Lecture 3 Flashcards
What is the functional unit of the cell?
Proteins
What are proteins?
Polymers of amino acids.
How are amino acids distinguished from one another?
Their side chains (R-groups)
Describe the kinds of R-groups
Non-polar (R is just C and H), polar uncharged (contains OH or SH), polar charged acidic (COO-), and polar charged basic (NH3+)
Name the unique characteristics of Cysteine and Tyrosine
Cysteine can form disulfide bonds and Tyrosine can be post-translationally modified by phosphorylation
What are peptide bonds?
They covalently link amino acid backbones
What dictates the folding of amino acid chains?
The primary structure of the protein
What drives the folding of proteins?
Hydrophobic associations- they want to move away from water.
What kinds of bonds occur between R-groups of amino acids?
Covalent and ionic bonds, hydrophobic associations, and hydrogen bonds.
What kind of side chain would form a covalent bond?
Cysteine-Cysteine (rare)
What kind of side chain would form ionic bonds?
A positive to a negative side chain
What kind of side chain would form hydrophobic associations?
Non-polar-Non-polar
What kind of side chain would form hydrogen bonds?
Polar uncharged-Polar uncharged
Name the factors that affect protein folding patterns and function
pH, temperature, ionic concentration and composition, and other cofactors
What is the function of a chaperone protein?
They assist in the folding of proteins or target misfolded proteins for degredation
What are the secondary functions of chaperone proteins?
They assemble multi-unit proteins, transport unfolded polypeptides to specific organelles, and target misfolded proteins for degredation
Where does transcription occur?
In the nucleus
Where does translation occur?
It starts on free ribosomes in the cytoplasm
What dictates the primary sequence of amino acids?
DNA
Describe proteolytic activation
When an inactive protein must be cleaved in order for it to be activated
Why are inactive proteins stored to be used instead of creating new active ones as we need them?
Translation is too slow
What is the inactive form of insulin?
Proinsulin
Where are sorting signals generally found?
At the N-terminus
What are the 3 mechanisms for moving proteins amongst compartments?
Nuclear import, protein translocators, and transport vesicles
Describe nuclear import
Soluble folded proteins are made in the cytoplasm and translated through NPC; requires NLS to be recognized by importins
Describe protein translocators
They transport unfolded proteins across the bilayer of the ER or mitochondria
Describe transport vesicles
They transport proteins made at the rough endoplasmic reticulum to the golgi which then sends them back or to lysosomes or the plasma membrane for export.
What does cotranslational mean?
Proteins destined for anywhere in the endomembrane system begin at the rough endoplasmic reticulum
What does post-translational mean?
The translation completes in the cytoplasm
Where is the signal sequence (SS) found on a protein destined for the endomembrane system?
The SS is found near the N terminus
What is the function of a signal sequence?
It directs the ribosome/mRNA/translating protein to dock at the rough endoplasmic reticulum
What is a signal recognition particle (SRP)?
It reads the signal sequence (SS) and binds N terminal SS to stop translation
Where does SRP dock?
To the translocon on the cytoplasmic surface of the ER at the SRP receptor
What is translocation?
It is ATP-dependant movement of the peptide into the lumen of the ER
What are the two mechanisms of membrane insertion?
There are proteins with an N terminal SS coupled with a stop transfer sequence and proteins with only a start transfer sequence anywhere in the protein
How are multipass proteins made?
The proteins have alternating start and stop transfer sequences
What is post-translational import?
When a protein is localized to its destination afer translation has completed.
For which organelles are proteins imported post-translationally?
Mitochondria, peroxisome, chloroplast, and the nucleus
What is the localization signal for mitochondria called?
The transit sequence
What is the default compartment in the mitochondria if only a transit sequence is present?
The matrix
What do TIM and TOM stand for?
Translocase of the inner membrane and translocase of the outer membrane
What is Hsp70?
Heat shock protein 70- a chaperone that prevents fully translated proteins from folding
Where is the N terminal transit sequence bound?
A receptor at TOM
What drives protein translocation in the mitochondria?
As part of cellular respiration, there is a voltage difference across the mitochondrial inner membrane that provides energy; ATP hydrolysis also helps
