Biochemistry-II - Protein Synthesis - Quick Guides | Biochemistry

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Study GuideBiochemistry-II–Protein Synthesis1.Fidelity in tRNA AminoacylationWhy Accuracy in Aminoacylation MattersBefore protein synthesis can begin, each tRNA must be attached to thecorrect amino acid.This process is calledaminoacylation.Accuracy here is critical because:•The ribosomedoes not checkwhich amino acid is attached to a tRNA•If a tRNA carries the wrong amino acid, that mistake will be built directly into the protein•Even small errors can produceharmful or nonfunctional proteinsAminoacyl-tRNA SynthetasesAminoacylation is carried out by enzymes calledaminoacyl-tRNA synthetases.Important points:•There are20 different synthetases, one for each amino acid in the genetic code•Each enzyme recognizes:oOne specific amino acidoOne or more specific tRNAs for that amino acidThese enzymes are responsible for maintaining theaccuracy of translation.Step 1: Formation of Aminoacyl AdenylateIn the first step:•The correctamino acidreacts withATP•This forms anaminoacyl adenylate intermediate•Inorganic pyrophosphate (PPi)is released

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Study GuideThe pyrophosphate is quickly broken down into phosphate byinorganic pyrophosphatase.This breakdown helps drive the reaction forward.The aminoacyl adenylate is ahigh-energy intermediate.Step 2: Transfer to tRNAIn the second step:•The amino acid is transferred from the aminoacyl adenylate to theacceptor end of tRNA•The amino acid attaches to theterminal adenosine (A) of the CCA sequence•No additional ATPis required for this stepThe final product is a correctly chargedaminoacyl-tRNA.

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Study GuideFigure 1Editing: Preventing MistakesEven with careful recognition, errors can occur—especially withchemically similar amino acids.To prevent these errors:•Aminoacyl-tRNA synthetases have asecond active site•This site performs anediting function

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Study GuideIf an incorrect amino acid is attached to a tRNA:•The enzyme recognizes the mistake•The incorrect aminoacyl-tRNA iscleaved•Free amino acid and free tRNA are releasedFigure 2Why Editing Is EssentialBecause:•The ribosome treatsall charged tRNAs the same•A mischarged tRNA would be used without question•This could lead to faulty proteinsEditing ensures thatonly correctly charged tRNAsenter protein synthesis.Energy Cost vs AccuracyThe editing process creates a“futile cycle”:

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Study Guide•Energy is used to form a bond•That bond is then broken if it is incorrectThis is similar to:•The3′–5′ exonuclease proofreadingactivity of DNA polymerase during DNA replicationAlthough energy is wasted:•The cost is small compared to the danger of errors•High fidelityin information processing is essential for survivalKey TakeawaysFidelity in tRNA aminoacylation is ensured by:•Highly specific aminoacyl-tRNA synthetases•A two-step, ATP-dependent reaction•An editing mechanism that removes incorrect amino acidsThis accuracy protects the cell from dangerous mistakes and ensuresreliable protein synthesis,even at the cost of extra energy.2.Initiation of Protein SynthesisChoosing the Correct Starting PointProtein synthesis must begin at thecorrect position on mRNA.In most cases, the start codon isAUG, and almost all proteins begin with the amino acidmethionine.However, AUG can also appearinside a gene.Therefore, cells need a way to:•Recognizewhich AUG starts translation•Distinguish it from AUG codons that appear later in the message

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Study Guide2.1Ribosomes and Their SubunitsIn prokaryotes, ribosomes consist oftwo subunits:•30S (small subunit)•50S (large subunit)Together, they form a70S ribosome.The “S” values refer to how fast particles sediment in a centrifuge; they arenot additive.Functions of the subunits:•30S subunit→ decoding and mRNA–tRNA interactions•50S subunit→ peptide bond formationBefore initiation begins, the ribosome exists asseparate subunits.Where Initiation BeginsTranslation begins at the5′ end of the mRNA.Because transcription occurs in the5′ → 3′ direction, bacteria can:•Begin translating mRNAbefore transcription is finishedThis coupling of transcription and translation plays an important role ingene regulationin bacteria.Special Initiator tRNAProtein synthesis begins with aspecial initiator tRNA, calledtRNAᵐᵉᵗᴵ(I = initiator)Key features:•In bacteria, this tRNA carriesN-formylmethionine (fMet)•The formyl group is added fromformyl-tetrahydrofolate•This initiator tRNA:oRecognizesonly start codonsoDoesnotinsert methionine at internal AUG codons

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Study GuideThe formyl group ensures that:•fMet can be placedonly at the amino (N-terminal) endof the protein2.2Identifying the Correct Start CodonShine–Dalgarno Sequence (RBS)To select the correct AUG, bacteria use an additional base-pairing step.•The mRNA contains apurine-rich sequenceupstream of the start codon•This sequence is called theShine–Dalgarno sequenceorribosome-binding site (RBS)•The RBS base-pairs with the3′ end of 16S rRNAin the 30S subunitFigure 1After this pairing:•Translation begins at thefirst AUG downstream of the RBSmRNAs with stronger complementarity between:•RBS and 16S rRNAare translatedmore efficientlyThis provides a mechanism fortranslational control.

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