Charging the tRNAEnzymes add amino acid to tRNAMustbe specific(mustconnectcorrectaminoacid)Esterbondformedwith3-OHoftRNAMost cells contain 20AARS's Isoacceptor tRNAs recognized by sameAARS Same amino acid attachedtRNAminoacidNHAminoacyl-tRNA
Charging the tRNA Enzymes add amino acid to tRNA Must be specific (must connect correct amino acid) Ester bond formed with 3’-OH of tRNA Most cells contain 20 AARS’s Isoacceptor tRNAs recognized by same AARS Same amino acid attached
AARS ReactionATPAminoacyl-tRNAsynthetasereactionNHAminoacidAmino acid “charged"" by addition of AMP from ATPSubsequent hydrolysis of PPi makes this irreversibleRibose-Adenineamino acid transferred to tRNA and AMP releasedNHAminoacyl-adenylate(aminoacyl-AMP)tRNAAMPtRNANH.Aminoacyl-tRNA
AARS Reaction Aminoacyl-tRNA synthetase reaction Amino acid “charged” by addition of AMP from ATP Subsequent hydrolysis of PPi makes this irreversible amino acid transferred to tRNA and AMP released
SpecificityofAARSAARS protein must recognize 2“business ends"of tRNA Contact anticodon loop and acceptor armEnsures correct amino acid connected to its cognate tRNAThe AARS may also have proofreading ability: example of IleRSist active site excludes any hydrophobic amino acid larger than Ilelle and those smaller could get activatedtRNAGIn2nd active site hydrolyzes any smaller than Ile(lle won't fit in this pocket)GInRS(ATP)
Specificity of AARS AARS protein must recognize 2 “business ends” of tRNA Contact anticodon loop and acceptor arm Ensures correct amino acid connected to its cognate tRNA The AARS may also have proofreading ability: example of IleRS 1st active site excludes any hydrophobic amino acid larger than Ile Ile and those smaller could get activated 2nd active site hydrolyzes any smaller than Ile (Ile won’t fit in this pocket)
Translating the MessageAnticodon of tRNA base pairs with codon in mRNAAminoacidCodonin5'to3'directionH,NAcceptorendAnticodonin3'to5'directionCCAAttachmentsitetRNAiconTVCloopDHUloopAnticodoloopUACAnticodonCodotLLLLLLLLLLAYG
Translating the Message Anticodon of tRNA base pairs with codon in mRNA Codon in 5’ to 3’ direction Anticodon in 3’ to 5’ direction