Antibodies are Y-shaped proteins that serve as a component of the immune system, identifying and neutralizing foreign invaders. Their structure has distinct domains, each contributing to function. This article focuses on the hinge region, exploring its structure and roles in antibody activity.
Structural Characteristics of the Hinge Region
The hinge region is a flexible segment between the CH1 and CH2 constant domains of the heavy chains. It connects the Fab arms to the Fc stem. Its amino acid composition, rich in proline, contributes to its flexible structure. Cysteine residues are also abundant, forming disulfide bonds that link the two heavy chains, providing stability.
The hinge divides into three sub-regions: the upper, core, and lower hinge. The upper hinge, on the N-terminal side, facilitates Fab arm movement. The core hinge contains varying cysteine residues depending on antibody subtype, forming inter-heavy chain disulfide bonds. The lower hinge, on the C-terminal side, enables the Fc region to move relative to the Fab arms, influencing immune interactions.
Functional Significance of Flexibility
The hinge region’s primary function is to provide segmental flexibility. This flexibility allows the two Fab (fragment antigen-binding) arms to move, rotate, and wave independently from each other and the Fc region. This movement, sometimes called “hinge-bending motion,” enables the antibody to adapt its shape.
This adaptability aids effective antigen binding. The independent movement of the Fab arms allows an antibody to simultaneously bind to two antigen epitopes even if they are located at varying distances or orientations on a pathogen’s surface. This bivalent binding increases overall binding strength (avidity), crucial for holding onto invaders. Beyond antigen recognition, the hinge’s flexibility also plays a role in initiating downstream immune responses by allowing the Fc region to properly interact with other immune system components, such as complement proteins or Fc receptors on immune cells.
Hinge Region Diversity in Antibody Isotypes
Not all antibody classes, or isotypes, possess identical hinge regions; their length and composition can vary significantly. For instance, among the immunoglobulin G (IgG) subclasses, there are notable differences in the number of amino acid residues and disulfide bridges within their hinges, which impacts their stability and flexibility. Human IgG1 has a relatively short hinge, providing a moderate level of flexibility.
In contrast, IgG3 possesses a much longer and more extended hinge region, containing more disulfide bonds, which can contribute to increased flexibility and potentially greater susceptibility to degradation. Immunoglobulin A (IgA) and Immunoglobulin D (IgD) also feature distinct hinge regions, contributing to their unique roles in immune defense. For example, the hinge of IgA can influence its susceptibility to cleavage by certain bacterial proteases.
Other antibody isotypes, such as IgM and IgE, do not have a classic hinge region. Instead, they contain an additional constant domain (CH4) that functionally provides a more limited form of flexibility compared to the distinct hinge found in IgG, IgA, and IgD. This structural variation contributes to the diverse biological activities and tissue distributions observed across different antibody classes.
Enzymatic Cleavage and Antibody Fragmentation
The exposed and relatively unstructured nature of the hinge region makes it a common target for proteolytic enzymes, known as proteases. This susceptibility to cleavage has been exploited in laboratory settings to generate specific antibody fragments, which are valuable tools in research and the development of therapeutics.
Two widely used proteases, papain and pepsin, cleave antibodies at distinct sites within or near the hinge. Papain cleaves the antibody above the disulfide bonds in the hinge region. This digestion yields three separate fragments: two identical Fab fragments, each capable of binding antigen, and one Fc fragment, which mediates effector functions. Conversely, pepsin cleaves the antibody below the hinge’s disulfide bonds. This results in a single F(ab’)2 fragment, which retains bivalent antigen-binding capability because the two Fab arms remain linked by the hinge disulfide bonds, while the Fc portion is degraded into smaller pieces (pFc’). These enzymatic fragmentations allow scientists to isolate specific functional parts of the antibody, enabling targeted studies or therapeutic applications where certain effector functions are not desired.