Exam 4: Protein Translation & post-translation Flashcards Preview

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Flashcards in Exam 4: Protein Translation & post-translation Deck (58):
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5' UTR

untranslated region at 5' end of mRNA

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3' UTR

untranslated region at 3' end of mRNA

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open reading frame

area of mRNA translated into protein

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monocistronic mRNA

mRNA contains only one open reading frame

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polycistronic mRNA

mRNA contains several open reading frames

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tRNA

anneal to 3-base codons on mRNA at 3-base anticodon region
have an amino-acid linked at the acceptor terminus

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Protein synthesis occurs in

the cytosol, on ribosomes

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Ribosomes

have two subunits, composed of 1-3 RNA molecules (rRNA) and dozens of proteins

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Two types of ribosomes in humans

Cytoplasmic - synthesis of bulk of proteins
Mitochondrial - protein synthesis inside mitochondria

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Translation initiation

IF2a activated by binding to GTP, binds to methionine-tRNA to form ternary complex
Terneray complex binds to a small ribosomal subunit
An mRNA molecule binds to structure to form pre-initiation complex
Pre-initiation complex binds to large ribosomal subunit to form initiation complex
eIF2a-GTP is hydrolyzed and GDP-eIF2a is released

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eukaryotic initiation factor 2a (eIF2a)

activates initiation of protein translation by binding to GTP

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Ternary complex

GTP-eIF2a and methionine-tRNA

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Pre-initiation complex

ternary complex, small ribosomal subunit, and mRNA

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Initiation complex

small & large ribosomal subunit, mRNA, tRNA-methionine

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Elongation phase of translation starts after

the initiator methionine-tRNA binds to the P site of the ribosome

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elongation factor (EF-1)

required to add a second tRNA to A site of ribosome

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eukaryotic release factor (eRF)

pairs with stop codon
when attached GTP is hydrolyzed, peptide is released from P site and ribosome separates

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Streptomycin

antibiotic - binds to small subunit and inhibits initiation, causes mistranslation of codons

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Neomycin and gentamicin

antibiotic - bind to ribosomes and cause mistranslation of codons

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Tetracycline

antibiotic - blocks A site of ribosomes and prevents tRNA binding

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Chloramphenicol

antibiotic - prevents peptidyl bond formation in protein translation

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Ricin toxin

removes adenine bases from various positions of the rRNA in large subunit, prevents translation

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Diphtheria toxin

inactivates EF-2 by ADP-ribosylation, prevents translation

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EF-2 (elongation factor)

provides energy for movement of mRNA one codon further, opening A site of ribosome

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Methods for regulating translation

1. Preventing the recognition of a start codon - binding protein to 5' UTR of mRNA prevents recognition of start codon
2. Regulating activity of initiation factors - phosphorylation of eIF-2 in response to certain stimuli inactivates initiation factor and turns off translation

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Heat shock proteins (HSPs)

chaperone proteins that repair proteins damaged by heat and other stresses - help proteins fold correctly

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Proteins destined for export from the cell are synthesized in the

endoplasmic reticulum
signal sequence peptide emerges from ribosome & recognized by SRP, moving the ribosome complex to the ER - binds to docking protein that transfers ribosome to transmembrane channel - translocon
As translation continues, peptide is threaded into ER

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signal recognition particle (SRP)

recognizes signal sequence that peptide is being transported out of cell and binds to docking protein - transferring ribosome to translocon

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translocon

transmembrane channel that threads newly synthesized proteins from ribosome into ER

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Unfolded protein response

triggered by accumulation of unfolded proteins in ER - such as during starvation and cholesterol overload
Inhibition of global protein translation
Induce chaperone production - improve chances of proper folding
Considers apoptosis - if unfolded protein amount exceeds capacity for repair

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Glycosylation is important because

1. it changes the physical properties of the protein
2. carbohydrates on protein surface are recognition sites for trafficking, protein interactions, recognition, and immune response

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Glycosyltransferases

transfer sugar from an activated sugar nucleotide to an acceptor substrate
Specific for sugar donor, acceptor molecule, and type of bond formed

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N-linked Glycosylation

Starts in ER before folding done, continues as protein goes through Golgi apparatus
oligosaccharide molecule is added to amino group of an asparagine residue

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Modifications in Golgi yield what two types of N-glycosylated proteins?

High mannose type
Complex type (mannose and 5 other charbohydrates)

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O-linked Glycosylation

occurs only on fully folded proteins after it has reaced the Golgi aparatus
transfer N-acetyl-galactosamine to hydroxyl group of serine or threonine resideues on surface of protein

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O-linked glycosylation is important in

blood types - attaches blood group antigens on red blood cells
H-antigen discriminates between self and foreign particles in blood - has 2 alleles, A & B

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A allele

adds N-acetylgalactosamine to H antigen

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B allele

adds galactose to H antigen

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O allele

non-functional and adds nothing to H antigen

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A & B alleles

express both forms of H antigen on surface of blood cells

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Failure to hydroxylate proline

collagen disorders - Scurvy, Ehlers-Danlos syndrome, etc

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failure to convert cysteine to formylglycine causes

multiple sulfatase deficiency (MSD) - sulfated glycosaminoglycans accumulate in lysosome

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Timming at N-terminus of protein

proteolysis removes N-terminal amino acids & modifies new terminus - many proteins need a different start than methionine

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Addition of hydrophobic moieties to protein

link membrane proteins to long-chain, hydrophobic molecules to change their surface properties

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C-terminus additions

glycosylphosphatidyl-inositol (GPI) anchor to C-terminus to tether to external side of plasma membrane

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Targeting of proteins to lysosome by

phosphorylating mannose residues of high-mannose glycoproteins
Mannose-6-P on protein surface targets vesicles to lysosome

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Key features of proteins targeted for mitochondria

Unfolded - stabilized by chaperones
Synthesized with large N-terminal presequence - interacts with receptor in outer mitochondrial membrane (cleaved off by matrix proteases in mitochondria)

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TOMs & TIMs

translocases of outer and inner mitochondrial membrane - channel through which preprotein enters mitochondrial matrix

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Deafness-dystonia syndrome

mitochondrial disorder caused by mutation in a TIM component - impairs cellular energy production by preventing assembly of fully functional mitochondria

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Cystic fibrosis

caused by deletion of one codon from CFTR1 gene
Interferes with folding and glycosylation of protein
CFTR protein is moved into cytosol and degraded instead of being sent to plasma membrane

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I-cell disease

transfer of phosphate to mannose is impaired
lysosomal proteins do not reach their compartment & function is compromised - leads to accumulation of undegraded proteins in lysosomes
In fibroblast - dense bodies of nonfunctional lysosomes & content
In serum - lysosomal proteins that did not reach lysosomes

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Mechanisms for protein degradation in cell

Lysosome & proteasome

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Lysosome

nonspecific degradation of extracellular and intracellular proteins
can digest itself - autophagy
contains high concentrations of diverse hydrolytic enzymes

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Proteasomes

required for specific degradation of cytoplasmic proteins

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Ubiquitin

mark protein for proteasomal destruction - needs multiple units of ubiquitin on same protein (poly-ubiquination) to signal for destruction
Activated by E1 enzymes, conjucated to E2 enzymes, and ligated to targets by E3 proteins
Recycled - not degraded by proteasome with protein

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Enzyme E3

identifies proteins for ubiquination & transfers ubiquitin to protein
Specific isozymes for certain classes of substrates

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Protein lifespan determined by:

Conformation (correct folding, no hydrophobic domains on surface of protein)
N-terminal residue (arginines or lysines are less stable than proteins with methionine or serine at N-terminal)
Other sequence elements (PEST sequences shorten lifespan of protein)

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PEST sequences

proline-glutamine-serine-threonine sequence in protein
Shorten life of protein