Scaling Proteome-Wide Reactions of Activity-Based Probes

Unified analysis of complex reactions of an activity-based probe with proteins in a proteome remains an unsolved challenge. We propose a power expression, rate = k^obs[Probe]^α, for scaling the progress of proteome-wide reactions and use the scaling factor (0 ≤ α ≤ 1) as an apparent, partial order with respect to the probe to measure the "enzyme-likeness" for a protein in reaction acceleration. Thus, α reports the intrinsic reactivity of the protein with the probe. When α = 0, the involved protein expedites the reaction to the maximal degree; when α = 1, the protein reacts with the probe via an unaccelerated, bimolecular reaction. The selectivity (β) of the probe reacting with two proteins is calculated as a ratio of conversion factors (k^obs values) for corresponding power equations. A combination of α and β provides a tiered system for quantitatively assessing the probe efficacy; an ideal probe exhibits high reactivity with its protein targets (low in α) and is highly selective (high in β) in forming the probe-protein adducts. The scaling analysis was demonstrated using proteome-wide reactions of HT-29 cell lysates with a model probe of threonine β-lactone.