Xa Protein Folding Turnover Degradation 1

Key Topics: Protein Folding, Protein Turnover, Protein Degradation, Clinical Significance

Definition: Protein turnover refers to the continuous cycle of synthesis and degradation of proteins in the cell, allowing the cellular concentration of each type of protein to be maintained.
Proteins have a half-life, defined as the time taken for the concentration of a specific protein to decrease to half its initial value.

Metabolic Flexibility:
- Protein turnover allows rapid adjustments in concentrations of regulatory enzymes, peptide hormones, and receptor molecules in response to physiological needs.
- Example: Cyclin proteins regulate cell cycle progression through their timely synthesis and degradation.
Developmental Processes:
- Protein degradation is required for normal cellular and embryonic development.
Amino Acid Supply:
- Provides amino acids for synthesizing new proteins when nutrients are limited.
Elimination of Damaged Proteins:
- Roughly one-third of synthesized proteins are rapidly degraded due to errors in synthesis or folding.
- Misfolded proteins accumulate from stress, requiring degradation through the Unfolded Protein Response (UPR) pathway.

Radioactive Labeling:
- Proteins are labeled with a radioactive amino acid (e.g., ^{35}S-Met) and sampled over time to measure degradation.
Cycloheximide Treatment:
- Cycloheximide inhibits protein synthesis, allowing analysis through Western blotting to determine protein depletion over time.

Insulin: Biological half-life of about 5 minutes.
- Required for blood sugar regulation, synthesized and degraded promptly.
Collagen & Histones: Very long half-life of about 72 days.
Regulatory enzymes have short half-lives, typically in minutes.

Proteins are ultimately broken down into amino acids by proteases that hydrolyze peptide bonds.

Endopeptidases: Hydrolyze peptide bonds within proteins.
Exopeptidases: Cleave residues from the N- or C-terminus of peptides.
Further classified by their catalytic centers:
- Serine Proteases: Utilize serine at the active site.
- Cysteine Proteases: Utilize cysteine at the active site (Example: Calpain).
- Aspartic Proteases
- Metalloproteases
Calpain: Ca$^{2+}$-dependent and intracellular cysteine protease, tightly regulated.

Involves covalent modification called ubiquitination, which tags proteins for degradation.
Ubiquitin: A highly conserved protein (76 amino acids, 8.5 kDa).

E1 (Ubiquitin-activating enzyme): Activates ubiquitin in an ATP-dependent manner.
E2 (Ubiquitin-conjugating enzyme): Receives ubiquitin from E1.
E3 (Ubiquitin ligase): Transfers ubiquitin to a specific lysine on target proteins via thioester-to-amide transition.
The protein is then polyubiquitinated, resulting in chains of 4-50 ubiquitin molecules.

N-terminal residues: Basic or bulky hydrophobic amino acids mark short-lived proteins.
Peptide motifs: Extended sequences (PEST sequences) lead to rapid degradation, often within 2 hours.

Autophagy: A "self-eating" cellular process for degrading long-lived proteins via lysosomal enzymes.
Provides recycling mechanisms and is vital during stress or nutrient deprivation.

Chaperone-mediated Autophagy: Transports specific proteins into lysosomes.
Microautophagy: Engulfs small cytoplasmic portions directly.
Macroautophagy: Involves larger bulk degradation and is the primary autophagic mechanism in eukaryotic cells.

Figures illustrating protein turnover, ubiquitination, proteasome processes, and autophagic mechanisms.

Protein Quality Control (PQC) systems manage folding, biogenesis, and turnover to maintain cellular homeostasis.
Misfolded proteins can lead to diseases and are directed to degradation or refolding pathways.

Specific gene mutations and associated proteins lead to various diseases through aggregation and misfolding.
- Alzheimer Disease: Amyloid-beta and Tau protein aggregation.
- Huntington Disease: Aggregation of huntingtin protein.
- Parkinson Disease: Aggregation of alpha-synuclein protein.

Amyloid accumulation primarily occurs in tissues where secretion levels are high, which can lead to organ dysfunction, as seen in Type 2 Diabetes.