Mechanisms of maintaining accuracy of protein biosynthesis on the ribosome

Da bi život bio moguć potreban je stalan protok genetičke informacije. Sintezu proteina na temelju informacije pohranjene u DNA omogućuju replikacija i transkripcija DNA te translacija mRNA. Točnost i brzina tih procesa bitni su za održavanje funkcionalnosti nekog organizma. Prilikom translacije može doći do ugradnje pogrešnih aminokiselina u polipeptidni lanac. No, postoji niz mehanizama na ribosomu koji omogućuju da učestalost ugradnje pogrešnih aminokiselina ne prelazi 1×10-3. Ovisno o sparivanju kodona mRNA i antikodona tRNA razlikujemo srodne, približno srodne i nesrodne aminoacilirane tRNA (aa-tRNA). Srodne aa-tRNA su one čije su baze antikodona potpuno komplementarne bazama kodona mRNA. Na ribosom aa-tRNA dolaze u obliku trojnog kompleksa s elongacijskim faktorom Tu (EF-Tu) i GTP-om. Ribosom odabire srodne aa-tRNA zahvaljujući mehanizmima kinetičke provjere i induciranog pristajanja. Ispravno sparene baze kodona i antikodona pokreću mehanizam induciranog pristajanja, koji dovodi do pravilne orijentacije aktivnih mjesta na ribosomu potrebnih za reakcije hidrolize GTP-a i akomodacije aa-tRNA u A mjestu. Konformacijske promjene odvijaju se u dekodirajućem centru male podjedinice ribosoma te se konformacijski signal prenosi do velike podjedinice koja je uključena u hidrolizu GTP-a i akomodaciju. Pokazano je da je komunikacija između male i velike podjedinice moguća zbog stvaranja ionskih mostova, koji nastaju nakon zatvaranja male podjedinice uslijed vezanja srodnog supstrata. Zahvaljujući GTP-aznoj aktivnosti EF-Tu, proces odabira srodne aa-tRNA rastavljen je na dva koraka: inicijalni odabir i kinetičku provjeru. Oni omogućuju ribosomu da u više navrata odbaci približno srodnu i nesrodnu aa-tRNA te na taj način višestruko povećavaju točnost translacijskog procesa.Constant flow of genetic information makes life possible. Protein biosynthesis based on information stored in DNA molecule is enabled by replication and transcription of DNA and translation of mRNA. Accuracy and speed of those processes are crucial for maintaining organism functionality. During translation, incorrect amino acids can be incorporated in a polypeptide chain. There are several mechanisms on the ribosome which prevent that incorporation frequency of incorrect amino acids rises above 1×10-3. Based on mRNA codon and tRNA anticodon pairing we can distinguish three types of aminoacylated tRNAs (aa-tRNA): cognate, near-cognate and non-cognate. Anticodon of cognate aa-tRNA is completely complementary with the codon of mRNA. Aa-tRNA is bound to the ribosome in the form of ternary complex together with the elongation factor Tu (EF-Tu) and GTP. Ribosome selects cognate aa-tRNA via mechanisms of kinetic proofreading and induced fit. Complementarity between codon and anticodon induces a conformational change that changes the orientation of active sites on the ribosome, required for reactions of GTP hydrolysis and accomodation of aa-tRNA in A site. Those conformational changes occur in the decoding center of small ribosomal subunit and conformational signal is transmitted to the large subunit which is involved in GTP hydrolysis and accomodation. It has been shown that communication between small and large ribosomal subunit is possible because of formation of intersubunit bridges. They are formed after small subunit closure, caused by cognate supstrate binding. Because of the GTPase activity of EF-Tu, process of cognate aa-tRNA selection has been separated in two stages: initial selection and kinetic proofreading. They enable ribosme to reject near-cognate and non-cognate aa-tRNA several times, and in that way they increase the accuracy of protein biosynthesis on the ribosome