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Escherichia coli K 12 appears to behave as an enzyme. We show in the present paper that, in fact, phosphofructokinase 2 also presents some regulatory properties in vitro: at high concentrations, ATP is an inhibitor of phosphofructokinase 2 and it provokes the tetramerization of the dimeric native enzyme. The binding of the two substrates to phosphofructokinase 2 is sequential and ordered as for phosphofructokinase 1, but in the former case fructose 6-phosphate is the first substrate to be bound and ADP the first product to be released. Each dimer of phosphofructokinase 2 binds two molecules of fructose 6-phosphate but only one molecule of the product fructose 1,6-bisphosphate. Although both phosphofructokinases of E. coli K 12 present regulatory properties in vitro, the mechanism of regulation of the activity of the two enzymes is strikingly different. It can be asked whether or not these mechanisms operate in vivo. [ABSTRACT FROM AUTHOR]