UDP-glucose pyrophosphorylase (UGP) occupies a central position in carbohydrate metabolism in all kingdoms of life, since its product UDP-glucose is essential in a number of anabolic and catabolic pathways and is a precursor for other sugar nucleotides. Its significance as a virulence factor in protists and bacteria has given momentum to the search for species-specific inhibitors. These attempts are, however, hampered by high structural conservation of the active site architecture. A feature discriminating UGPs of different species is the quaternary organization. While UGPs in protists are monomers, di- and tetrameric forms exist in bacteria, and crystal structures obtained for the enzyme from yeast and human identified octameric UGPs. These octamers are formed by contacts between highly conserved amino acids in the C-terminal β-helix. Still under debate is the question if octamerization is required for functionality of the human enzyme. Here we used single amino acid replacements in the C-terminal β-helix to interrogate the impact of highly conserved residues on octamer formation and functional activity of human UGP. Replacements were guided by the sequence of Arabidopsis thaliana UGP, known to be active as a monomer. Correlating the data obtained in Blue Native PAGE, size exclusion chromatography and enzymatic activity testing, we prove that the octamer is the active enzyme form. This new insight into structure-function-relationships in human UGP does not only improve the understanding of the catalysis of this important enzyme, but in addition broadens the basis for studies aimed at designing drugs that selectively inhibit UGPs from pathogens.