Comprehensive Study Notes on Protein Degradation: Proteases, Lysosomes, UPS, and ER Processes

WHY CELLS DEGRADE PROTEINS

Protein degradation is a fundamental cellular process required for maintenance, regulation, and survival. The primary reasons cells degrade proteins include:

• Recycling Amino Acids: Cells degrade short peptides to recover amino acids for new protein synthesis.

• Biological Activation and Localization: Newly synthesized proteins often undergo partial degradation (e.g., removal of N-terminal or C-terminal signal peptides) to:

  • Acquire their specific biological function.

  • Be correctly localized to their designated cellular compartment.

• Quality Control: Eliminating damaged or misfolded proteins. Misfolded proteins are dangerous because they can form non-physiological interactions with other proteins, leading to cellular dysfunction.

• Regulation of Protein Lifespan: Every protein has a consistent half-life, which can range from a few minutes (short-lived) to several days (long-lived). Degradation regulates the duration of protein signals and activities.

PROTEASES: CLASSIFICATION AND GENERAL FUNCTIONS

Proteases are the enzymes responsible for protein degradation.

• Genomic Prevalence: There are approximately $588$ proteases in humans, representing about $3\%$ of the genome.

• Definition: Proteases are enzymes that catalyze the irreversible hydrolysis of peptide bonds, resulting in either total or partial protein degradation.

• Biological Importance: They are essential for cellular health and viability, participating in:

  • Core Genetic Processes: DNA replication and transcription.

  • Cellular Behavior: Proliferation, differentiation, migration, and apoptosis.

  • Tissue Maintenance: Remodeling of the extracellular matrix (ECM).

  • Signaling: Processing of hormones and other bioactive peptides.

• Physiological and Pathological Roles: Proteases are involved in blood coagulation, digestion, morphogenesis during development, and inflammation. Dysregulation is linked to tumor development and metastasis.

CATEGORIZATION BY ACTION AND CATALYTIC TYPE

Proteases are categorized based on where they cut the polypeptide chain and their chemical mechanism.

• Action Categories:

  • Endopeptidases: Break peptide bonds located within the interior of the protein molecule.

  • Exopeptidases: Remove amino acids from the ends of the molecule. They can remove up to 3 amino acids from the N-terminal (amino-peptidases) or C-terminal (carboxy-peptidases).

• Catalytic Classes (Mammals):

  1. Metalloproteases ($192$): Use a metal ion in catalysis.

  2. Serine Peptidases ($184$): Use a serine residue as a nucleophile.

  3. Cysteine Peptidases ($164$): Use a cysteine residue (thiol group).

  4. Threonine Peptidases ($27$): Use an N-terminal threonine.

  5. Aspartic Peptidases ($21$): Use aspartate residues to activate water.

  6. Glutamate Peptidases: Use glutamic acid.

  7. Asparagine Lyases: Use asparagine for self-cleavage (no water required).

GENERAL MECHANISMS OF PROTEASE ACTION

The catalytic mechanism varies depending on whether the protease uses a covalent or non-covalent strategy.

• Covalent Catalysis (Serine, Cysteine, Threonine):

  • The nucleophile is part of an amino acid side chain (the -OH of Ser/Thr or the -SH of Cys).

  • Histidine ($\text{His}$) residues usually act as general bases to activate the nucleophile.

• Non-Covalent Catalysis (Metalloproteases and Aspartic Proteases):

  • The nucleophile is an activated molecule of $H_2O$.

  • In aspartic proteases, $Asp$ or $Glu$ residues activate the water. In metalloproteases, a Zinc ($Zn^{2+}+$) ion serves as the acid/base activator.

• Universal Three-Step Process:

  1. Nucleophilic Attack: The nucleophile (O- from Ser, S- from Cys, or O- from water) attacks the carbonyl carbon of the peptide bond ($C$-terminal of the first amino acid).

  2. Tetrahedral Intermediate: This attack forms an unstable tetrahedral intermediate at the carbonyl carbon.

  3. Proton Transfer and Bond Rupture: A proton is ceded to the nitrogen of the peptide bond ($N$-terminal of the second amino acid), breaking the bond. In covalent catalysis, a second nucleophilic attack by a water molecule is required to break the covalent bond between the protease and the substrate, releasing the second fragment and regenerating the enzyme.

DETAILED PROTEASE CLASSES AND CLINICAL EXAMPLES

• Metalloproteases: These use a divalent metal ion, typically $Zn^{2+}$, coordinated by three amino acids (often $His$, $Glu$, $Asp$, or $Lys$) and one water molecule.

  • Matrix Metalloproteinases (MMPs): A group of $23$ enzymes that degrade collagen and remodel the ECM. Associated with arthritis, cancer, and fibrosis.

  • ADAMs (A Disintegrin And Metalloproteinases): Transmembrane proteins with cytoplasmic tails for signaling. Their ectodomains modify growth factors and receptors.

  • ADAMTS: Secreted proteins that remodel connective tissue (degrading procollagen, aggrecan, or versican).

• Aspartic Peptidases: Function optimally at low pH. Use two aspartic acid residues to activate $H_2O$. Often secreted as zymogens (e.g., pepsinogen). Example: HIV Protease ($Asp25$).

• Serine Peptidases: Use a catalytic triad (usually $Asp-His-Ser$). $Asp$ orients $His$, allowing $His$ to act as a base to deprotonate $Ser$. Form an acyl-enzyme intermediate. Example: Chymotrypsin.

• Cysteine Peptidases: Use a thiolate ($-S^{-}$) nucleophile. Examples: Papain and human Cathepsins (B, C, F, H, K, L).

• Clinical Inhibitors:

  • ACE Inhibitors: Target Zinc metalloproteinases (Peptidyl dipeptidase A) to manage blood pressure by preventing vasoconstriction.

  • HIV Protease Inhibitors: Block the viral aspartyl protease.

  • Bortezomib: A threonine protease inhibitor used to block the proteasome.

LYSOSOMAL DEGRADATION PATHWAY

Lysosomes are the "digestive system" of the cell, handling long-lived proteins and materials from outside the cell.

• Structure and Environment: Membrane-bound organelles containing ~$60$ different hydrolytic enzymes (acid hydrolases). They maintain an acidic pH ($\approx 5$) via a proton pump, which is necessary for enzyme activity and protects the cytoplasm (pH $\approx 7.2$) from accidental digestion if a lysosome ruptures.

• Lysosomal Storage Diseases (LSD): Result from mutations in lysosomal enzymes. Example: Gaucher Disease, caused by a defect in glucosylceramide (glucocerebroside) degradation. Symptoms include splenomegaly ($95\%$), hepatomegaly ($87\%$), anemia ($40\%$), and bone pain ($27\%$). It is treated with Enzyme Replacement Therapy (ERT).

• Entry Routes:

  • Endocytosis: Extracellular molecules enter via vesicles, move to early endosomes, then late endosomes, and finally lysosomes. Acid hydrolases are tagged with Mannose 6-Phosphate in the Golgi for delivery to these endosomes.

  • Phagocytosis: Large particles or bacteria are engulfed by specialized cells (macrophages, neutrophils) into phagosomes, which fuse with lysosomes to form phagolysosomes.

  • Autophagy: Recycling the cell's own components. A portion of cytoplasm or an organelle (e.g., mitochondria) is enclosed in an autophagosome, which then fuses with a lysosome.

THE UBIQUITIN-PROTEASOME SYSTEM (UPS)

The UPS is the primary route for the degradation of short-lived, regulatory, and cytosolic proteins.

• Ubiquitin (Ub): A highly conserved $76$-amino acid protein ($8.6\,kDa$). It marks proteins for degradation via an isopeptide bond between its $Gly76$ C-terminal and a Lysine ($Lys$) on the target protein.

• The Ubiquitin Cycle:

  1. E1 (Activation): Ubiquitin is activated by E1 in a process requiring $ATP \rightarrow AMP + PP_i$ (energy cost equivalent to $2 \times ATP$).

  2. E2 (Conjugation): Activated Ub is transferred to an E2 enzyme.

  3. E3 (Ligation): An E3 ligase catalyzes the transfer of Ub to the substrate. E3s determine substrate specificity.

  4. DUBs (Deubiquitinating enzymes): Remove Ub from substrates, making the process reversible.

• Polyubiquitination Codes:

  • $Lys48$ (K48): Canonical signal for degradation by the proteasome.

  • $Lys63$ (K63): Non-canonical signal for DNA damage repair and signaling.

  • $Met1$: Linear chains used in signaling.

  • $Lys11$: Involved in cell cycle and ERAD (ER-associated degradation).

• The Proteasome: A large complex where the chain of Ub is recognized by the "lid." The protein is unfolded (using $ATP$), translocated into the $20S$ core, and hydrolyzed by threonine proteases into fragments of $7-9$ amino acids. Ubiquitin is released and recycled.

PROTEIN DEGRADATION IN THE ENDOPLASMIC RETICULUM (ER)

The ER has a dedicated system to handle misfolded proteins, as the folding process is often slow and prone to error.

• ERAD (ER-Associated Degradation): Misfolded proteins are identified by chaperones (Calnexin, Calreticulin), exported to the cytosol, polyubiquitinated, and degraded by the proteasome. EDEM1 recognizes severely misfolded proteins by removing mannose residues.

• UPR (Unfolded Protein Response): Triggered by an accumulation of misfolded proteins. It aims to expand the ER, increase chaperone production, and reduce new protein entry. If the stress is not resolved, apoptosis is triggered. The UPR activates three receptors:

  1. IRE1: Excises an intron from the pre-mRNA of the transcription factor XBP1. Spliced XBP1 stimulates UPR gene transcription.

  2. ATF6: A transcription factor sequestered in the ER; it moves to the Golgi to be cleaved and activated upon stress.

  3. PERK: A kinase that phosphorylates translation factor eIF2, which inhibits general translation to reduce the protein load entering the ER.