Protein SynthesisIswar Hazarika, Date:18/09/20121styr. M.Pharm, Pharmacology
• Proteins are composed of amino acids – there are 20different amino acids• Different proteins are made by combining these 20amino acids in different combinations
• Proteins are manufactured (made) by the ribosomes
•Function of proteins:1.Help fight disease2.Build new body tissue3.Enzymes used for digestion and other chemical reactionsare proteins(Enzymes speed up the rate of a reaction)4. Component of all cell membranes
Making a Protein—Transcription•First Step: Copying of genetic information from DNA to RNA calledTranscriptionWhy? DNA has the genetic code for the protein that needsto be made, but proteins are made by the ribosomes—ribosomesare outside the nucleus in the cytoplasm.DNA is too large to leave the nucleus (double stranded), butRNA can leave the nucleus (single stranded).
• Part of DNA temporarily unzips and is used as atemplate to assemble complementary nucleotidesinto messenger RNA (mRNA).
• mRNA then goes through the pores of the nucleus withthe DNA code and attaches to the ribosome.
Making a Protein—Translation•Second Step: Decoding of mRNA into a protein is calledTranslation.•Transfer RNA (tRNA) carries amino acids from thecytoplasm to the ribosome.
These amino acids come from the food we eat. Proteinswe eat are broken down into individual amino acids andthen simply rearranged into new proteins according to theneeds and directions of our DNA.
•A series of three adjacent basesin an mRNA molecule codes fora specific amino acid—called acodon.•A triplet of nucleotides in tRNAthat is complementary to thecodon in mRNA—called ananticodon.•Each tRNA codes for a differentamino acid.Amino acidAnticodon
• mRNA carrying the DNA instructions and tRNA carryingamino acids meet in the ribosomes.
• Amino acids are joined together to make a protein.Polypeptide = Protein
5’-ATGCCTAGGTACCTATGA-3’3’-TACGGATCCATGGATACT-5’5’-AUGCCUAGGUACCUAUGA-3’5’-AUG CCU AGG UAC CUA UGA-3’N-MET-PRO-ARG-TYR-LEU-CDNATranscriptiondecoded asTranslationmRNAProtein
tRNAs are activated by amino-acyl tRNA synthetasesAmino-acyl tRNAsynthetase
Amino-acyl tRNA synthetases:One synthetase for each amino acida single synthetase may recognize multiple tRNAsfor the same amino acidTwo classes of synthetase.Different 3-dimensional structuresDiffer in which side of the tRNA they recognizeand how they bind ATPClass I - monomeric, acylates the 2’OH on the terminal riboseArg, Cys , Gln, Glu, Ile, Leu, Met, Trp Tyr, ValClass II - dimeric, acylate the 3’OH on the terminal riboseAla, Asn, Asp, Gly, His, Lys, Phe, Ser, Pro, Thr
Two levels of control to ensure that the proper amino acidis incorporated into protein: 1) Charging of the proper tRNA
2) Matching thecognate tRNA to themessenger RNA
The association of the large and small subunits creates thestructural features on the ribosome that are essential forprotein synthesisThree tRNA bindingsites:A site = amino-acyltRNA binding siteP site = peptidyl-tRNAbinding siteE site = exit site
In addition to the APE sites there is an mRNA binding groovethat holds onto the message being translated
Incorporation of the correct amino acyl-tRNA is determinedby base-pairing interactions between the anticodon of thetRNA and the messenger RNASTEPS OF TRANSLATION:1. Initiation2. Elongation3. Termination
Initiation of TranslationInitiation is controlled differently in prokaryotic andeukaryotic ribosomesIn prokaryotes a single transcript can give rise to multiple proteins
In prokaryotes, specificsequences in the mRNAaround the AUG codon, calledShine-Delgarno sequences,are recognized by an intiationcomplex consisting of a Metamino-acyl tRNA, InitiationFactors (IFs) and the smallribosomal subunit
GTP hydrolysis byIF2 coincident withrelease of the IFs andbinding of the largeribosomal subunit leadsto formation of a completeribosome,on the mRNAand ready to translate.
Eukaryotic mRNAs have a distinct structure at the 5’ end
In contrast, Eukaryotesuse a scanning mechanismto intiate translation.Recognition of the AUGtriggers GTP hydrolysisby eIF-2
GTP hydrolysis byeIF2 is a signal forbinding of the largesubunit and beginningof translation
Proper reading of theanticodon is the secondimportant quality controlstep ensuring accurateprotein synthesis=EF-1Elongation factorsIntroduce a two-step“Kinetic proofreading”STEP 2:Elongation
A second elongation factorEF-G or EF-2, drives thetranslocation of the ribosomealong the mRNATogether GTP hydrolysisby EF-1 and EF-2 help driveprotein synthesis forward
STEP 3: Termination oftranslation is triggered bystop codonsRelease factor entersthe A site and triggersHydrolysis of the peptidyl-tRNAbond leading to release ofthe protein.
Release of the protein causesthe disassociation of theribosome into its constituentsubunits.