The tetramer bovine liver catalase molecule, with its tyrosines presented by their phenyl rings, most of them coloured orange. From neutron scattering it appears that one of the following tyrosines (coloured blue) may have become a radical: tyr-324, tyr-357 or tyr-369. Tyr-136 (marked by an arrow) is another slightly less probable candidate. From EPR (Svistunenko & Cooper, 2004), tyrosyl-369 is suggested (dark blue, closest to the centre of the catalase molecule). Tyr-357 is very close to the iron atom (black filled circles) of the porphyrin ring.