L-ATP is recognized by some cellular and viral enzymes: does chance drive enzymic enantioselectivity?

We demonstrate that L-ATP is recognized by some enzymes that are involved in the synthesis of nucleotides and nucleic acids. L-ATP, as well as its natural D-enantiomer, acts as a phosphate donor in the reaction catalysed by human deoxycytidine kinase, whereas it is not recognized by either enantioselective human thymidine kinase or non-enantioselective herpes virus thymidine kinase. L-ATP strongly inhibits (K-i 80 mu M) the synthesis of RNA primers catalysed by DNA primase associated with human DNA polymerase a, whereas RNA synthesis catalysed by Escherichia coli RNA polymerase is completely unaffected. Moreover, L-ATP competitively inhibits ATP-dependent T4 DNA ligase (K-i 25 mu M), suggesting that it interacts with the ATP-binding site of the enzyme. Kinetic studies demonstrated that L-ATP cannot be used as a cofactor in the ligase-catalysed joining reaction. On the other hand: L-AMP is used by T4 DNA ligase to catalyse the reverse reaction, even though a high level of intermediate circular nicked DNA molecules accumulates. Our results suggest that a lack of enantioselectivity of enzymes is more common than was believed a few years ago, and, given the absence of selective constraints against L-nucleosides in Nature, this may depend on chance more than on evolutionary strategy.