Cytoplasmic serine hydroxymethyltransferase (cSHMT) is a tetrameric, pyridoxal phosphate (PLP)-dependent enzyme that catalyzes the reversible interconversion of serine and tetrahydrofolate to glycine and methylenetetrahydrofolate. The enzyme has four active sites and is best described as a dimer of obligate dimers. Each monomeric subunit within the obligate dimer contributes catalytically important amino acid residues to both active sites. To investigate the interchange of subunits among cSHMT tetramers, a dominant-negative human cSHMT enzyme (DNcSHMT) was engineered by making three amino acid substitutions: K257Q, Y82A, and Y83F. Purified recombinant DNcSHMT protein was catalytically inactive and did not bind 5-formyltetrahydrofolate. Coexpression of the cSHMT and DNcSHMT proteins in bacteria resulted in the formation of heterotetramers with a cSHMT/DNcSHMT subunit ratio of 1. Characterization of the cSHMT/DNcSHMT heterotetramers indicates that DNcSHMT and cSHMT monomers randomly associate to form tetramers and that cSHMT/DNcSHMT obligate dimers are catalytically inactive. Incubation of recombinant cSHMT protein with recombinant DNcSHMT protein did not result in the formation of hetero-oligomers, indicating that cSHMT subunits do not exchange once the tetramer is assembled. However, removal of the active site PLP cofactor does permit exchange of obligate dimers among preformed cSHMT and DNcSHMT tetramers, and the formation of heterotetramers from cSHMT and DNcSHMT homodimers does not affect the activity of the cSHMT homodimers. The results of these studies demonstrate that PLP inhibits dimer exchange among cSHMT tetramers and suggests that cellular PLP concentrations may influence the stability of cSHMT protein in vivo.