Ubiquitination, a post-translational modification, regulates numerous cellular processes by attaching ubiquitin molecules to the target proteins, thereby altering their cellular levels and functions. A central aspect of ubiquitin-mediated signaling is the formation of different types of polyubiquitin (polyUb) chains, which can be either homotypic or heterotypic, generating a variety of cellular signals that activate distinct downstream pathways. Homotypic chains are linked via the same lysine on each molecule, whereas heterotypic chains are conjugated via multiple lysines. The synthesis of these diverse polyubiquitin chains is driven by interactions between substrate-conjugated ubiquitin molecules, ubiquitin-conjugating enzymes (E2s), and ubiquitin ligases (E3s). However, the molecular details of these interactions and how they govern the synthesis of different homotypic and heterotypic chains remain poorly understood. The E2 enzyme E2-25K preferentially extends K48-linked polyubiquitin chains on K63-linked template polyubiquitin chains, creating branched K48/K63 chains. In this study, we investigated the role of dynamic interactions between E2-25K and the K63-linked diubiquitin substrate to assess the molecular mechanism of branched-chain assembly. Our data reveal that E2-25K shows no preference for binding to the proximal or distal ubiquitin of a template chain. However, binding to the distal unit activates the complex; binding to the proximal unit does not. This study highlights the critical role of stereospecificity in enzyme/substrate interactions for branched ubiquitin chain synthesis and provides insights into the mechanisms of ubiquitin signaling.