Scientists have identified three conformational states in Src kinase: a dominant active state, one known inactive state, and a novel intermediate (intermediate) state located between the active and inactive states[1]. This intermediate state is short-lived and allows for the rapid release of ADP after ATP hydrolysis, providing the high catalytic cycle rate required for the processive phosphorylation of multisite substrates[2]. A Src mutant with a single amino acid change that decreased the intermediate state population and increased the active state population had slower ADP release and lower catalytic turnover compared to wild-type Src[1]. In this variant, the turnover approached the rate of dissociation from the p130Cas substrate, which in in vitro experiments led to the accumulation of partially phosphorylated p130Cas - a typical feature of the distributional (non-processive) mechanism[1]. The authors demonstrated that the intermediate state is essential for the processive phosphorylation not only of Src but also of the related kinases Hck and Lck[2]. Stabilization of the active state at the expense of the intermediate state impaired SFK-dependent functions: Src-driven cell migration and Lck-mediated T-cell activation[1][5]. The study used nuclear magnetic resonance (NMR), biochemical and cellular assays to map the conformational ensemble and assess the functional consequences of the conformational changes[1]. These results show that short-lived conformational intermediates enable rapid ADP/ATP turnover and are crucial for the ability of SFK to perform rapid multisite phosphorylation[2].