Topic 5 - Synthesis and Fate of Protiens Flashcards Preview

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What are post-translational modifications?

Biochemical modifications to a protein that occur after translation of the polypeptide, e.g. glycosylation or phosphorylation.


What advantages might there be in the local translation of mRNA?

It is more efficient to move a single mRNA molecule to where the protein that it encodes is needed, and to translate it there, than to transport many proteins from a more distant site. The proteins might interfere with cell function if they were located in the wrong place in a cell.


What are miR response elements (MREs)?

Segments of nucleotides in mRNA that act as targets for microRNAs.


What is an isoacceptor?

A tRNA molecule that can bind to more than one specific codon.


What are aminoacyl tRNA synthases?

Enzymes that are responsible for tRNA charging, i.e. the joining of the appropriate tRNA molecule to its corresponding amino acid. There is one such enzyme for each of the 20 amino acids incorporated into proteins.


What is the wobble hypothesis?

An explanation for the fact that that several different codons may encode the same amino acid, because of a lack of requirement for precise base-pairing (wobble) in the third nucleotide of the codon.


What is an initiator tRNA (tRNAi)?

A specific tRNA molecule used only for the initiation of translation. Initiator tRNAs in prokaryotes carry the modified amino acid formylmethionine, whereas in eukaryotes they carry methionine. The initiator tRNAs interact with a specific initiation factor before binding to the ribosome.


What is elongation factor (EF)?

The term used to describe the ancillary protein that assists with the elongation phase of translation through binding to a charged tRNA. The elongation factor responds to the appropriate pairing of the rRNA anticodon with the mRNA codon by releasing the tRNA amino acid into the ribosome peptide linkage site.


What is the Kozak sequence?

A short sequence of ribonucleotides within a eukaryotic mRNA chain, lying upstream from the AUG initiation codon, which assists with translation by directing initiation at the AUG site by the scanning initiation complex.


The amino acid is bound to the tRNA via its carboxyl group. What free group is exposed on the amino acid bound to the tRNA, which will form the peptide bond?

The amino group (–NH2) is available to form the peptide bond with the terminal carboxyl group on the growing peptide chain.


What is the evolutionary implication of the observation that protein synthesis is catalysed by rRNA?

It suggests that during the early evolution of life, catalysis mediated by RNA preceded catalysis mediated by proteins.


What are termination factors (TFs)?

Proteins that bind to actively translating ribosomes at stop codons. These proteins have one surface that specifically recognises stop codons in the mRNA and a region that interacts with the large ribosomal subunit. The tertiary shape of termination factors mimics those of the elongation factor–tRNA complex.


What is non-stop decay?

The process whereby eukaryotic mRNAs with no stop codons are degraded in the cell. The stalled mRNA-ribosome complex is recognised and the mRNA targeted for degradation.


What is no-go decay?

The process where ribosomes are stalled on an mRNA, usually due to protein binding or secondary structure in the mRNA.


What is co-translational folding?

The term used to describe the folding of the newly synthesised polypeptide chain as it is being synthesised on the ribosome, i.e. as it emerges from the tunnel on the ribosome.


Where are secreted proteins synthesised within a cell?

They are synthesised on ribosomes located on the endoplasmic reticulum.


What is a nuclear localisation signal (NLS)?

A short sequence of amino acid residues within the N-terminal region of a polypeptide chain that is recognised by importin A and targets the polypeptide for transport into the nucleus.


What large biomolecules move through the nuclear pore complexes?

Messenger RNAs.


What is co-translational localisation?

The term used to describe the first stage in the localisation of a newly synthesised polypeptide to cellular subcompartments or for export from the cell. Proteins destined for certain compartments are actively synthesised across the ER membrane and are processed for downstream destinations.


What is glycosylation?

The addition of carbohydrate units to a polypeptide (glycoprotein) or lipid (glycolipid).


What are lectins?

The general term used to describe a class of adhesion molecules that bind glycosyl (i.e. carbohydrate) chains (and therefore the proteins they are attached to). Calreticulin is an example of a lectin.


What is the glycocalyx?

A tangled layer on the outside of the cell formed by the carbohydrate groups that are attached to many of the proteins and lipids that comprise the membrane. The glycocalyx has an important role in recognition and adhesion.


What is axonal transport?

The movement of organelles and proteins along the microtubular network in the axons of neuronal cells. (T5P2)


What is vesicular transport?

The mechanism by which molecules are ferried between the membrane-bound compartments of a cell, or between these compartments and the plasma membrane.


What are transport vesicles?

Vesicles that pinch off from one compartment and diffuse, or are more often actively transported, to another compartment, where they fuse and discharge their cargo.


Give one example of a membrane molecule, that you have already encountered, that can facilitate transport of proteins from the cytosol across a membrane.

Signal recognition factor (ER-SRF), which recognises the signal sequence on membrane and secreted proteins, interacts with a transport channel for transfer of the translating polypeptide across the ER membrane (Figure 3.1). You may also have identified the TOM–TIM complex on mitochondria (Figure 2.1) or components of the nuclear pore complex (Figure 2.2).


What are signal patches?

Sequences of amino acids within a polypeptide chain that are recognised by (for example) chaperone proteins, which promote transport to the correct cellular compartment.


What are sorting receptors?

Membrane proteins of the endosome that help sort proteins for their correct cellular compartment.


What is the basolateral domain?

The area of the cell membrane facing the extracellular matrix and adjacent cells, in contradistinction to the apical domain.


What are secretory vesicles?

Vesicles that release hormones, cytokines and small signalling molecules from cells.


What is constitutive secretion?

One of the two basic types of exocytosis carried out by all cells and which transfers molecules from the Golgi network to the outer surface of the cell.


What is regulated secretion?

Secretion that occurs in response to specific conditions, signals or biochemical triggers, and is the process underlying the release of cytokines, hormones, neurotransmitters and other small signalling molecules, such as histamine.


What is endocytosis?

The process by which cells internalise molecules from the outside. It includes pinocytosis, the uptake of small soluble molecules in vesicles, and phagocytosis, the internalisation of large insoluble particles.


What is phagocytosis?

The process by which bacteria, cell debris and other larger items are internalised by cells.


What are caveolae?

Flask-shaped invaginations of the cell membrane, and the vesicles that are derived from them, which are surrounded by the protein caveolin. Receptors located in caveolar membranes bind their specific ligand, and endocytosis then follows. Caveolae also function to transfer molecules from one side of a cell to the other or between the cell surface and the large endosomal compartment called the caveosome.


What is the ubiquitin-mediated pathway?

The protein degradation pathway by which proteins that are to be degraded are tagged with chains of ubiquitin by ubiquitin ligases.


What is ubiquitin?

A small, highly conserved protein that is attached to proteins through the action of ubiquitin ligases and targets them for degradation in the proteasome.


What is a proteasome?

A large multisubunit protease. A component of the ubiquitin-mediated pathway for protein degradation. The proteasome is a barrel structure into which proteins are placed for degradation. The process yields short peptide chains, which are further degraded by cellular proteases.


Explain macroautophagy

Process by which defunct macromolecules are taken up directly from the cytosol by lysosomes (by invagination of the lysosomal membrane) and broken down.


What is chaperone-mediated autophagy?

A mechanism by which defunct proteins are directed to lysosomes (for degradation) by binding to chaperones.


What are lysosomal storage diseases?

A group of conditions in which a lysosomal enzyme is defective, resulting in the progressive accumulation of undegraded biomolecules. Many of these conditions produce nerve cell death.