Fig. 4. Primary kinetic plots from Limulus polyphemus arginine kinase (AK) WTrev. This AK is a typical 40-kDa monomer. Its kinetic parameters were determined to serve as a source of comparison for Ensis AK. Limulus AK displays significant synergy of substrate binding, which is characteristic of both arginine kinase and creatine kinase. The intersection of the six lines in each plot above the x-axis is indicative of this substrate binding synergy. To determine kinetic constants, the slopes and y-intercepts from these primary plots were replotted in secondary plots.