Question

What is the order of binding of a given tRNA on the ribosome during translation?

Is this order true for all tRNAs? If not, Why?

Answer 1

The cellular factory responsible for synthesizing proteins is ribosome. It has got three sites where amino acyl tRNA bind namely, P-site (peptidyl site or donor site), A-site (aminoacyl or acceptor site) and E site (exit site). tRNA brings amino acids to the ribosomes in the order (APE) during translation.

But the order for delivering amino acids is not true for all tRNAs. The initiating formyl or non-formylated methionine or methionine tRNA can bind only with P site which remains in the middle of the ribosome slot for binding of tRNAs. The site adjacent to P site where new aminoacyl tRNA complex reaches is the A-site and forms codon-anticodon bonding. A peptide bond is formed between COOH group of first amino acid (methionine) and NH2 group of the incoming second amino acid. This is followed by the transfer of methionine from the first tRNA onto the amino acid of the second tRNA in the A-site. With the formation of peptide bond, the ribosome is moved on mRNA exactly by one codon. This shift allows the first, empty tRNA to drift out through E site and the second tRNA consisting of amino acids shifts to P-site, with the A-site being exposed to a new codon such that the whole cycle can repeat.

Hence, this is the reason why all tRNAs does not bind to the ribosome in the same order. All tRNA except the initiating methionine tRNA follows the same order of binding on ribosomes (from A to P). The methionine tRNA always start the translation process binding only with P-site, so it moves in the order from P to E site.