This diagram(0.12MB) is taken from Molecular Cell Biology by H. Lodish et al. Third Edition. In the first drawing (top) the orientation of tRNA molecules are shown relative to the large and small ribosomal subunits in a prokaryote. The small subunit is shown as a "transparent white" in front of the "opaque grey" of the larger subunit. The upper parts of the tRNA molecules are "behind" the small subunit, and the P and A and E binding sites are seen "through" the small subunit. The lower part of the tRNA molecules show the P and A binding sites on the small subunit, with the tRNA molecules placed on either side of the "pedestal" of the small subunit. The mRNA is shown on the "near" side of the small subunit. The orange-shaded region shows the "entrance path" of each tRNA to the A site, or the P site in the case of the first tRNA charged with fMET amino acid.
Recall that the ribosome has a lot of protein and critical rRNA molecules in its makeup. It is flexible and can move in, as yet, unknown ways. It is not likely to be a rigid structure, and it does have a tunnel through it, as shown in the middle image, where the ribosome is shown from a different perspective. The green line is the growing polypeptide.
In the third drawing (below) the diagramatic features of the shifts in binding are shown. The ribosome "walks" along the mRNA, and one amino acid per codon is added. There are six binding configurations. Three are shown as the tRNA's are drawn "vertical" with binding sites on both subunits the "same," named E, P, and A. Binding of the initial tRNA with fMET is directly to the P site, using both subunits. Thereafter, the new tRNA binds first with the A site of the small subunit, then the large subunit. After the connection of the new amino acid to the peptide chain by the peptide bond, there is a "side step" to the P site after the old tRNA (now without anything attached to it's 3' end but the OH) has made room by side stepping to the E site. This resembles a chorus line of dancers. Then they side step on the small subunit, leaving the A site open for the new tRNA to bond with the next codon. The new tRNA with a new amino acid will "flow" through the orange tinted region shown on the right side of the ribosome in the first image, to bond with the next mRNA codon and small subunit A site (designated A/T) and then side step to the A site of the large subunit.
Maintained by Dick Richardson
Last updated 11/7/96