattice interactions arise through formation of VH antiparallel beta ribbons whose strands are contributed by pseudo dyad related H2, and by L3 hypervariable loops from neighboring molecules. While one CH2 domain was mobile, Fabs and three domains of the Fc were well defined, as were hinge polypeptides connecting Fabs to the Fc, and the covalently attached oligosaccharides. Direct interactions are observed between hinge polypeptides, the glycosylated loop of one CH2 domain, and the oligosaccharide. Lattice interactions clearly influence, perhaps even determine the overall conformation of the antibody observed in this crystal. Comparison of this IgG1 with previously determined intact antibody structures extends the conformational range arising from segmental flexibility.