These are very undetailed views of the ribosome, showing the large subunit in green and the small subunit in red. In this tutorial, we will be examining the functioning of the large (green) subunit. From this view, you should be able to get a general understanding of what the ribosome looks like as a whole, and in its component parts, without being confused by too much detail.

The large subunit is roughly hemispherical, but with three protrusions, called protuberances. We will be referring to these in the future, so be sure that you understand where they are located on the molecule. The large subunit is also referred to as the 50S subunit, and the small subunit is referred to as the 30S subunit. This is a reference to the sedimentation coefficient of the molecule (don't worry about what this means exactly), but is essentially a measure of how fast it would move in solution. Larger molecules typically sediment faster, and thus have larger coefficients (S). Also note that these figures (50S and 30S) refer to prokaryotic ribosomes. The ribosomes of eukaryotes are somewhat larger (40S and 60S in rat liver (4)), but not studied in great depth. This tutorial examines the large subunit of the bacterium Haloarcula marismortui, and draws much information from work published during the summer of 2000. (1,2 ,3)

This is a more detailed view of the large (50S) subunit, positioned similarly to the lower right image in the figure above. The complete ribosome (large and small subunits) is comprised of three separate rRNA chains, and more than 50 proteins (1). The large subunit contains two rRNA chains, 23S and 5S, and 26 individual proteins (1). The image at left depicts both rRNA chains of the large subunit, as well as its 27 proteins. The 5S rRNA chain is colored yellow, and proteins are colored blue. The 23S rRNA chain has been divided into two colors -- the sugar-phosphate backbone is white, and the bases are red. Again, you should be able to see the three protrusions (protuberances), which disrupt the somewhat regular structure of the molecule. Also note that the cartoon image from Voet & Voet has been distorted for visibility of the protuberances.

In this side view, the 100-Å deep active cleft of the large subunit has been highlighted in green (3). This cleft is almost entirely lacking in proteins, in particular, the active site itself. This finding has led researchers to conclude that the peptide bonds between amino acids are catalyzed entirely by the rRNA (3).

This view is looking from above, into the active cleft of the molecule. In the center of this 100-Å deep region is the active site, which we will examine later in the tutorial. In red are several key nucleotides which are responsible for catalyzing the peptide bond formation. The most important of these is Adenine 2486 (A2486) (3).

This rear view of the subunit shows clearly the 5S rRNA that forms a good deal of the central protuberance. The entire structure is approximately 250 Å across, and has a molecular mass of 2.5 X 10⁶D (2, 4).