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Anomalous behaviour of a protein during SDS/PAGE corrected by chemical modification of carboxylic groups
Abstract:
The 29,000-Mr Actinomadura R39 beta-lactamase exhibited a remarkably low electrophoretic mobility on SDS/PAGE, yielding an Mr value almost twice that computed from the corresponding gene sequence. We showed that chemical modification of the carboxylic groups of glutamic acid and aspartic acid residues restored a normal electrophoretic mobility and that the anomalous behaviour of that protein on SDS/PAGE was due to its very large negative charge at neutral pH. We also compared the behaviour of the same enzyme on gel filtration in the presence of SDS with those of other class A beta-lactamases (Mr approx. 30,000). These experiments suggested that the very low electrophoretic mobility of the Actinomadura R39 beta-lactamase upon SDS/PAGE was more probably due to a low degree of SDS binding rather than to an unusual shape of the SDS-protein complex.
Insights
The Actinomadura R39 beta-lactamase shows unusual low mobility on SDS-PAGE due to its high negative charge. Chemical modification corrected this, indicating charge, not shape, explains the anomaly.
Area of Science:
- Biochemistry
- Molecular Biology
- Enzymology
Background:
- Beta-lactamase enzymes are crucial in antibiotic resistance.
- The Actinomadura R39 beta-lactamase presents an unusual electrophoretic mobility.
- Standard molecular weight estimation via SDS-PAGE is complicated by this anomaly.
Purpose of the Study:
- To investigate the cause of the anomalous low electrophoretic mobility of Actinomadura R39 beta-lactamase on SDS-PAGE.
- To determine if the protein's charge or shape contributes to its behavior in SDS-PAGE.
- To compare its behavior with other class A beta-lactamases.
Main Methods:
- Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was used to assess electrophoretic mobility.
- Chemical modification of carboxylic acid groups (glutamic acid, aspartic acid) was performed.
- Gel filtration chromatography in the presence of SDS was employed for comparison.
Main Results:
- Actinomadura R39 beta-lactamase exhibited an apparent molecular weight nearly double its calculated value.
- Chemical modification of acidic residues normalized the electrophoretic mobility.
- The enzyme possesses a high negative charge at neutral pH, explaining the anomalous SDS-PAGE behavior.
- Comparison with other beta-lactamases suggested low SDS binding, not an unusual complex shape, as the primary cause.
Conclusions:
- The anomalous electrophoretic mobility of Actinomadura R39 beta-lactamase is primarily attributed to its high net negative charge.
- This charge significantly impacts SDS binding, leading to underestimated molecular weights in SDS-PAGE.
- Understanding these properties is vital for accurate protein characterization and comparative studies of beta-lactamases.