Courses/Bioc 406
How to make proteins from mRNAs
Small subunit decodes mRNA Large subunit Makes polypeptide Contains peptidyl transferase center
Eukaryotic ribosome just a little bigger then prokaryote
Ribosomes are fundamentally RNA, proteins don't change structure (tune function)
mRNA with multiple translating ribosomes: a polysome
Three stages in translation initiation: ribosome is places on start codon elongation: mRNA-templated polypeptide polymerization termination: the polypeptide and mRNA are released
How is start sites recognized by ribosome? Prokaryotes: Shin-Darlgarn sequence: a few pairs down from initiation codon. Pairs with 16S rRNA Eukaryotes: the small subunit binds the 7-methyl-G cap, the scans 5'-3' to find a start codon
Ingredients: mRNA, fMet-tRNA (fMet), initiation factors, IF-1, IF-2, IF-3, GTP and MG2+, Small (30S) subunit
Bacteria sets a specialized iniator tRNA charged with a modified amino acid, N-formylmethionine (fMet). Eukaryotes use plain old Met.
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Ribosome has three tRNA binding sites Amino acyl-tRNA (A) Peptidyl-tRNA (P) Exit (E) ![[Pasted image 20260303120025.png]] Steps
Elongation cycle ![[Pasted image 20260303121037.png]]
Elongation: aa-tRNA selection The tetracycline binds the 30S subunits A-site, blocking entry of the aa-tRNA-EF-Tu complex GTP hydrolyzes Acts as a timer mechanism, when GTP can hydrolyze, slows mechanism, allowing correct pairing
These are two steps at which the aa-tRNA can dissociate (fall off) before peptidyl transfer!
Elongation: Peptidyl transfer On A site, nucleophilic attacks P site
Puromycin is a chain terminator mimics an aminoacytylated tRNA
Fragment raction mimics peptdyl transfer f-Met is a radioactive form of the amino acid formylmethionine peptidyl transferase enzyme is RNA - not protein!!
Release polypeptide from tRNA in P site