Answer:
Thioredoxin
Explanation:
Ribonucleotides serve as precursors for synthesis of deoxyribonucleotides and the reaction is catalyzed by enzyme ribonucleotide reductase. The reduction of a ribonucleotide into deoxyribonucleotide requires a pair of hydrogen atoms.
These hydrogen atoms are supplied from NADPH via ubiquitous protein thioredoxin. The oxidized form of thioredoxin is reduced back in a reaction catalyzed by thioredoxin reductase.