Structure-activity relationships between nonenzymatic or enzymatic kinetic parameters and the local electrophilicity ωk of the α-carbon. (a) Regarding the second-order rate constant knonc of the nonenzymatic reaction between GSH and probes. Data for other probes are unavailable due to extremely low reactivity. (b–d) Regarding the catalytic constant kcat of (b) GSTA1-1, (c) GSTM1-1, and (d) GSTP1-1, respectively, towards different probes. Linear fittings were based on mean values. Error bars represent SD.