H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 30
Текст из файла (страница 30)
This type of post-translational alteration is sometimes called processing. The most common form is enzymaticcleavage of a backbone peptide bond by proteases, resultingin the removal of residues from the C- or N-terminus of a713.2 • Folding, Modification, and Degradation of Proteins(a)NH2UbAMP+ PPi+ ATPCO E2E1UbE11CCytosolictarget proteinOUb23E3E2OE1 = Ubiquitin-activating enzymeNHE2 = Ubiquitin-conjugating enzymeCUbE3 = Ubiquitin ligaseUb = UbiquitinSteps 1, 2, 3(n times)(b)UbUbUbnCapATP4ADPCoreUbiquitin Marks Cytosolic Proteinsfor Degradation in ProteasomesIn addition to chemical modifications and processing, the activity of a cellular protein depends on the amount present,which reflects the balance between its rate of synthesis andrate of degradation in the cell.
The numerous ways that cellsregulate protein synthesis are discussed in later chapters. Inthis section, we examine protein degradation, focusing onthe major pathways for degrading cytosolic proteins.The life span of intracellular proteins varies from as shortas a few minutes for mitotic cyclins, which help regulate passage through mitosis, to as long as the age of an organism forproteins in the lens of the eye. Eukaryotic cells have severalintracellular proteolytic pathways for degrading misfolded ordenatured proteins, normal proteins whose concentrationmust be decreased, and extracellular proteins taken up by thecell. One major intracellular pathway is degradation by enzymes within lysosomes, membrane-limited organelles whoseacidic interior is filled with hydrolytic enzymes. Lysosomaldegradation is directed primarily toward extracellular proteins taken up by the cell and aged or defective organelles ofthe cell (see Figure 5-20).Distinct from the lysosomal pathway are cytosolic mechanisms for degrading proteins.
Chief among these mechanismsis a pathway that includes the chemical modification of a lysine side chain by the addition of ubiquitin, a 76-residuepolypeptide, followed by degradation of the ubiquitin-taggedprotein by a specialized proteolytic machine. Ubiquitinationis a three-step process (Figure 3-13a):■ Activation of ubiquitin-activating enzyme (E1) by theaddition of a ubitiquin molecule, a reaction that requiresATPTransfer of this ubiquitin molecule to a cysteine residuein ubiquitin-conjugating enzyme (E2)■ProteasomeUbCapUb5UbPeptides▲ FIGURE 3-13 Ubiquitin-mediated proteolyticpathway. (a) Enzyme E1 is activated by attachment of aubiquitin (Ub) molecule (step 1 ) and then transfers this Ubmolecule to E2 (step 2 ).
Ubiquitin ligase (E3) transfers thebound Ub molecule on E2 to the side-chain —NH2 of a lysineresidue in a target protein (step 3 ). Additional Ub moleculesare added to the target protein by repeating steps 1 – 3 ,forming a polyubiquitin chain that directs the tagged proteinto a proteasome (step 4 ). Within this large complex, theprotein is cleaved into numerous small peptide fragments(step 5 ).
(b) Computer-generated image reveals that aproteasome has a cylindrical structure with a cap at each endof a core region. Proteolysis of ubiquitin-tagged proteinsoccurs along the inner wall of the core. [Part (b) fromW. Baumeister et al., 1998, Cell 92:357; courtesy of W.
Baumeister.]Formation of a peptide bond between the ubiquitinmolecule bound to E2 and a lysine residue in the targetprotein, a reaction catalyzed by ubiquitin ligase (E3)■This process is repeated many times, with each subsequentubiquitin molecule being added to the preceding one. The resulting polyubiquitin chain is recognized by a proteasome,another of the cell’s molecular machines (Figure 3-13b). Thenumerous proteasomes dispersed throughout the cell cytosolproteolytically cleave ubiquitin-tagged proteins in an ATPdependent process that yields short (7- to 8-residue) peptidesand intact ubiquitin molecules.MEDIA CONNECTIONSE2E1Overview Animation: Life Cycle of a Proteinpolypeptide chain. Proteolytic cleavage is a common mechanism for activating enzymes that function in blood coagulation, digestion, and programmed cell death (Chapter 22).Proteolysis also generates active peptide hormones, such asEGF and insulin, from larger precursor polypeptides.An unusual and rare type of processing, termed proteinself-splicing, takes place in bacteria and some eukaryotes.This process is analogous to editing film: an internal segmentof a polypeptide is removed and the ends of the polypeptideare rejoined.
Unlike proteolytic processing, protein selfsplicing is an autocatalytic process, which proceeds by itselfwithout the participation of enzymes. The excised peptideappears to eliminate itself from the protein by a mechanismsimilar to that used in the processing of some RNA molecules (Chapter 12). In vertebrate cells, the processing of someproteins includes self-cleavage, but the subsequent ligationstep is absent. One such protein is Hedgehog, a membranebound signaling molecule that is critical to a number of developmental processes (Chapter 15).72CHAPTER 3 • Protein Structure and FunctionCellular proteins degraded by the ubiquitin-mediatedpathway fall into one of two general categories: (1) native cytosolic proteins whose life spans are tightly controlled and(2) proteins that become misfolded in the course of their synthesis in the endoplasmic reticulum (ER). Both contain sequences recognized by the ubiquitinating enzyme complex.The cyclins, for example, are cytosolic proteins whoseamounts are tightly controlled throughout the cell cycle.These proteins contain the internal sequence Arg-X-X-LeuGly-X-Ile-Gly-Asp/Asn (X can be any amino acid), which isrecognized by specific ubiquitinating enzyme complexes.
Ata specific time in the cell cycle, each cyclin is phosphorylatedby a cyclin kinase. This phosphorylation is thought to causea conformational change that exposes the recognition sequence to the ubiquitinating enzymes, leading to degradationof the tagged cyclin (Chapter 21). Similarly, the misfolding ofproteins in the endoplasmic reticulum exposes hydrophobicsequences normally buried within the folded protein.
Suchproteins are transported to the cytosol, where ubiquitinating enzymes recognize the exposed hydrophobic sequences.The immune system also makes use of the ubiquitinmediated pathway in the response to altered self-cells, particularly virus-infected cells. Viral proteins within the cytosolof infected cells are ubiquitinated and then degraded in proteasomes specially designed for this role. The resulting antigenic peptides are transported to the endoplasmic reticulum,where they bind to class I major histocompatibility complex(MHC) molecules within the ER membrane.
Subsequently,the peptide-MHC complexes move to the cell membranewhere the antigenic peptides can be recognized by cytotoxicT lymphocytes, which mediate the destruction of the infectedcells.Alternatively Folded Proteins Are Implicated inSlowly Developing DiseasesAs noted earlier, each protein species normally foldsinto a single, energetically favorable conformationthat is specified by its amino acid sequence. Recentevidence suggests, however, that a protein may fold into an alternative three-dimensional structure as the result of mutations, inappropriate post-translational modification, or otheras-yet-unidentified reasons. Such “misfolding” not only leadsto a loss of the normal function of the protein but also marksit for proteolytic degradation.
The subsequent accumulationof proteolytic fragments contributes to certain degenerativediseases characterized by the presence of insoluble proteinplaques in various organs, including the liver and brain. ❚Some neurodegenerative diseases, including Alzheimer’sdisease and Parkinson’s disease in humans and transmissiblespongiform encephalopathy (“mad cow” disease) in cows(b)(a)Digestive Proteases Degrade Dietary ProteinsThe major extracellular pathway for protein degradation is thesystem of digestive proteases that breaks down ingested proteins into peptides and amino acids in the intestinal tract.Three classes of proteases function in digestion. Endoproteasesattack selected peptide bonds within a polypeptide chain.
Theprincipal endoproteases are pepsin, which preferentiallycleaves the backbone adjacent to phenylalanine and leucineresidues, and trypsin and chymotrypsin, which cleave thebackbone adjacent to basic and aromatic residues. Exopeptidases sequentially remove residues from the N-terminus(aminopeptidases) or C-terminus (carboxypeptidases) of aprotein. Peptidases split oligopeptides containing as many asabout 20 amino acids into di- and tripeptides and individualamino acids. These small molecules are then transportedacross the intestinal lining into the bloodstream.To protect a cell from degrading itself, endoproteases andcarboxypeptidases are synthesized and secreted as inactiveforms (zymogens): pepsin by chief cells in the lining of thestomach; the others by pancreatic cells.
Proteolytic cleavageof the zymogens within the gastic or intestinal lumen yieldsthe active enzymes. Intestinal epithelial cells produceaminopeptidases and the di- and tripeptidases.20 m100 nm▲ EXPERIMENTAL FIGURE 3-14 Alzheimer’s disease ischaracterized by the formation of insoluble plaquescomposed of amyloid protein.
(a) At low resolution, an amyloidplaque in the brain of an Alzheimer’s patient appears as a tangleof filaments. (b) The regular structure of filaments from plaquesis revealed in the atomic force microscope. Proteolysis of thenaturally occurring amyloid precursor protein yields a shortfragment, called -amyloid protein, that for unknown reasonschanges from an -helical to a -sheet conformation. Thisalternative structure aggregates into the highly stable filaments(amyloid) found in plaques.
Similar pathologic changes in otherproteins cause other degenerative diseases. [Courtesy of K. Kosik.]3.3 • Enzymes and the Chemical Work of Cellsand sheep, are marked by the formation of tangled filamentous plaques in a deteriorating brain (Figure 3-14). The amyloid filaments composing these structures derive fromabundant natural proteins such as amyloid precursor protein, which is embedded in the plasma membrane, Tau, amicrotubule-binding protein, and prion protein, an “infectious” protein whose inheritance follows Mendelian genetics.Influenced by unknown causes, these helix–containing proteins or their proteolytic fragments fold into alternative sheet–containing structures that polymerize into very stablefilaments.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
