Parole chiave: Enzyme - Crystal Structure - Trna - Tuberculosis,
*** IBB - CNR *** a CIRPeB , University of Naples Federico II , via Mezzocannone 16 , 80134 Naples IT ., b Department of Pharmacy , University of Naples Federico II , via Mezzocannone 16 , 80134 Naples IT ., c Institute of Bio-structures and Bio-imaging- CNR-IBB , via Mezzocannone 16 , 80134 Naples IT ., d Department of Life Sciences , Imperial College London , London , SW7 2AZ UK .,
Ribonuclease AS (RNase AS) is a crucial enzyme for virulence of Mycobacterium tuberculosis. We previously observed that RNase AS structurally resembles RNase T from E. coli, an important enzyme for tRNA maturation and turnover. Here, we combine x-ray crystallography and molecular dynamics (MD) to investigate the specificity and dynamic properties of substrate binding. Both x-ray and MD data provide structural determinants that corroborate the strict substrate specificity of RNase AS to cleave only adenosine residues, due to the structural features of adenine base. Beside suggesting tRNA as most likely substrate of RNase AS, MD and modelling studies identify key enzyme-ligand interactions, both involving the catalytic site and the double helix region of tRNA, which is locked by interactions with a set of arginine residues. The MD data also evidence a ligand induced conformational change of the enzyme which is transferred from one chain to the adjacent one. These data well explain the dimeric nature of both RNase AS and RNase T, with two catalytic grooves composed of both Chains. Also, they account for the dichotomy of tRNA, which contains both the substrate poly(A) chain and an inhibiting double strand RNA. Indeed, they provide a possible mechanism of allosteric regulation, which unlocks one catalytic groove when the second groove is inhibited by the double strand region of tRNA. Finally, a full comprehension of the molecular details of tRNA maturation processes is essential to develop novel strategies to modulate RNA processing, for therapeutic purposes.