Phosphofructokinase (PFK) is a tetrameric enzyme that catalyzes a key step in glycolysis, namely the conversion of D-fructose 6-phosphate to D-fructose 1,6-bisphosphate. PFK from muscle is a homotetramer of M subunit, PFK from liver is a homotetramer of L-subunits, while PFK from platelets can be composed of any tetrameric combination of M and L subunits. PFKL represents the L subunit.