This study was designed to understand the mechanism and functional implication of the two heterozygous mutations (H391Y and K422R) of human pyruvate kinase M2 isozyme (PKM 2 ) observed earlier in a Bloom syndrome background. The co-expression of homotetrameric wild type and mutant PKM 2 in the cellular milieu resulting in the interaction between the two at the monomer level was substantiated further by in vitro experiments. The cross-monomer interaction significantly altered the oligomeric state of PKM 2 by favoring dimerization and heterotetramerization. In silico study provided an added support in showing that hetero-oligomerization was energetically favorable. The hetero-oligomeric populations of PKM 2 showed altered activity and affinity, and their expression resulted in an increased growth rate of Escherichia coli as well as mammalian cells, along with an increased rate of polyploidy. These features are known to be essential to tumor progression. This study provides insight in understanding the modulated role of large oligomeric multifunctional proteins such as PKM 2 by affecting cellular behavior, which is an essential observation to understand tumor sustenance and progression and to design therapeutic intervention in future.A variety of genetic diseases and experimental situations within the heterozygous state depict an allelic relationship where the mutant recessive allele overrides the function of its normal (wild type) dominant allele. This condition, referred to as dominant negative, is usually observed in the case of oligomeric or multidomain proteins with a possibility of cross-monomer interaction. Such mutant proteins by acting as competitive inhibitors of the normal protein function could generate polymorphic forms in a single cell. A phenomenon observed in collagen, where dominant negative mutations cause the production of abnormal oligomers (1, 2), and in transcription factors like helix-loop-helix and leucine zippers, where mutant monomers sequestering the function of wild type in a dimer bound to DNA, leads to an altered gene expression (3-5). Dominant negative mutations are also reported to affect some multifunctional molecules like p53 with differential impact on cell physiology (6 -10).Pyruvate kinase (EC 2.7.1.40) catalyzes irreversibly the transphosphorylation from P-enolpyruvate to ADP-generating pyruvate and ATP in glycolysis (11,12). Depending upon the differential metabolic requirements of the tissues, the enzyme is expressed in four different isoforms, L, R, M 1 , and M 2 in vertebrates (13). PKM 2 2 is a ubiquitous, prototype enzyme, present in all tissues during embryonic stage, and is gradually replaced by other isozymic forms in specific tissues, during development. It is necessary for cellular division irrespective of the type of tissue and reappears during cellular division and tumor formation (14 -17). PKM 2 is known to regulate its activity by switching between an active tetramer and inactive dimer form in a fructose 1,6-bisphosphate-dependent manner to shift the cellular met...