(52)
2. Angive ribosomets tre sites for tRNA (A, amino; P, peptide, og E, exit site). Definere funktionen af disse tre sites
Stryer, s. 828-829, fig. 29.22; fig. 29.23; fig. 29.24
Devlin, s 250

Ribosomes have three tRNA-binding sites that bridge the small and large subunit.

The binding sites for the amino acids carried by tRNA, (the amino acids of the growing polypeptide chain) are located on the large subunit, while the binding site for the codon-anticodon interactions is located on the small subunit. This is possible since tRNA has a L-from tertiary structure, which allows that the acceptor stem is in one side of the L, while the anticodon on the other.

Thus, each tRNA molecule contacts both the large and the small subunit and both subunits have an A, P and E site.

A-site - for aminoacyl. This is also the so-called acceptor-site.
It is called like that because the aminoacyl-tRNA specified by the next codon in the mRNA message is bound to it. After translocation it is empty.  
In the A-site of the small subunit tRNAs anticodon is bound to the mRNA codon. 

P-site - for peptidyl, also called the donor site. At the beginning of the translation, the initiating methionyl-tRNA is placed into this position so that its methionyl residue may be transferred to the free α-amino group of the incoming aminoacyl-tRNA. As the translation goes on, new tRNA molecules carrying the adjacent a. a. occupy this site.
The P-site of the small unit contains the anticodon of the donor-tRNA before the translocation and afterwards, it contains the anticodon of the ex-acceptor tRNA.

 
E-site - for exit, the binding place for tRNA after it has completed its role in translation. When bound to this site, tRNA is freed from the ribosome.
 

The mechanism of protein synthesis is the following: The cycle begins with a peptidyl tRNA in the P site. An aminoacyl tRNA binds to the A site. When both sites are occupied, a new peptide bond is formed. The tRNAs and the mRNA are translocated through the action of an elongation factor, which moves the deacylated tRNA to the E site. Once there, it is free to disassociate and complete the cycle.

The ex-aminoacyl tRNA, which is now the peptidyl-tRNA occupies the P site, while a new aminoacyl t-RNA comes into the A site and the cycle begins again.   

The other end of each tRNA molecule interacts with the large subunit, where the amino acids are bound in the growing polypeptide chain. The acceptor stems of the tRNA molecules occupying the A and the P site converge at a site where a peptide bond is formed. The site is connected to the back of the ribosome by a channel, through which the nascent polypeptide chain passes during synthesis.    

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