Protein Name

Ribonuclease inhibitor (RI)


Sus scrofa (pig)

Biological Context

We often think of protein inhibitors as small molecules which bind to the active site of a protein and in this way block its activity. However protein inhibitors are quite often proteins themselves. They can influence the activity of other proteins by changing their shape or blocking access to the active site.

Structure Description


The structure that we see here is a mammalian ribonuclease inhibitor (RI). RI is a large molecule with a rather unusual shape. It is actually much larger than the protein that it inhibits. RI is shaped like a horseshoe, with a large cavity at the center of this horseshoe. Another structure, PDB:1DFJ, shows the same molecule with a ribonuclease bound inside the large cavity in the center. 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) a├Źnd 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)



  • Kobe, B. Deisenhofer, J.; "Mechanism of ribonuclease inhibition by ribonuclease inhibitor protein based on the crystal structure of its complex with ribonuclease A."; J. Mol. Biol.; (1996) 264:1028-1043 PubMed:9000628.


author: Arno Paehler

Japanese version:PDB:2BNH