The article discusses the role of E1 enzymes in activating ubiquitin-like proteins (UBLs) and their importance in downstream signaling pathways. E1 enzymes are responsible for activating UBLs through a process involving adenylation and thiol transfer, and they coordinate the use of UBLs in specific pathways by charging cognate E2 enzymes. The activation of UBLs is a central mechanism for modulating protein functions, and defects in UBL pathways are associated with various diseases, including cancer, neurodegenerative disorders, and muscle atrophy. UBLs are attached to other proteins or lipids through a multienzyme cascade involving E1, E2, and E3 enzymes. The E1 enzyme is at the apex of each UBL cascade and activates the UBL before directing it to downstream pathways. The ubiquitin system is the best understood UBL pathway, and the activation of ubiquitin involves the E1 enzyme binding MgATP and ubiquitin, and catalyzing ubiquitin C-terminal acyl-adenylation. The E1 enzyme then transfers the activated ubiquitin to E2 enzymes, which interact with the downstream ubiquitylation machinery to modify the target. The article also discusses the diversity and specificity of E1 enzymes, highlighting that different E1 enzymes are responsible for activating different UBLs. For example, UBA1 is responsible for activating ubiquitin, while NAE1-UBA3 is responsible for activating NEDD8. The article describes the catalytic mechanisms of canonical E1 enzymes, such as UBA1, and the structural insights into these enzymes. It also discusses the regulation of E1 enzymes in vivo and the role of E1 enzymes in various biological pathways, including autophagy and the interferon system. The article concludes by highlighting the importance of E1 enzymes in the UBL conjugating pathway and their potential as targets for therapeutic inhibitors. The development of specific inhibitors of E1 enzymes, including cell-permeable small-molecule inhibitors, is an important area of current research. The potential for the use of such inhibitors in therapy is high, given the links seen between components of UBL cascades and human disease.The article discusses the role of E1 enzymes in activating ubiquitin-like proteins (UBLs) and their importance in downstream signaling pathways. E1 enzymes are responsible for activating UBLs through a process involving adenylation and thiol transfer, and they coordinate the use of UBLs in specific pathways by charging cognate E2 enzymes. The activation of UBLs is a central mechanism for modulating protein functions, and defects in UBL pathways are associated with various diseases, including cancer, neurodegenerative disorders, and muscle atrophy. UBLs are attached to other proteins or lipids through a multienzyme cascade involving E1, E2, and E3 enzymes. The E1 enzyme is at the apex of each UBL cascade and activates the UBL before directing it to downstream pathways. The ubiquitin system is the best understood UBL pathway, and the activation of ubiquitin involves the E1 enzyme binding MgATP and ubiquitin, and catalyzing ubiquitin C-terminal acyl-adenylation. The E1 enzyme then transfers the activated ubiquitin to E2 enzymes, which interact with the downstream ubiquitylation machinery to modify the target. The article also discusses the diversity and specificity of E1 enzymes, highlighting that different E1 enzymes are responsible for activating different UBLs. For example, UBA1 is responsible for activating ubiquitin, while NAE1-UBA3 is responsible for activating NEDD8. The article describes the catalytic mechanisms of canonical E1 enzymes, such as UBA1, and the structural insights into these enzymes. It also discusses the regulation of E1 enzymes in vivo and the role of E1 enzymes in various biological pathways, including autophagy and the interferon system. The article concludes by highlighting the importance of E1 enzymes in the UBL conjugating pathway and their potential as targets for therapeutic inhibitors. The development of specific inhibitors of E1 enzymes, including cell-permeable small-molecule inhibitors, is an important area of current research. The potential for the use of such inhibitors in therapy is high, given the links seen between components of UBL cascades and human disease.