Carboxylate-bridged non-heme diiron (I1) centers occur in several metalloenzymes involved in the biological utilization of di~ xygen. l-~ X-ray structural information is now available for three proteins in this class, hemerythrin (Hr), 6 the R2 protein of Escherichia coli ribonucleotide reductase (RR),'and the hydroxylase protein of soluble methane monooxygenase (MM0). 8 In each of the three biological systems, 0 2 binds to the ...