Ribonucleotide reductase is a heteromeric complex that maintains balanced deoxynucleoside triphosphate pools by reducing ribonucleoside diphosphates to deoxyribonucleoside diphosphates. The alpha/RR1/L subunit is the catalytic subunit, while the beta/RR2/S subunit houses the free radical required for catalysis. The complex is allosterically regulated by its activator ATP and its inhibitor dATP. (Adapted from PMID: 21336276.)