Inhibition of chlamydial class Ic ribonucleotide reductase by C-terminal peptides from protein R2

Chlamydia trachomatis ribonucleotide reductase (RNR) is a class Ic RNR. It has two homodimeric subunits: proteins R1 and R2. Class Ic protein R2 in its most active form has a manganese-iron metal cofactor, which functions in catalysis like the tyrosyl radical in classical class Ia and Ib RNRs. Oligopeptides with the same sequence as the C-terminus of C. trachomatis protein R2 inhibit the catalytic activity of C. trachomatis RNR, showing that the class Ic enzyme shares a similar highly specific inhibition mechanism with the previously studied radical-containing class Ia and Ib RNRs. The results indicate that the catalytic mechanism of this class of RNRs with a manganese-iron cofactor is similar to that of the tyrosyl-radical-containing RNRs, involving reversible long-range radical transfer between proteins R1 and R2. The competitive binding of the inhibitory R2-derived oligopeptide blocks the transfer pathway. We have constructed three-dimensional structure models of C. trachomatis protein R1, based on homologous R1 crystal structures, and used them to discuss possible binding modes of the peptide to protein R1. Typical half maximal inhibitory concentration values for C. trachomatis RNR are about 200 µ m for a 20-mer peptide, indicating a less efficient inhibition compared with those for an equally long peptide in the Escherichia coli class Ia RNR. A possible explanation is that the C. trachomatis R1/R2 complex has other important interactions, in addition to the binding mediated by the R1 interaction with the C-terminus of protein R2.