2-4.3 Ribonuclease (RNase):

• Nuclease that can catalyze hydrolysis of ribonucleotides from either single stranded or double stranded RNA sequence are called ribonucleotides (RNase).
  
  
• RNase are classified into two types depending on position of cleavage, i.e. endoribonuclease (cleave internal bond) and exoribonuclease (cleave terminal bond).
  
  
• RNase is important for RNA maturation and processing.
  
  
• RNaseA and RNaseH play important role in initial defence mechanism against RNA viral infection.

Two common types of ribonucleases are discussed below:

 

2-4.3.1 RibonucleaseA (RNaseA):

• An endo-ribonuclease that cleaves specifically single-stranded RNA at the 3' end of pyrimidine residues.
  
  
• The RNA is degraded into 3'-phosphorylated mononucleotides C and U residues and oligonucleotides in the form of 2', 3'-cyclic monophosphate intermediates.
  
  
• Optimal temperature for RNaseA is 60°C (activity range 15-70°C) and optimal pH is 7.6.
  
  
• RNaseA has two histidine residues in its active site (His12 and His119). In the first step, His12 acts as a base; accepting proton forming a nucleophile which then attacks positively charged phosphorus atom. His119 acts as an acid in this case, donating a proton to oxygenated P-O-R' bond. The imidazole side chain acts as base in His 12 here.
  
  
• The side chain of Lys41 and Phe120 further stabilize the transition state. Nitrogen of the main chain of Phe120 donates hydrogen, thus bonding with the unbound oxygen atom.
  
  
• In the second step the acid base activities get reversed and His119 accepts proton from water causing hydroxyl attack on cyclic intermediate.
  
  
• Activity of RNaseA can be inhibited by alkylation of His12 and His119 residue essential for activity of the enzyme.