Phosphofructokinase (PFK) deficiency due to a catalytically inactive mutant M-type subunit

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• Phosphofructokinase (PFK) is a key enzyme in glycolysis, catalyzing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate.
• M-type PFK is primarily found in muscle and heart tissue, playing a crucial role in energy metabolism.
• Deficiency in PFK can lead to impaired glycolysis and various clinical manifestations, particularly affecting muscle function.

Observation:

• A patient presented with M-type PFK deficiency characterized by reduced PFK activity in red blood cells (39% of normal) and absent activity in muscle tissue.
• Leukocytes and platelets exhibited normal PFK activity.
• The red cell enzyme showed normal biochemical properties and was immunologically distinct from the muscle PFK, suggesting a specific defect in the M-isoenzyme.

Findings:

• The muscle M-subunit of PFK was found to be structurally abnormal and catalytically inactive, despite being present and immunologically identical to the normal enzyme.
• Histological preparations, muscle extracts, electromyography, ischemic exercise testing, histochemistry, and electron microscopy all confirmed the complete absence of PFK activity in muscle.
• Myoblast cultures derived from the patient also showed no detectable PFK activity, even after prolonged cultivation.

Implications:

• This case highlights a novel mechanism of M-type PFK deficiency involving a catalytically inactive but immunologically present enzyme subunit.
• Understanding the molecular basis of this deficiency can aid in diagnosing and potentially treating glycogen storage diseases and other metabolic myopathies.
• Further research into the structure-function relationship of M-type PFK may reveal new therapeutic targets for energy metabolism disorders.