= Papain digestion of IgG
{c}
Limited <Papain digestion of IgG> cuts each <immunoglobulin heavy chain> on the <N-terminus> side of the hinge crosslinks, releasing two <Fab fragments> and one <Fc fragment>.<Immunobiology: The Immune System in Health and Disease, 5th edition by Charles A. Janeway, Jr. and others/The structure of a typical antibody molecule>[Immunobiology: The Immune System in Health and Disease, 5th edition by Charles A. Janeway, Jr. and others; §3-3 and Figure 3.3: papain cut positions and two Fab plus one Fc]{ref}
\Image[/igg-papain-fragment-products.png]
{title=Two backbone cuts give two <Fab fragments> and one <Fc fragment>}
{description=Original schematic of ideal limited <Papain digestion of IgG>, with cuts distinguished from retained <protein disulfide bonds>. The lower panel counts products for a constructed partial-digestion example, not a measured digest. Small released <peptides>, incomplete cleavage of one arm and secondary cleavage are excluded from that model.}
A connectivity argument explains the three products. Treat each continuous backbone segment as a <graph vertex>, and retained interchain <disulfide bonds> as <edges>. After the two stipulated cuts, six backbone segments remain: two complete <immunoglobulin light chains>, two <immunoglobulin heavy chain> arm segments and two <immunoglobulin heavy chain> stem segments. Each light chain remains attached to its own arm segment, producing two separate <Fab fragments>. The two stem segments remain attached to each other, producing one <Fc fragment>. Thus six backbone segments form three <connected components of a graph>, not six independently released products. Removing crosslinks instead, while keeping each backbone intact, would leave four chain components in this simplified covalent-connectivity model.
For a constructed mixture, start with $5\,\mathrm{pmol}$ of intact <IgG> and stipulate that $60\%$ undergoes the complete two-cut event while the rest remains intact. Count <amount of substance> of each product:
$$
n_{\mathrm{intact}}=2\,\mathrm{pmol},\qquad n_{\mathrm{Fab}}=6\,\mathrm{pmol},\qquad n_{\mathrm{Fc}}=3\,\mathrm{pmol}.
$$
The total amount of separately connected products becomes $11\,\mathrm{pmol}$, although the starting amount was $5\,\mathrm{pmol}$. Each digested <IgG> replaces one connected assembly with three. The <antibody binding site> inventory is conserved in this model:
$$
2n_{\mathrm{intact}}+n_{\mathrm{Fab}}=(2\cdot2+6)\,\mathrm{pmol}=10\,\mathrm{pmol}.
$$
The factor two counts sites per intact <IgG>, not extra <immunoglobulin light chains> per <Fab fragment>. Fragment counts, backbone-segment counts and <antibody binding site> counts must be kept distinct. A real digest's retained binding would additionally depend on preservation of the <paratopes>.
The historical functional test was more subtle than asking whether a fragment still formed a visible precipitate. <Rodney Robert Porter>'s earlier rabbit anti-<ovalbumin> preparation was digested with <papain> powder. The resulting fragment did not precipitate <ovalbumin> itself, but specifically inhibited <ovalbumin> <precipitation (chemistry)> with undigested antiserum. The intact-antiserum reaction supplied the positive comparison: failure to precipitate alone did not mean failure to recognize the <antigen>.<Separation and Isolation of Fractions of Rabbit Gamma-Globulin containing the Antibody and Antigenic Combining Sites by Rodney Robert Porter>[Rodney R. Porter, Separation and Isolation of Fractions of Rabbit Gamma-Globulin containing the Antibody and Antigenic Combining Sites, 1958, opening paragraph: earlier papain-powder digest and inhibition of precipitation]{ref}
The inference is that binding and assembly of a precipitating network are different abilities. A one-site <Fab fragment> can occupy an <epitope> without providing the two-arm bridge available to intact <IgG>. The inhibition readout supports retained recognition under the tested conditions; it does not locate individual contacts or show that every <papain>-generated product binds. This is the conceptual benefit of fragmenting an <antibody>: one can ask which functions survive loss of a structural connection.
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