The image shown to the right is the spacefilled model of the complete ricin molecule. The Jsmol image is based on pdb file 2aai.The ricin molecule is made of two chains, one that is critical for entry into the cell, the other that wreaks havoc once it is inside.

The two chains within the ricin molecule represented with a backbone model. The pink is the A chain with the catalytic site that cleaves RNA. The blue is the b chain that has the necessary receptors present for entry of the toxin into the cell.

The two chains are held together loosely with one disulfide bond(picture here lime green). This bond is broken in the endoplasmic reticulum by human protein disulfide isomerase. Notice that there are other disulfide bonds(pictured green) present in the b chain.

The highlighted residues here in dark red, using a spacefilled model, are the receptors on the b chain essential to allow for the ricin toxin to enter the cell. These sites link to galactose/n-acytaylgalactosamine on the cell surface initiating endocytosis.

Once the B chain has been cleaved by HPDI, it is possible for the A subchain to be translocated to the cytosol. There it is able to wreak havoc, catalytically clipping a weak spot (sometimes called the Sarcin/Ricin loop) on the 28S ribosome subunit, inactivating the ribosome. What is amazing is that a single ricin molecule can inactivate 25 ribosomes per second, thus stopping all translation in the cell, effectively killing the cell. This button illustrates the groove containing the catalytic site. The red residues show the catalytic sites glutamic acid 177 and arginine 180. The purple residues are sidechains important in positioning the RNA thus permitting catalysis.

Another view showing the catalytic site, this time using a spacefilled model.