Immunology—Enzymatic cleavage of antibodies: When the protease papain digests an IgG molecule (under non-reducing conditions), into which defined fragments is IgG typically split?

Introduction / Context:Understanding how antibodies are enzymatically cleaved is foundational in immunology and biotechnology. The classic papain digestion of immunoglobulin G (IgG) produces well-defined fragments that retain specific biological properties used in diagnostics, structural studies, and therapeutics.

Given Data / Assumptions:

• Enzyme: papain (a cysteine protease) acting on IgG.
• Conditions: non-reducing; disulfide bonds between heavy chains remain relevant for fragment definitions.
• Goal: identify the named fragments generated from intact IgG.

Concept / Approach:Papain cleaves IgG above the hinge region. This releases two identical antigen-binding fragments (Fab) and a single crystallizable fragment (Fc). Each Fab contains one light chain and the corresponding variable and constant domains of a heavy chain, preserving monovalent antigen binding. The Fc fragment contains the effector-mediating CH2–CH3 domains of both heavy chains but lacks antigen-binding sites.

Step-by-Step Solution:Recognize papain cleavage site: above the hinge region.Predict products: two Fab (each with one antigen-binding site) + one Fc (effector region).Match to options: two Fab fragments and one Fc fragment.

Verification / Alternative check:Contrast with pepsin cleavage (below the hinge) that yields one F(ab')2 fragment plus degraded Fc pieces. This difference confirms the papain-specific pattern.

Why Other Options Are Wrong:

• Three Fab / multiple Fc: IgG has only two antigen-binding arms; cannot produce three intact Fab.
• Two Fab and two Fc / F(ab')2 only: Do not match papain's above-hinge cleavage signature.

Common Pitfalls:Confusing papain with pepsin results; forgetting that Fab is monovalent whereas F(ab')2 is divalent.

Final Answer:Two Fab fragments and one Fc fragment.