Building a mathematical model of a biological system requires input of experimental data for each networked component, ultimately generating a model that can be used to test scientific hypotheses. A fundamental requirement in the computation of these systems is that the total amount of each component can be specified precisely. An added level of complexity occurs because a vast number of protein posttranslational modifications modulate protein function. Each of these modified forms therefore needs to be considered as a separate system component, and must therefore be quantified individually. In this chapter, we describe how designer QconCAT proteins can be used to determine the absolute amounts of both the polypeptide components and their covalently modified derivatives in both yeast and mammalian extracts derived from living cell populations.
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