Ribonuclease inhibitor

Ribonuclease inhibitors (RI) are a family of large (~450 residues, ~49 kDa), acidic (pI ~4.7), proteins that catalyze the degradation of ribonucleases. Human RI(hRI) is a major cellular protein, comprising ~0.1% of all cellular protein by weight. ^[1] Human RI is shown to the right complexed as a dimer with RNase 1.

Ribonucleases (RNase) are enzymes that degrade RNA and are often cytotoxic which gives them chemotherapeutic properties. However, when bound to an RI they are no longer functional. Understanding the mechanism through which RI identifies and binds to RNases will allow scientists to design/modify RNases to evade hRI. In fact, one drug, Onconase (ONC), a ribonuclease from the Northern Leopard Frog (Rana pipiens), is now in Phase III clinical trials as a cancer chemotherapeutic agent ^[2].

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Interactions between hRI and RNase 1

Arg39 and Arg91 are proposed to be “electrostatic targeting residues” a term used by Johnson et. al to define residues that push the formation of protein complexes^[3]. Mutagenic studies that changed Arg39 and Arg91 to leucines strongly affected the association rate of the complex. As shown, and form multiple hydrogen bonds to hRI, keeping the RNase in place, allowing the formation of salt bridges that further lock hRI and RNase together. ^[4]

Mechanism of Inhibition

A citrate molecule bound to hRI forms hydrogen bonds with His12, Lys41, and His119, three key catalytic residues of RNase 1. This interferes with the substrate-binding pocket of RNase 1. ^[5]