Protein biosensors serve as an analytical device combining a biological component with a physiochemical detector. Through protein engineering we are able to utilize pores to be used as stochastic sensors, for single molecule detection. The applications of biosensors range from fundamental research, clinical diagnosis, and even advances in homeland security. PCR (Polymerase chain reaction) and ELISA (Enzyme- linked immunosorbent assay) are current sensitive detection methods. However, these methods are time consuming and require laborious effort, where results are provided hours or days later. For this reason alternate approaches are mounting in demands that are rapid in detection time, highly sensitive and reliable.
Through stochastic sensing, we utilize the passage of ionic current through a protein pore containing engineered recognition sites, allowing for monitoring of analytes present. We propose the use of monomeric protein Outer membrane protein G (OmpG) will allow us to tailor and fine tune properties of this pore in detection of analytes.
This has features that can be customized as a sensor component, using protein engineering we can develop this protein to be a successful biosensor. Utilizing the flexibility of an extracellular loop, we will append a ligand to help detect our target analyte.
Structural Features of OmpG
OmpG is a 14-stranded beta-barrel and in contrast to most porins, appears to function as a monomer.[1] The central pore of OmpG is wider than other E. coli porins and it is speculated that it may form a non-specific channel for the passive transport of larger oligosaccharides.[1] OmpG adopts two conformations: open and closed. At neutral pH the porin displays an open conformation. However at a more acidic pH the closed conformation is adopted. This closed conformation is a result of OmpG's flexible extracellular loop 6, which folds across the channel blocking the pore opening. The rearrangement of loop 6 appears to be triggered by a pair of histidine residues, which repel each another at acidic pH, resulting in the breakage of neighbouring H-bonds and a lengthening of loop 6 from 10 to 17 residues[2,3].
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The loop environment of OmpG carries a net negative charge. This highly negative character may be implicated in the gating behavior of OmpG. If we can gain a deeper understanding of OmpG gating, we could could use this knowledge to tune it for specific detection of target analytes.
Current OmpG Projects in the Chen Lab
Engineering loop 6 to detect large protein analytes
Investigating the effect of loop charge on the gating behavior of OmpG
Additional Resources
Current applications using nanopores for protein detection [2]
For additional information, see: Nanobiotechnology Review [3]
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References
[1] Chen M, Li Q-H and Bayley, H (2008) "Orientation of the monomeric porin OmpG in planar lipid bilayers." ChemBioChem 9(18):3029-36
[2] Chen M, Khalid S, Sansom M and Bayley H (2008) "Outer membrane protein G: engineering a quiet pore for biosensing." Proc Natl Acad Sci U S A 105: 6272-6277
[3] Damaghi M, Bippes C, et al. (2010) "pH-dependent interactions guide the folding and gate the transmembrane pore of the beta-barrel membrane protein OmpG." J Mol Biol 397(4):878-82.
