Transthiolation is a fundamental biological reaction and is utilized by many enzymes involved in the conjugation of ubiquitin and ubiquitin-like proteins. However, tools that enable selective profiling of this activity are lacking. Transthiolation requires cysteine-cysteine juxtaposition; therefore a method that enables irreversible "stapling" of proximal thiols would facilitate the development of novel probes that could be used to profile this activity. Herein, we characterize biocompatible chemistry that enables sequential functionalization of cysteines within proteins at a single atomic center. We use our method to develop a new class of activity-based probe that profiles transthiolation activity of human E1 activating enzymes. We demonstrate use in vitro and in situ and compatibility with competitive activity-based protein profiling. We also use the probe to gain insight into the determinants of transthiolation between E2 and a RING-in-between-RING (RBR) E3 ligase. Furthermore, we anticipate that this method of thiol functionalization will have broad utility by enabling simple redox-stable cross-linking of proximal cysteines in general.