Ubiquitin itself is not a catalyst but acts as a substrate or a modifier in catalytic processes. In the context of catalysis, ubiquitin modifies target proteins to regulate their activity, stability, and interactions. This modification can alter enzymatic activities and hence, influence various catalytic pathways within the cell. Enzymes that facilitate ubiquitination, such as E3 ligases, play a catalytic role by transferring ubiquitin to the substrate protein.