Chlamydial ribonucleotide reductase: Tyrosyl radical function in catalysis replaced by the Fe III -Fe IV cluster

• Published in: Proceedings of the National Academy of Sciences - PNAS 2006-06, Vol.103 (26), p.9850-9854
• Main Authors: Voevodskaya, N., Narvaez, A.-J., Domkin, V., Torrents, E., Thelander, L., Gräslund, A.
• Format: Article
• Language: English
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0600603103

Ribonucleotide reductase (RNR) from Chlamydia trachomatis is a class I RNR composed of proteins R1 and R2. In protein R2, the tyrosine residue harboring the radical that is necessary for catalysis in other class I RNRs is replaced by a phenylalanine. Active C. trachomatis RNR instead uses the Fe III -Fe IV state of the iron cluster in R2 as an initiator of catalysis. The paramagnetic Fe III -Fe IV state, identified by 57 Fe substitution, becomes electron spin resonance detectable in samples that are frozen during conditions of ongoing catalysis. Its amount depends on the conditions for catalysis, such as incubation temperature and the R1/R2 ratio. The results link induction of the Fe III -Fe IV state with enzyme activity of chlamydial RNR. Based on these observations, a reaction scheme is proposed for the iron site. This scheme includes ( i ) an activation cycle involving reduction and an oxygen reaction in R2 and ( ii ) a catalysis cycle involving substrate binding and turnover in R1.