Signal transduction mechanisms limitations

MAPK is activated by a unique dual function kinase called MEK, which is an acronym for MAPK and ERK-kinase (Crews et al, 1992 Zheng and Guan, 1993). MEK has also been named MAPK-kinase (or MAPKK) and is unusual because it phosphorylates MAPK on both a threonine and a tyrosine amino acid. MEK consists of a family of at least three protein kinase isoforms, each of which shows differential reactivity toward the different members of the MAPK family. Each MEK isoform is activated through phosphorylation mechanisms. When fully dephosphorylated, MEK is inactive. When phosphorylated by either MEK-kinase (MEKK) or the proto-oncogene raf, the phosphotransferase activity of MEK is "turned on." Although both raf and MEKK can activate MEK, differential activation of either of these kinases can lead to the activation of different downstream signal transduction pathways. Therefore, the roles of raf and MEKK in the cell are not limited to MEK and, therefore, MAPK activation. 

Because e-noses are composed of an array of chemical sensors, the performance of the sensor array is limited by the properties (sensitivity, selectivity, reproducibility, reversibility, etc.) of the individual sensors. Therefore, this chapter will focus on the individual chemical sensing approaches and their transduction mechanisms, rather than the aggregate performance of the array coupled with signal processing techniques. 

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