Lecture 2: Mitochondria - Exam 5 Flashcards
Discuss Vacuolation (Paraptosis).
Paraptosis is an alternative cell death pathway characterized by vacuolation and damage to the endoplasmic reticulum and mitochondria.
-Paraptosis is a backup mode for cancer therapy.
-Paraptosis has a role in cancer cells, immune cells, other cells, and response to therapies.
-The biomarker of paraptosis is underdetermined.
-The interrelationship between paraptosis and other programmed cell deaths.
Give an overview of the Ubiquitin-proteasome pathway.
-Ubiquitin is first activated by the enzyme E1.
-Activated ubiquitin is then transferred to one of several different ubiquitin-conjugating enzymes (E2).
-A ubiquitin ligase (E3) then associated with both E2 and a substrate protein to direct the transfer of ubiquitin to a specific target.
Describe Ubiquitin Activating Enzyme E1.
- E1 binds ubiquitin (ATP hydrolyzed to AMP). The C-terminus of the ubiquitin molecule binds AMP (adenosine monophosphate). Thus, E1 binds to both ubiquitin and AMP.
- Catalytic cysteine residues attack the ubiquitin causing release of AMP and formation of an E1-ubiquitin thioester intermediate**.
- Reaction is repeated as another ubiquitin is adenylated the charged ubiquitin is transferred to an E2 forming another thioester bond.
-The human genome contains two genes that produce E1 enzymes capable of activating ubiquitin: UBA1 and UBA6**.
Ubiquitination requires three types of enzymes:
Ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), and ubiquitin ligases (E3s).
Ubiquitination requires three types of enzymes: Ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), and ubiquitin ligases (E3s). The process consists of three main steps:
- Activation by E1
- Conjugation by E2
- Ligation by E3… Ubiquitin is attached to the amino group of the side chain of a lysine residue.
The ubiquitin chain grows!
Describe the Ubiquitin Conjugating Enzyme E2.
-E2 ubiquitin-conjugating enzymes catalyze the transfer of ubiquitin from E1 to the active site cysteine of the E2 via a trans (thio)esterification reaction.
-Each E2 has a ubiquitin-conjugating catalytic (UBC) domain that has a Cys residue located in a shallow groove at its active site which will form a thioester bond with the ubiquitin molecule being transferred from the E1.
-Humans possess 35 different E2 enzymes. Highly conserved among eukaryotic cells.
Humans possess only 2 ____ enzymes.
E1
Describe Ubiquitin Ligating Enzyme E3.
- Transfer ubiquitin from E2 enzymes to substrates. Ubiquitin is attached to the amino group of the side chain of a lysine residue.
- Involved in final target selection and specificity of the reaction.
What are Degrons?
Any motif or structure on the substrate that targets proteins for degradation.
A degron is a portion of a protein that is important in regulation of protein degradation rates. Known degrons include short amino acid sequences, structural motifs and exposed amino acids (often lysine or arginine)
E3 enzymes possess one of two domains:
E3 HECT DOMAIN or the E3 RING DOMAIN
Describe the E3 HECT DOMAIN. Describe the E3 RING DOMAIN.
HECT DOMAIN: forms a thioester bond with ubiquitin prior to transferring it to the substrate.
RING DOMAIN: Does not form a thioester intermediate but promotes ubiquitin conjugation by bridging the interaction between E2 and substrate proteins.
Substrate selection depends on…?
Ubiquitin ligases (E3)!!
E3 ligases are critical to determine substrate specificity by the covalent attachment of ubiquitin to substrate proteins.
-Many are poorly characterized, particularly with respect to their target* proteins.
Targeting processes are regulated by?
Example?
Intracellular signals or signals from the environment.
Example of signal: Phosphorylation of a specific site on a protein that unmasks a normally hidden degradation signal (Degrons).
