Abnormal enzyme activity in muscle tissue Reduced muscle 6-phosphofructokinase activity 2024-02-24T10:53:49Z Activity of the enzyme 6-phosphofructokinase in muscle tissue is below the lower limit of normal. 6-phosphofructokinase (EC 2.7.1.11) is also known as phosphofructokinase I and phosphohexokinase. The PFKM gene encodes the muscle isoform of phosphofructokinase. PFK catalyzes the irreversible conversion of fructose-6-phosphate to fructose-1,6-bisphosphate and is a key regulatory enzyme in glycolysis.