Ribonucleotide reductase (RNR) catalyses the reduction of ribonucleotides to deoxyribonucleotides. In conventional class I RNRs the active site is located in the R1 subunit, and the R2 subunit contains a diiron cofactor and a stable tyrosyl radical essential for activity.

Class Ic Chlamydia trachomatis RNR lacks the tyrosyl radical and uses a Mn(IV)Fe(III) cofactor for catalysis. The requirement for metals for RNR activation was studied in C. trachomatis F127Y and Y129F R2, and in mouse wild type and Y177F R2 proteins. The results indicate that R2 affinity for metals is determined by the amino acid located next to the metal site, at the position of the radical carrying tyrosyl. Specifically, R2 proteins that contain phenylalanine in this place bind Mn and Fe, and the tyrosyl containing R2s bind only Fe.

Further results show that C. trachomatis RNR can be inhibited by R2 C-terminal oligopeptides. The highest inhibition was observed for a 20-mer peptide indicating that the oligopeptide inhibition mechanism of class Ic is similar to that of the class Ia and b.

The second part of the thesis deals with class Ia RNR inhibition. The results show that a lanthanum complex containing three 1,10-phenantroline molecules (KP772) which has showed promising cytotoxic activity in cancer cell lines inhibits mouse R2 protein in the presence of external reductants by iron-chelation. It is suggested that KP772 has several synergistic inhibition mechanisms that contribute to its overall anticancer activity. Moreover, other results show that Triapine, a promising chemotherapeutic compound, and its Fe, Ga, Zn, and Cu complexes, inhibit mouse R2 in both reducing and non-reducing conditions. Inhibition by Triapine involves the binding of the drug to the surface of the R2 protein leading to labilization of the Fe-center and subsequent Fe-chelation by Triapine. Formation of the Fe(II)-Triapine complex which reacts with O₂ to produce reactive oxygen species results in complete RNR inactivation.