Pathway: Synthesis of active ubiquitin: roles of E1 and E2 enzymes
Reactions in pathway: Synthesis of active ubiquitin: roles of E1 and E2 enzymes :
Synthesis of active ubiquitin: roles of E1 and E2 enzymes
Ubiquitin monomers are processed from larger precursors and then activated by formation of a thiol ester bond between ubiquitin and a cysteine residue of an E1 activating enzyme (UBA1 or UBA6, Jin et al. 2007). The ubiquitin is then transferred to the active site cysteine residue of an E2 conjugating enzyme (reviewed in van Wijk and Timmers 2010, Kleiger and Mayor 2014, Stewart et al. 2016). Precursor proteins containing multiple ubiquitin monomers (polyubiquitins) are produced from the UBB and UBC genes. Precursors containing a single ubiquitin fused to a ribosomal protein are produced from the UBA52 and RPS27A genes. The proteases OTULIN and USP5 are very active in polyubiquitin processing, whereas the proteases UCHL3, USP7, and USP9X cleave the ubiquitin-ribosomal protein precursors yielding ubiquitin monomers (Grou et al. 2015). Other enzymes may also process ubiquitin precursors. A resultant ubiquitin monomer is activated by adenylation of its C-terminal glycine followed by conjugation of the C-terminus to a cysteine residue of the E1 enzymes UBA1 or UBA6 via a thiol ester bond (Jin et al. 2007, inferred from rabbit homologues in Haas et al. 1982, Hershko et al. 1983). The ubiquitin is then transferred from the E1 enzyme to a cysteine residue of one of several E2 enzymes (reviewed in van Wijk and Timmers 2010, Stewart et al. 2016).
After translation, many newly formed proteins undergo further covalent modifications that alter their functional properties. Modifications associated with protein localization include the attachment of oligosaccharide moieties to membrane-bound and secreted proteins (N-linked and O-linked glycosylation), the attachment of lipid (RAB geranylgeranylation) or glycolipid moieties (GPI-anchored proteins) that anchor proteins to cellular membranes, and the vitamin K-dependent attachment of carboxyl groups to glutamate residues. Modifications associated with functions of specific proteins include gamma carboxylation of clotting factors, hypusine formation on eukaryotic translation initiation factor 5A, conversion of a cysteine residue to formylglycine (arylsulfatase activation), methylation of lysine and arginine residues on non-histone proteins (protein methylation), protein phosphorylation by secretory pathway kinases, and carboxyterminal modifications of tubulin involving the addition of polyglutamate chains.
Protein ubiquitination and deubiquitination play a major role in regulating protein stability and, together with SUMOylation and neddylation, can modulate protein function as well.
Metabolism of proteins, as annotated here, covers the full life cycle of a protein from its synthesis to its posttranslational modification and degradation, at various levels of specificity. Protein synthesis is accomplished through the process of Translation of an mRNA sequence into a polypeptide chain. Protein folding is achieved through the function of molecular chaperones which recognize and associate with proteins in their non-native state and facilitate their folding by stabilizing the conformation of productive folding intermediates (Young et al. 2004). Following translation, many newly formed proteins undergo Post-translational protein modification, essentially irreversible covalent modifications critical for their mature locations and functions (Knorre et al. 2009), including gamma carboxylation, synthesis of GPI-anchored proteins, asparagine N-linked glycosylation, O-glycosylation, SUMOylation, ubiquitination, deubiquitination, RAB geranylgeranylation, methylation, carboxyterminal post-translational modifications, neddylation, and phosphorylation. Peptide hormones are synthesized as parts of larger precursor proteins whose cleavage in the secretory system (endoplasmic reticulum, Golgi apparatus, secretory granules) is annotated in Peptide hormone metabolism. After secretion, peptide hormones are modified and degraded by extracellular proteases (Chertow, 1981 PMID:6117463). Protein repair enables the reversal of damage to some amino acid side chains caused by reactive oxygen species. Pulmonary surfactants are lipids and proteins that are secreted by the alveolar cells of the lung that decrease surface tension at the air/liquid interface within the alveoli to maintain the stability of pulmonary tissue (Agassandian and Mallampalli 2013). Nuclear regulation, transport, metabolism, reutilization, and degradation of surfactant are described in the Surfactant metabolism pathway. Amyloid fiber formation, the accumulation of mostly extracellular deposits of fibrillar proteins, is associated with tissue damage observed in numerous diseases including late phase heart failure (cardiomyopathy) and neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's.