ubiquitin

Ubiquitination is an ATP-dependent enzymatic cascade involving three enzyme classes: E1 (ubiquitin-activating enzyme) activates ubiquitin using ATP; E2 (ubiquitin-conjugating enzyme) carries activated ubiquitin; and E3 (ubiquitin ligase) transfers ubiquitin to the target protein's lysine residue, conferring substrate specificity. Polyubiquitination (typically via Lys48-linked chains) targets proteins for proteasomal degradation, while monoubiquitination or Lys63-linked chains serve non-degradative signaling roles (DNA repair, endocytosis, NF-κB activation).

Clinical relevance: - Cell cycle regulation: Cyclins are ubiquitinated by APC/C and SCF complexes to allow mitotic progression - Tumor suppressor regulation: MDM2 ubiquitinates p53, targeting it for degradation; loss of this regulation contributes to cancer - Von Hippel-Lindau (VHL) disease: VHL protein is an E3 ligase component that normally targets HIF-1α for degradation under normoxia; VHL mutations lead to HIF-1α accumulation, causing hemangioblastomas, renal cell carcinoma, and pheochromocytoma - Parkin (PARK2) mutations: Parkin is an E3 ligase; loss-of-function mutations cause autosomal recessive juvenile Parkinson disease due to accumulation of misfolded proteins - Angelman syndrome: caused by loss of UBE3A (an E3 ligase) expression from the maternal allele - Proteasome inhibitors (bortezomib, carfilzomib) are used in multiple myeloma treatment by blocking degradation of pro-apoptotic factors, inducing cancer cell death - Ubiquitin-like modifiers (SUMO, NEDD8) participate in related but distinct signaling pathways

Ubiquitin is highly conserved across eukaryotes, reflecting its fundamental role in cellular homeostasis.