Computational Study of Uracil Tautomeric Forms in the Ribosome: The Case of Uracil and 5-Oxyacetic Acid Uracil in the First Anticodon Position of tRNA.

Tautomerism is important in many biomolecular interactions, not least in RNA biology. Crystallographic studies show the possible presence of minor tautomer forms of transfer-RNA (tRNA) anticodon bases in the ribosome. The hydrogen positions are not resolved in the X-ray studies, and we have used ab initio calculations and molecular dynamics simulations to understand if and how the minor enol form of uracil (U), or the modified uracil 5-oxyacetic acid (cmo5 U), can be accommodated in the tRNA-messenger-RNA interactions in the ribosome decoding center. Ab initio calculations on isolated bases show that the modification affects the keto-enol equilibrium of the uracil base only slightly; the keto form is dominant (>99.99%) in both U and cmo5 U. Other factors such as interactions with the surrounding nucleotides or ions would be required to shift the equilibrium toward the enol tautomer. Classical molecular simulations show a better agreement with the X-ray structures for the enol form, but free energy calculations indicate that the most stable form is the keto. In the ribosome, the enol tautomers of U and cmo5 U pair with a guanine forming two hydrogen bonds, which do not involve the enol group. The oxyacetic acid modification has a minor effect on the keto-enol equilibrium.