L41-42: Translation and Protein Processing I-II Flashcards Preview

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Flashcards in L41-42: Translation and Protein Processing I-II Deck (38)
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1
Q

Describe and contrast the composition of eukaryotic and prokaryotic ribosomes

A

1.) Eukaryotic - small subunit = 40 S - large subunit = 60 S - assembled size = 80 S * Mitochondrial: small = 30-35S, large = 40-45S, total = 55S 2.) Prokaryotic - small subunit = 30 S - large subunit = 50 S - assembled size = 70 S

2
Q

Name ribosome sites

A
  • E: exit site - P: peptidyl site - A: acceptor site
3
Q

Describe the progression of ribosome assembly

A

1.) GTP binds eIF2a 2.) GTP:eIF2a becomes bound to Met-tRNA to form ternary complex 3.) 40S:eIF3 binds ternary complex (with eIF1 and eIF1alpha) 4.) mRNA now binds small subunit and pre-initiation complex is formed (with aid of eIF-4a, eIF-4b, eIF-4f, eIF-5 and PAB) 5.) eIF-5b:GTP are added to this complex displacing hydrolyzed GDP:eIF2a and 60 S subunit is recruited and positioned with met-tRNA in P site. Elongation can now ensue

4
Q

Describe elongation and termination of translation

A

1.) EF-1-GTP charges tRNA molecule (EF1-GDP = product). AA-tRNA moves into A site 2.) Peptide bond formation occurs 3.) EF-2-GTP hydrolysis allows ribosomal complex to move one codon down with mRNA-peptidyl-complex now occupying the P site and the A site is empty. Uncharged tRNA leaves through E site 4.) Ribosome is now ready to repeat the cycle 5.) Once the stop codon is moved into the A site, eRF bound to GTP is hydrolyzed and the ribosomal complex dissociates

5
Q

Names of 70S ribosome inhibitors

A
  • Streptomycin - Neomycin - Gentamicin - Tetracycline - Chloramphenicol
6
Q

Action of streptomycin

A
  • 70 S ribosome inhibitor - Specifically binds to small subunit (30 S) and inhibits initiation and causes mistranslation of codons
7
Q

Action of neomycin

A
  • 70 S ribosome inhibitor - Specifically causes mistranslation of codons
8
Q

Action of gentamicin

A
  • 70 S ribosome inhibitor - Specifically causes mistranslation of codons
9
Q

Action of tetracycline

A
  • 70 S ribosome inhibitor - Specifically blocks A site and prevents tRNA binding
10
Q

Action of chloramphenicol

A
  • 70 S ribosome inhibitor - Specifically prevents peptidyl bond formation
11
Q

Action of ricin

A
  • potent ribosome inactivating protein (RIP) found in castor beans - it removes adenine bases from rRNA
12
Q

Action of diphtheria toxin

A
  • protein produced by C. diphtheriae that inactivates EF-2 by ADP ribosylation, preventing elongation
13
Q

Explain the regulation of translation

A
  • Points of regulation are at 1.) recognition of start codon and 2.) activity of initiation factors 1.) Recognition of start codon: bind of regulatory protein in 5’ UTR can mask start codon 2.) eIF-2a can be inactivated by phosphorylation
14
Q

Role of chaperones. Example

A
  • Proteins emerging from ribosome need to fold correctly - Folding is aided by chaperones – example = Hsp 90. Hsp90 binds ATP and misfolded proteins, loosens up protein and gives it another chance to fold correctly - These are important for survival of stress – heat shock
15
Q

Charcot Marie Tooth Disease

A
  • Congenital chaperone defects cause protein folding disorder
16
Q

Explain the synthesis of exported proteins. Where does this occur?

A
  • Synthesis of exported proteins begins at ER - Protein emerging from ribosome has signal peptide sequence - Signal recognition particle binds to signal peptide (stalls translation), binding ribosome to docking protein and positioning exiting protein sequence within translocon to ER lumen - Translation resumes, protein sequence is fed into ER lumen and signal peptidase inside lumen cleaves signal peptide - Protein is modified in ER and Golgi apparatus
17
Q

Describe the unfolded protein response

A
  • Accumulation of unfolded proteins in ER due to physiological stressors triggers unfolded protein response, which is general inhibition of translation, specific induction of HSP and / or apoptosis
18
Q

Function of protein glycosylation

A
  • Protein glycosylation confers: - 1.) physical changes: solubility, structure and bulk - 2.) generation of individual surface signatures
19
Q

How are glycosyltransferases specific?

A

1.) specific for activated sugar 2.) specific for acceptor 3.) specific for linkage formed

20
Q

Types of glycosylation. Describe

A
  • N-linked: starts in ER before protein folding is complete, adds sugars to Asn residues in protein in predictable fashion, modification of this can occur in Golgi - O-linked: starts in Golgi after protein folding is complete, adds sugars to serine or threonine residues, but not in predictable fashion
21
Q

Describe N-linked glycosylation mechanism

A
  • Dolichol phosphate in ER membrane acts as site for oligosaccharide in ER - Glycosyltransferase adds two GlcNAcs to dolichol - Glycosyltransferase adds five mannose - Dolichol phosphate linked to above CHOs reorientates from cytoplasm into ER lumen - 4 more mannoses are added onto oligosaccharide using dolicholphosphomannose - 3 glucoses are added to mannose forming universal oligosaccharide containing 14 sugars - Highly specific modification of universal oligosaccharide occurs in Golgi apparatus by addition or removal of CHOs, yielding high mannose type or complex type (sialic acid, fucose, N-acetyl glucosamine, N-acetyl-galactosamine, galactose)
22
Q

Disorders of glycosylation

A
  • CDG = congenital disorders of glycosylation - CDG-I: defective synthesis of lipid-linked oligosaccharide precursor (12 variants) - CDG-II: defective trimming of oligosaccharide chain (6 variants)
23
Q

What is CDG-I?

A
  • Defective synthesis of lipid-linked oligosaccharide precursors – 12 variants
24
Q

What is CDG-II?

A
  • Defective trimming of oligosaccharide chains – 6 variants
25
Q

Where are O-glycosylated proteins seen?

A
  • Proteoglycans of the ECM - H-antigen on surface of RBCs (predictable)
26
Q

RBC surface antigens. Core? Sugar residues for O, A and B antigen?

A
  • Core: Serine – Gal – GlcNAc – Gal X – Fucose - O: Serine – Gal – GlcNAc – Gal X – Fucose - A: Core – Gal X attached to GalNAc as well as Fucose - B: Core – Gal X attached to Gal as well as Fucose
27
Q

Examples of post-translational modifications

A
  • Glycosylations - Hydroxylation of proline - Acetylation of lysine - Cysteine to formylglycine - N-terminal trimming (removal of methionine) - Addition of hydrophobic moieties - Addition of GPI anchor to C-terminus
28
Q

Give examples for the post-translational modification of amino acids

A
  • Hydroxylation of proline - Acetylation of lysine - Cysteine to formylglycine
29
Q

Explain the four ways by which hydrophobic molecules are added to proteins. What is the function of this?

A

1.) N-terminal myristoylation 2.) Palmitoylation at cysteine 3.) Prenylation at cysteine close to C-terminus 4.) Addition of GPI anchor to c-terminus - Function: anchor proteins to PM for hanging structure into cytoplasm or into ECF

30
Q

How are proteins directed to lysosomes

A
  • Proteins directed to lysosome require phosphomannose
31
Q

Describe protein import into mitochondria and insertion of proteins into mitochondrial membranes

A
  • Mitochondrial pre-sequence is present on polypeptide sequence - Hsp 70 chaperones prevent premature folding in cytoplasm - TOM/TIM complex spanning outer to inner MM are present and feed polypeptide into mitochondrial matrix - Presequence is cleaved by matrix proteases. Hsp 60 proteins associate with sequence inside the mitochondrial matrix - Other signal sequences mediate their insertion into the mitochondrial membranes and what direction they hang
32
Q

Why are Hsp70 proteins necessary for mitochondrial proteins?

A
  • Hsp70 proteins associate with mitochondrial polypeptide sequences and prevent folding within the cytoplasm. - Folding will prevent movement of these proteins into the mitochondrial matrix
33
Q

What is cystic fibrosis? What is the defect?

A
  • CF is most often caused by deletion prevents correct glycosylation of CFTR1 protein. As a result, it isn’t moved to the cell surface and is degraded in the cytosol
34
Q

Name two protein sorting disorders

A
  • Cystic fibrosis - I-cell disease
35
Q

What is I-cell disease? Describe the defect

A
  • Surface mannose residues are not phosphorylated and therefore lysosomal proteins don’t reach the correct compartment and appear in serum - Result: lysosomal degradation of proteins and CHOs is impaired, inclusion bodies are seen in lysosomes making them appear dense
36
Q

Compare and contrast protein degradation in lysosomes and proteasomes

A

1.) Lysosomes - Lysosomes contain acid hydrolases for all types of molecules - Functions in autophagy and in the endocytic pathways where clathrin associates with endocytosed vesicles directing them to lysosome 2.) Proteasomes - Proteasome is multiprotein complex in cytoplasm and nucleus - It selectively degrades poly-ubiquinated proteins (ubiquitination regulates protein activity ie. cyclins and misfolded proteins)

37
Q

Explain the role of ubiquitin in protein degradation

A
  • Ubiquitin is a small protein that is transferred to target proteins, specifically an amide bond is formed bw c-terminal glycine and a lysine in target molecule - Ubiquitination is regulatory (mono-ubiquitin protein are not necessary degraded) - Poly-ubiquitination leads to proteasomal destruction of proteins, using activation and transfer of ubiquitin by E1, E2 and E3 enzymes
38
Q

Describe the factors that determine protein half-life

A
  • Conformation: eg. Misfolding results in hydrophobic domains being placed on surface and leads to degradation - N-terminus: Ser/Met proteins are more stable than Arg/Lys proteins - Other sequences: eg. PEST sequences shorten lifespan

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