University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2020
Views:

131

Pages:

22
Author:

Donavan Williams

Section 19.2: Protein Synthesis 1. Initiation — N-terminal (5’) mRNA à C-terminal (3’) § Requires RNAmet – N-terminal AA of all proteins Initiation tRNA: Eukaryotes – tRNAmet; Prokaryotes – tRNAfmet § § Formation of initiation complex - binding of small ribosomal subunit to mRNA, subsequent binding of initiator tRNA § § § § § Initiator tRNA binds to AUG codon on the mRNA Ends with binding of large ribosomal subunit P (peptidyl) site occupied by initiator tRNA A (aminoacyl) site binds 2nd aminoacyl-tRNA Polysome - multiple ribosomes can read an mRNA simultaneously Figure 19.4a Protein Synthesis Section 19.2: Protein Synthesis 2. Elongation - mRNA is read in 5′à3′ direction protein is assembled from N-terminus to the C-terminus 1. 2. 3. Figure 19.4b Protein Synthesis Addition of 2nd amino acid in A site ü Specified by mRNA in A-site Peptidyl transferase catalyzes peptide bond formation between aas in P-site & A-site ü Dipeptidyl-tRNA in A-site ü Uncharged tRNA at P-site Translocation - transfer of peptidyltRNA to the P site ü GTP provides energy ü A-site peptide chain shifted to Psite ü Uncharged tRNA in P-site released Section 19.2: Protein Synthesis 3. Termination – no aminoacyl-tRNA to bind with stop codon §Protein-releasing factor binds to A site ü Cleaves bond between protein and last tRNA §Ribosome releases mRNA ü Dissociates large & small subunits ü GTP required ü Wide variety of protein factors Figure 19.4b Protein Synthesis Post-translation modifications § Amino acid removal § Side chain modifications § Combining with other polypeptides Section 19.2: Protein Synthesis Prokaryotic Protein Synthesis – rate of 20 aa/sec §70S Ribosome composed of a 50S/30S subunit §peptidyl transferase center (PTC) binds 3′ ends of aminoacyl- and peptidyl-tRNAs for peptide bond formation ü Located on 50S in 23S rRNA subunit §GTPase associated region (GAR) is a set of overlapping binding sites of 23S structural elements ü Drives GTP hydrolysis (acts as GAP) causing conformational change §decoding center (DC) in 30S located in A site ü mRNA codon is matched with a tRNA anticodon Section 19.2: Protein Synthesis Initiation complex formation §IF-3 binds to 30S subunit ü Prevents premature binding to 50S subunit ü Promotes mRNA binding §IF-1 binds to A site blocking tRNA binding ü Shine-Dalgarno sequence guides binding of 30S subunit to AUG ü Located on 16S region ü Upstream from AUG ü Unique to each mRNA §IF-2 GTPasebinds to initiator fmet-tRNAfmet, P-site entry ü GTP hydrolysis conformational change ü 50S subunit binds Section 19.2: Protein Synthesis Elongation – addition of amino acids to growing protein 1. Positioning an aminoacyl-tRNA in the A site § EF-Tu-GTP binds to aminoacyl-tRNA then guides to A site § Positions the aminoacyl-tRNA in the A site ü Protects aminoacyl linkage from hydrolysis § After binding, GTP hydrolysis occurs; EF-Tu is released 2. Peptide bond formation - catalyzed by peptidyl transferase §A Site – dipeptidyl-tRNA; P Site – uncharged tRNA ü Catalyzes nucleophilic attack of A-site a-amino group on carbonyl carbon of p-site amino acid ü Release of polypeptides from ribosome 3. Translocation - movement of mRNA by ribosome §Uncharge tRNA moves from P site to E site; released §Peptidyl-tRNA translocates from A site to P site ü EF-G required - another GTP-binding protein §A site occupied by next aminoacyl-tRNA Section 19.2: Protein Synthesis E P A E P A E P A E P A Section 19.2: Protein Synthesis §Termination - termination codon enters A site ü UAA, UAG, UGA §Three release factors (RF1, RF2, and RF3) are involved §Both RF1 and RF2 resemble tRNAs in shape and size §RF1 recognizes UAA and UAG §RF2 recognizes UAA and UGA §RF3 - GTPase necessary for RF1 and RF-2 binding to ribosome §Complex dissociates – freeing release factors, tRNA, mRNA, 30S/50S subunits Section 19.2: Protein Synthesis Posttranslational Modifications §Trigger factor (TF) - molecular chaperone helps begin folding process as each nascent polypeptide emerges from exit tunnel §Covalent alteration –removal of signal peptides; formylmethionine residue §Chemical modifications - methylation, phosphorylation, carboxylation, & covalent linkage to lipid molecules Section 19.2: Protein Synthesis Figure 19.10 Transcription and Translation in E. coli Translational Control §Prokaryotes occurs at transcription initiation §Transcription and translation are coupled §Prokaryotic mRNA has a short half-life (1-3 minutes) §Rates of translation also vary §Differences in Shine-Dalgarno sequences Section 19.2: Protein Synthesis §Functional and structural differences between prokaryotic and eukaryotic protein synthesis are the basis of the therapeutic and research uses of antibiotics Section 19.2: Protein Synthesis Eukaryotic Protein Synthesis Chain Initiation Extra processing mRNA secondary structure – methylguanosine cap, poly(A) tail, removal of introns ü Associates with ribosome after leaving nucleus; complexed with several proteins § Scan for TSS – no Shine-Dalgarno sequence, ribosome migrates in 5’ -> 3’ direction §Initiation – begins with assembly of pre-initiation complex (PIC) §Pre-initiation complex (PIC) - binding of 40S subunit to eIF1 A, eIF2 (GTP-binding protein), GTP, and methionyl-tRNAmet §eIF2-GTP mediates the binding of the initiator tRNA to the 40S subunit §eIF3 (bound to the 40S subunit) prevents association with large subunit (60S) §43S preinitiation complex - binds to 5’-cap, has cap-binding complex (CBC or eIF4F) associated § Section 19.2: Protein Synthesis Binding 40S to elf1 A, elF2, GTP, tRNAmet to AUG 3’-poly(A) tail is brought to close proximity to 5’capped end by EIF4G & PAPB Forms circular mRNA, scan for AUG near 5’ end Section 19.2: Protein Synthesis §80S complex - initiation complex binds the 60S subunit §Hydrolysis of GTP bound to eIF2 §eIF5-acts as a guanine nucleotide activating protein §Initiation factors are released from the ribosome Figure 19.12 mRNA Scanning and 80S Initiation Complex Formation Section 19.2: Protein Synthesis Elongation – no E site, A & P sites only, 2 elongation factors § eEF1a mediates binding of aminoacyl-tRNAs to A site ü Correct match – eEF1a leaves site ü Wrong – complex leaves site § Peptidyl transferase of 60S subunit catalyzes peptide bond formation § eEF2-GTP binds to ribosome ü GTP hydrolysis - energy to move mRNA § Stops at stop codons Section 19.2: Protein Synthesis Termination - release factors 1. eRF1 – recognizes/binds to stop codons & to eRF3 2. eRF1 binding causes peptidyl transferase to hydrolyze the ester linkage, releasing polypeptide 3. eRF3 triggers dissociation of eRF1 and ribosomal subunits §Translation efficiency may be related to circular conformation of polysomes Figure 19.14a Eukaryotic Protein Synthesis Termination Section 19.2: Protein Synthesis Posttranslational Modifications – prepares protein for functional role, folding into native conformation §Proteolytic cleavage – common regulatory mechanism §Remove N-terminal methionine and signal peptides §Convert proproteins into their active forms §Preproprotein – inactive precursor with removable signal peptide §Preproinsulin – proinsulin - insulin Figure 19.16 Proteolytic Processing of Insulin Section 19.2: Protein Synthesis Posttranslational Modifications §Glycosylation – catalyzed by glycosyl transferase §N-linked oligosaccharide is assembled in association with phosphorylated dolichol (polyisoprenoid) §Vital role in protecting ER from misfolded glycoproteins §Cannot be correctly folded are targeted for ER-associated protein degradation by ubiquitin proteasome in cytoplasm §Hydroxylation - proline and lysine is required for structural integrity collagen and elastin §Vitamin C (ascorbate) is required to hydroxylate proline §Inadequate dietary intake of vitamin C can result in scurvy, which is caused by weak collagen fiber structure §Phosphorylation plays critical roles in metabolic, control, signal transduction, and protein-protein interaction Section 19.2: Protein Synthesis Posttranslational Modifications §Lipophilic Modifications - covalent attachment of lipid moieties to proteins §Acylation and prenylation are the most common §Methylation - marking proteins for repair or degradation or changing their cellular function §Carboxylation - increases a protein’s sensitivity to Ca2+dependent modulation §Disulfide Bond Formation – conformation stabilization Biochemistry in Perspective Targeting - directing protein to proper destination §Transcript localization - specific mRNA is bound to receptors creating protein gradients §mRNA is moved by motor proteins (e.g., dynein and kinesin) along cytoskeletal filaments §Signal hypothesis – polypeptides targeted to proper location by sorting signals (signal peptides) §Facilitate insertion of polypeptide into the appropriate membrane Biochemistry in Perspective Figure 19.19 Cotranslational Transfer across the RER Membrane a) Polypeptide protrudes from ribosome, SRP binds to signal sequence causing a transient cessation of translation b) Subsequent binding of SRP to SRP receptor results in binding of ribosome to translocon complex in RER membrane c) Translation restarts; polypeptide inserts into the membrane Biochemistry in Perspective Figure 19.19 Cotranslational Transfer across the RER Membrane d) Dissociation of SRP from receptor e) Continuation of elongation f) Signal peptide removal by signal peptidase