Purified (unconjugated) Thermo Scientific Pierce Recombinant Protein G is useful as the basis for preparing various kinds of probes or affinity media for detection or purification of rabbit and human antibodies, especially IgG isotypes, in immunoassays and antibody purification protocols.

Features of Recombinant Protein G:

• Contains two Fc-binding domains per protein
• Better than Protein A for mouse (incl. IgG1), human (incl. IgG3) and rat, goat and cow antibodies
• Poorer than Protein A for guinea pig, pig, dog and cat
• Does not bind human IgM, IgD or IgA

Protein G is a bacterial cell wall protein isolated from group G Streptococci. DNA sequencing of native Protein G identifies two IgG-binding domains and sites for albumin and cell surface binding. The albumin and cell surface binding domains have been eliminated from Recombinant Protein G to reduce nonspecific binding and, therefore, can be used to separate IgG from crude samples. Optimal binding occurs at pH 5, although binding is also effective at pH 7.0 to 7.2.

Properties of Recombinant Protein G:
• Source: E. coli
• Molecular Weight: ∼21,600 (Apparent MW by SDS-PAGE: 32,000)
• Form: salt-free powder
• A280 of 0.1% solution: 1.0
• Isoelectric point (pI): 4.5

Because Protein G has greater affinity than Protein A for most mammalian IgGs, it may be used for the purification of mammalian IgGs that do not bind well to Protein A. Protein G binds with significantly greater capacity than Protein A to several IgG subclasses such as human IgG3, mouse IgG1 and rat IgG2a. However, Protein G does not bind to human IgM, IgD and IgA. Differences in binding characteristics between Protein A and Protein G may be explained by the differing compositions in the IgG-binding sites of each protein. The tertiary structures of these proteins are very similar although their amino acid compositions are significantly different.

There are inconsistencies in reported binding properties of IgG to Protein G. Variations in isolation and manufacturing methods for Protein G may affect IgG binding, partially because there are differing numbers of IgG-binding sites on various sources of Protein G. Binding studies have been performed using native Protein G and several different recombinant forms. Several assay methods have been used to determine relative affinity, including radiolabeling experiments and ELISA techniques. The differing affinity assays may explain some of the inconsistencies. In addition, there are significant binding differences when different buffers are used. Approximately 44% more IgG from rat serum bound to Protein G when Protein G Binding Buffer was used as compared with 20 mM Tris Buffer, pH 7.5.

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