Adaptive metabolic reprogramming gives cancer cells a proliferative advantage. 
Tumour cells extensively use glycolysis to sustain anabolism and produce serine, 
which not only refuels the one-carbon units necessary for the synthesis of 
nucleotide precursors and for DNA methylation, but also affects the cellular 
redox homeostasis. Given its central role in serine metabolism, serine 
hydroxymethyltransferase (SHMT), a pyridoxal 5'-phosphate (PLP)-dependent 
enzyme, is an attractive target for tumour chemotherapy. In humans, the 
cytosolic isoform (SHMT1) and the mitochondrial isoform (SHMT2) have distinct 
cellular roles, but high sequence identity and comparable catalytic properties, 
which may complicate development of successful therapeutic strategies. Here, we 
investigated how binding of the cofactor PLP controls the oligomeric state of 
the human isoforms. The fact that eukaryotic SHMTs are tetrameric proteins while 
bacterial SHMTs function as dimers may suggest that the quaternary assembly in 
eukaryotes provides an advantage to fine-tune SHMT function and differentially 
regulate intertwined metabolic fluxes, and may provide a tool to address the 
specificity problem. We determined the crystal structure of SHMT2, and compared 
it to the apo-enzyme structure, showing that PLP binding triggers a 
disorder-to-order transition accompanied by a large rigid-body movement of the 
two cofactor-binding domains. Moreover, we demonstrated that SHMT1 exists in 
solution as a tetramer, both in the absence and presence of PLP, while SHMT2 
undergoes a dimer-to-tetramer transition upon PLP binding. These findings 
indicate an unexpected structural difference between the two human SHMT 
isoforms, which opens new perspectives for understanding their differing 
behaviours, roles or regulation mechanisms in response to PLP availability 
in vivo.