30 - Antibiotics and Toxins Flashcards
What is a common target for natural toxins?
The ribosome, because of its critical function.
How did toxins/antibiotics evolve?
As natural weapons used by organisms to kill or outcompete other organisms
What are three uses humans have for antibiotics and toxins?
- To combat pathogens
- To conduct research
- To kill other humans
What is the only common feature between all antibiotics or toxins?
That they can impair some biochemical activity
True or false? Antibiotics and toxins usually have very precise activity and some biological specificity (eg. prokaryote, type of cell etc.).
True
How does puromycin inhibit translocation in translation?
It is an aminoacyl-tRNA analog. Both these have a reactive amino group that acts as a nucleophile when appropriately positioned in the A site for attack on the ester linkage of peptidyl-tRNA in the P site. The puromycin reaction is a dead end reaction leading to chain termination.
How does the ricin toxin kill organisms?
It cleaves the purine (depurination) from an A residue of the 28S rRNA in a universally conserved region essential for function.
Interaction with elongation factor eEF2 is abolished and translocation cannot occur
What is the most common type of post-translational modification of proteins? What enzymes do this?
Proteolytic cleavage (eg. procollagen to collagen)
Aminopeptidases or carboxypeptidases remove only one or a few amino or carboxy terminal residues (eg. initial Met or fMet is often removed)
The formation of S-S disulfide bonds in protein is an example of a _____
Post-translational modification of protein
List specific post-translational modifications of certain amino acid side chains (7)
- Hydroxylation of Pro and Lys
- Attachment of sugars to Asn, Ser and Thr (glycosylation)
- Phosphorylation
- Acetylation
- Methylation
- Addition of prosthetic groups (eg. heme to globin)
- Attachment of lipids (ef. farnesyl to Cys residues)
What type of proteins are synthesized in free polysomes?
Soluble proteins that remain inside the cell
What type of proteins are synthesized in membrane-bound polysomes?
Proteins secreted or directed to various subcellular compartments (organelles)
What is the signal hypothesis about ribosomes and the ER?
Signal for ribosomal attachment is provided by a hydrophobic sequence of amino acids at the NH2 terminal end of the newly synthesized polypeptide chain
True or false? Some ribosomes are always attached to the ER and some are always free.
False. All ribosomes are free unless a protein contains a signal (ER signal peptide) for attachment to the ER. Once done translating the ribosome dissociates from the ER.
The mRNA, however may stay permanently bound to the ER as part of a polyribosome.
What is an amino-terminal signal sequence?
A sequence that is near the N end of a protein and normally indicates where this protein will go
What is a signal recognition particle (SRP)?
It binds to the signal sequence and ribosome, immediately halts translation and binds to a receptor protein located in the ER.
- Small cytoplasmic RNA species
- Composed of Small cytoplasmic RNA species
What are the 5 steps of co-translational targeting of secretory proteins to the ER?
- Signal sequence emerges from ribosome, it is recognized and bound by signal recognition particle (SRP)
- SRP escorts complex to ER membrane, SRP binds to SRP receptor
- SRP released, ribosome binds to membrane translocation complex of 3 proteins (Sec61, ribosome receptor), signal sequence inserted into membrane channel
- Translation resumes, growing polypeptide chain translocated across membrane
- Cleavage of signal sequence by signal peptidase releases polypeptide into ER lumen
What happens to mature glycoproteins?
They are sorted and packaged for extracellular secretion or for transport to other intracellular locations
What do carbohydrate units do for glycoproteins? (2)
- Help glycoproteins fold correctly in ER
- Final modifications direct them to their ultimate destinations (eg. C terminal KDEL sequence have the retained in ER, or else they go to Golgi)
What are the steps of protein secretion by bacteria?
- The nascent polypeptide complexes with SecB, which prevents complete folding during transport to the membrane
- At the membrane, an ATPase (SecA) derives translocation of the pro-protein through the membrane with the aid of membrane proteins SecE and SecY, which form a membrane pore
- The leader sequence is then cleaved off the secreted protein by a membrane peptidase
What type of transport do proteins destined for the nucleus take?
Gated transport
Transmembrane transport is used to transfer proteins to ____? (4)
- Peroxisomes
- Mitochondria
- Endoplasmic reticulum
- Plastids
Vesicular transport is used to transfer proteins to ____? (5)
- Golgi (from ER)
- Lysosome (from golgi or endosome)
- Endosome (from cell surface)
- Cell surface (from golgi or secretory vesicles)
- Secretory vesicles (from golgi)
What recognizes the mitochondrial outer membrane receptor (TOM) complex? What does this do?
N-terminal leader sequence
Leads to transport of the protein through both receptor (TIM and TOM) into the matrix, where the matrix targeting signal is cleaved off by a matrix protease.
How do proteins get to the mitochondrial intermembrane space?
The TOM complex is recognized by the N-terminal leader sequence and the protein is shuttled through both TIM and TOM receptors into the mitochondrial matrix.
In the matrix, the matrix targeting signal is cleaved off by a matrix protease. Cleavage may expose a new membrane targeting signal that directs the protein back to the inner membrane, the intermembrane space or to the outer membrane.This second targeting signal is then removed by the action of a specific processing peptidase.
