Structure and Stability Changes of Human IgG1 Fc as a Consequence of Methionine Oxidation

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Abstract
The Fc region has two highly conserved methionine residues, Met 33 (CH2 domain) and Met 209 (CH3 domain), which are important for the Fc’s structure and biological function. To understand the effect of methionine oxidation on the structure and stability of the human IgG1 Fc expressed in Escherichia coli, we have characterized the fully oxidized Fc using biophysical (DSC, CD, and NMR) and bioanalytical (SEC and RP-HPLC−MS) methods. Methionine oxidation resulted in a detectable secondary and tertiary structural alteration measured by circular dichroism. This is further supported by the NMR data. The HSQC spectral changes indicate the structures of both CH2 and CH3 domains are affected by methionine oxidation. The melting temperature (Tm) of the CH2 domain of the human IgG1 Fc was significantly reduced upon methionine oxidation, while the melting temperature of the CH3 domain was only affected slightly. The change in the CH2 domain Tm depended on the extent of oxidation of both Met 33 and Met 209. This was confirmed by DSC analysis of methionine-oxidized samples of two site specific methionine mutants. When incubated at 45 °C, the oxidized Fc exhibited an increased aggregation rate. In addition, the oxidized Fc displayed an increased deamidation (at pH 7.4) rate at the Asn 67 and Asn 96 sites, both located on the CH2 domain, while the deamidation rates of the other residues were not affected. The methionine oxidation resulted in changes in the structure and stability of the Fc, which are primarily localized to the CH2 domain. These changes can impact the Fc’s physical and covalent stability and potentially its biological functions; therefore, it is critical to monitor and control methionine oxidation during manufacturing and storage of protein therapeutics.