Ribonucleotide reductase (RNR) is the enzyme responsible for de novo synthesis of deoxyribonucleotides, needed for both synthesis and repair of cellular DNA. The RNRs known so far are divided into three distinct classes; I, II and III. The conventional class I enzyme is composed of two separate subunits. The larger R1 subunit contains the active site, whereas the smaller R2 subunit contains a system specialized in forming, transporting and stabilizing a tyrosyl free radical.

Recently a new class Ic RNR was discovered in the bacterium Chlamydia trachomatis. It differs from the conventional class Ia and b RNRs in that it has a phenylalanine at the otherwise conserved tyrosyl radical harboring residue in its R2 subunit. Additionally the metal cluster shows some unusual aspects, of which the most striking perhaps is that the most red-ox active form is a mixed Mn-Fe cluster, instead of the normal Fe-Fe counterpart.

In this work several biochemical and biophysical methods were used to study activation and inhibition mechanisms in RNR from various class I species. The results from studying the oxygen activation confirm the role of the iron ligand E238 as a key residue for controlling the outcome of the reaction in E. coli protein R2. The finding of a stable sulfinyl radical after reconstitution of the R2 Y177F/I263C variant from mouse indicates that sulfinyl radicals may possibly be considered as stabilized forms of very short-lived thiyl radicals, proposed to be important in the radical chemistry of RNR. The investigation of the role of the proposed radical transfer pathway during chemical reduction of the iron/radical center shows that no specific pathway is required for the reduction of protein R2 from mouse. The results from inhibition studies of C. trachomatis demonstrate that the same mechanism of inhibition functions on this new class Ic RNR, however less efficiently than in class Ia and b.