Provides the precursors necessary for DNA synthesis. Catalyzes the biosynthesis of deoxyribonucleotides from the corresponding ribonucleotides. [thioredoxin]-disulfide + a 2'-deoxyribonucleoside 5'-diphosphate + H2O = [thioredoxin]-dithiol + a ribonucleoside 5'-diphosphate Under complex allosteric control mediated by deoxynucleoside triphosphates and ATP binding to separate specificity and activation sites on the M1 subunit. The type of nucleotide bound at the specificity site determines substrate preference. It seems probable that ATP makes the enzyme reduce CDP and UDP, dGTP favors ADP reduction and dTTP favors GDP reduction. Stimulated by ATP and inhibited by dATP binding to the activity site, the dATP inhibition is mediated by AHCYL1 which stabilizes dATP in the site (By similarity). Genetic information processing; DNA replication. Heterodimer of a large and a small subunit. Interacts with RRM2B. Interacts with AHCYL1 which inhibits its activity (By similarity). Two distinct regulatory sites have been defined: the specificity site, which controls substrate specificity, and the activity site which regulates overall catalytic activity. A substrate-binding catalytic site, located on M1, is formed only in the presence of the second subunit M2. The level of the enzyme activity is closely correlated with the growth rate of a cell and appears to vary with the cell cycle. Belongs to the ribonucleoside diphosphate reductase large chain family.