Structural information about crucial cell growth enzymes might lead to new cancer treatments say researchers in Germany.

Ribonucleotide reductase (RNR) is the enzyme that catalyses the reduction of nucleotides. This is the rate-limiting step in DNA synthesis, and makes RNR an important target for cell growth control. RNR inhibitors have been proposed for treatments where cell proliferation needs to be inhibited, such as for cancer and bacterial and viral infections.

Mouse RNR, a mammalian enzyme closely related to human RNR, has two nonidentical subunits, R1 and R2. The R2 protein has a paramagnetic di-iron site which generates stable tyrosyl radicals that are essential for enzymatic activity. Previous studies have provided important but incomplete information on the structure of mammalian RNRs.

The paramagnetic centre in the protein can be probed with EPR spectroscopy. Daniele Biglino and colleagues at the Max Planck Institute for Bioinorganic Chemistry have used a pulsed electron-electron double resonance (PELDOR) technique to measure accurately the distances between the tyrosyl radicals of mouse R2.

'The weak magnetic (dipole-dipole) interaction between the two radicals can be used to measure the distance between them, allowing conclusions about the interaction between the two halves of the protein to be made,' said Biglino. 

The PELDOR spectra showed that the enzyme forms a dimer under physiological conditions. It also showed that the distance between the two radicals present in the two halves of this dimer is similar to that found from crystallographic studies in the bacterial protein E. coli. This implies that mouse R2 subunit forms a dimer that closely resembles the E. coli R2 subunit. 

'These results demonstrate the potential of the PELDOR technique to obtain structural information on enzymes containing paramagnetic centres for which crystal structures are difficult to obtain, such as mammalian RNRs,' said Biglino. The method can also be used to study many similar systems in biology and materials science, he said. 

Debora Giovanelli