16 Questions
Which organelle contains digestive enzymes such as lipases, nucleases, and proteases?
Lysosomes
What is the tissue half-life of Lactic Acid Dehydrogenase in the heart?
1.6 days
Which proteins have a short half-life and include regulatory proteins and enzymes that catalyze committed steps?
Regulatory proteins
In which compartment does protein degradation occur through the ubiquitin-dependent proteasome system?
Cytosol
What is responsible for the breakdown of most short-lived proteins in human cells?
Proteosomal degradation
How are proteins selected for degradation?
Tagged by covalent attachment of ubiquitin and then degraded by proteosome
Which enzyme catalyzes the transfer of ubiquitin from E2 to a Lysine amino group of the protein destined for degradation?
E3: ubiquitin ligase
What is the function of the 19S regulatory particle in the proteasome?
Recognition and binding of polyubiquitinated proteins
What determines the protein function by monoubiquitinating some proteins?
Regulation of ubiquitination
What is the minimum number of ubiquitin molecules critical for efficient degradation of a protein?
Four
Which enzyme family can reverse the degradation effects by cleaving the bond between ubiquitin and its substrate protein?
UCH (Ubiquitin Carboxyl-terminal Hydrolases)
Explain the process of protein degradation through the ubiquitin-proteasome system, including the role of ubiquitin and the proteasome components.
The process involves tagging proteins destined for degradation with ubiquitin, which is a three-step process involving E1, E2, and E3 enzymes. The ubiquitinated proteins are then recognized and bound by the 19S regulatory particle of the 26S proteasome. The protein is unfolded, translocated into the central core, and hydrolyzed into smaller peptides within the proteasome.
Describe the role of deubiquitinating enzymes (DUBs) in the ubiquitin-proteasome system.
DUBs can reverse the degradation effects by cleaving the peptide or isopeptide bond between ubiquitin and its substrate protein. They are important for regulating the ubiquitin-proteasome system and can affect protein degradation.
What are the major biological events controlled by the proteasomal pathway?
The proteasomal pathway controls cell cycle progression, transcriptional control, cell development and differentiation, and signal transduction via the breakdown of specific proteins such as p53, Rb, cyclins, CDK inhibitors, and transcription factors.
Explain the role of the 19S regulatory particle in the 26S proteasome.
The 19S regulatory particle is important for the recognition and binding of polyubiquitinated proteins, removal of ubiquitin, unfolding the protein substrate, and translocation into the central core of the proteasome. It plays a crucial role in the degradation of proteins.
What is the function of ubiquitin in the process of protein degradation?
Ubiquitin tags proteins destined for degradation by covalently attaching to them, marking them for recognition and degradation by the proteasome. This process is essential for maintaining cellular homeostasis by regulating protein levels and functions.
Study Notes
Intracellular Protein Degradation
- Intracellular protein degradation is essential for maintaining a constant supply of amino acids and preventing the accumulation of abnormal proteins within the cell.
Protein Turnover
- Protein turnover is not constant and varies greatly among individual proteins, reflecting their role within the cell.
- Half-lives of proteins range from: • 100-200 hours for "normally" proteins • Short-lived proteins (regulatory proteins, enzymes, and transcription factors) • Long-lived proteins (structural proteins and crystallins)
- Tissue distribution affects protein degradation rates, as exemplified by Lactic Acid Dehydrogenase: • Heart: 1.6 days • Muscle: 31 days • Liver: 16 days
Regulation of Protein Degradation
- Protein degradation is a regulated process, influenced by factors such as nutritional state.
- Example: Acetyl CoA carboxylase has a half-life of: • 48 hours when fed • 18 hours when fasted
Compartments of Protein Degradation
- Protein degradation occurs in two main compartments:
• Lysosomes (10-20%): acidic compartment containing digestive enzymes (lipases, nucleases, proteases), responsible for degrading:
- Extracellular proteins
- Some intracellular proteins • Cytosol (80-90%): ubiquitin-dependent proteasome system, responsible for degrading most intracellular proteins.
Test your knowledge of intracellular protein degradation with this overview quiz. Learn about the process, its importance for the cell, and the variation in protein half-life.
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