Ribonuclease A / ribonuclease inhibitor (RI) complex
Chemical processes like the catalytic activity of enzymes need to be controlled. They should proceed when necessary but they should be inhibited when their activity is not needed. Human chemists usually try to design enzyme inhibitors that are small molecules. Nature however controls many processes by the interaction of large molecules. Ribonucleases digest ribonucleic acids (RNA), an important intermediate in the translation of DNA to proteins.
The structure seen here is that of the complex of a mammalian ribonuclease inhibitor (RI) together with ribonuclease A (RNAse A). RI is a large molecule with a rather unusual shape. It is shaped like a horseshoe, with a large cavity at the center of this horseshoe. Inside this cavity sits the RNAse A molecule. The RNAse A molecule is not entirely surrounded by the ribonuclease inhibitor, but the insertion of part of RNAse A into the RI cavity blocks the access of RNA to the active site of RNAse A and in this way prevents RNAse A from cleaving RNA. The interaction between RNAse A and RI is very tight, with a large contact area between the two molecules and many electrostatic interactions between polar side chains. In addition to RNAse A RI can inhibit other ribonucleases. RI can bind various RNAses despite their shape differences and at the same time do so as if it were specific for individual RNAses. This is accomplished by the flexibility of the ribonuclease inhibitor. The horseshoe shape is formed by 15 very similar structures, called leucine-rich repeats (LRR) and the LRRs can, by moving ever so slightly, one at a time, change the shape of the inner cavity to adapt to different types of RNAses.
Protein Data Bank (PDB)
author: Arno Paehler