E1 (ubiquitin-activating enzyme)

E1, or ubiquitin-activating enzyme, is a protein that kickstarts the process of marking other proteins for degradation or modification. It uses energy from ATP (a type of cellular fuel) to attach a small protein called ubiquitin to itself. This activation step is essential because it prepares ubiquitin for transfer to target proteins through other enzymes, ultimately directing those proteins for recycling or specific functions within the cell. Essentially, E1 acts as the first step in a carefully regulated system that helps maintain cellular health by controlling protein quality and turnover.