E1 enzymes activate ubiquitin or related proteins and pass them to E2 enzymes. A recent structure and associated biochemical studies (Huang et al., 2005) show how an E1 binds its cognate E2 and indicates that large, conformational changes will be an integral component of the E1 reaction cycle.
VanDemark, A. P., & Hill, C. P. (2005, February 18). E1 on the move. Molecular Cell. Cell Press. https://doi.org/10.1016/j.molcel.2005.02.004