Participates as a hydrogen donor in redox reactions through the reversible oxidation of its active center dithiol to a disulfide, accompanied by the transfer of 2 electrons and 2 protons. It is involved in many cellular processes, including deoxyribonucleotide synthesis, repair of oxidatively damaged proteins, protein folding, sulfur metabolism, and redox homeostasis. Thioredoxin-dependent enzymes include phosphoadenosine-phosphosulfate reductase MET16, alkyl-hydroperoxide reductase DOT5, thioredoxin peroxidases TSA1 and TSA2, alkyl hydroperoxide reductase AHP1, and peroxiredoxin HYR1. Thioredoxin is also involved in protection against reducing stress. As part of the LMA1 complex, it is involved in the facilitation of vesicle fusion such as homotypic vacuole and ER-derived COPII vesicle fusion with the Golgi. This activity does not require the redox mechanism. Monomer. Part of the heterodimeric LMA1 complex together with the proteinase inhibitor PBI2. Most of the thioredoxin of yeast is in this complex rather than the well-studied monomer. LMA1 binds to the ATPase SEC18. Reversible disulfide bond formation between Cys-30 and Cys-33, reverted by thioredoxin reductase TRR1 using NADPH as hydrogen donor. Present with 8579 molecules/cell in log phase SD medium. Yeast has two cytoplasmic thioredoxins, TRX1 and TRX2, and one mitochondrial, TRX3. Belongs to the thioredoxin family.