Molecule of the Month: Pyruvate Kinase M2

Cancer cells make many radical changes to support their uncontrolled growth. Surprisingly, one of these changes is reprogramming energy production pathways. In normal cells, conversion of glucose to energy begins with glycolysis in the cell cytoplasm. Pyruvate produced at the end of glycolysis normally enters the mitochondria for the citric acid cycle. In the presence of oxygen this pathway powers aerobic energy production through the action of the electron transfer chain (Complex I, III, and IV) and ATP synthase. Cancer cells take in large amounts of glucose, but paradoxically block the last step of glycolysis. Instead of entering the mitochondria, pyruvate that is made in these cells is converted to lactate.

The last step in glycolysis is catalyzed by the enzyme pyruvate kinase (PDB ID 4fxf). It catalyzes the transfer of a phosphate group from phosphoenolpyruvate to form ATP (the energy currency of cells) and pyruvate. Since it is vital for cell survival, the activity of pyruvate kinase is highly regulated. The enzyme forms two types of assemblies: a high-activity tetramer and a low-activity dimer. If more energy is required, fructose bisphosphate (a molecule formed earlier in glycolysis) promotes tetramer formation. A variety of other regulatory molecules can bind to specific allosteric sites to promote dimer formation, and decrease enzyme activity. Four different types of tissue-specific pyruvate kinases are made in the body: PKM1 is made by cells that need a lot of energy, PKR is found in red blood cells, PKL in liver cells, while PKM2 is normally made by embryonic and other proliferating cells. The PKM1 enzyme is commonly in its tetrameric form and is constitutively active. However, the PKM2 enzyme binds to many allosteric regulators to affect its tetramer-to-dimer transition and regulate its action. Cancer cells take advantage of this regulation to reprogram their own metabolism.

Many cancer cells produce high levels of the PKM2 enzyme, shifting their energy production pathways to glycolysis. This metabolic reprogramming is paradoxical for two reasons - firstly these cells do not perform aerobic energy production, even when oxygen is plentiful; secondly, even though these rapidly dividing cells require more energy than normal cells, they use glycolysis, a markedly less energy-efficient process, as the major source of energy. The solution to this paradox lies in another consequence of the metabolic bottleneck created by slowing PKM2: it causes a build up of various metabolic intermediates from previous steps in glycolysis. These intermediates are directed to many biosynthetic pathways, allowing cancer cells to build protein and nucleic acid building blocks needed for rapid growth. Thus, cancer cells prioritize growth over energy production. This reprogramming of glucose utilization is referred to as the Warburg Effect, named after Otto Heinrich Warburg, the researcher who discovered it.

The Warburg Effect is a common hallmark of tumorigenesis. Since its discovery, researchers have found that PKM2 is phosphorylated and acetylated in cancerous cells (PDB entry 6wp3). These modifications affect the binding sites of activators such as fructose bisphosphate. Without these activators, PKM2 cannot efficiently form the high-activity tetramers. The dimeric form of the enzyme is not only less active in glycolysis but can also move into the nucleus to promote the transcription of other cancer-causing genes. These changes in turn cause a buildup of lactate that supports tumor progression and resistance to cancer therapies. Since PKM2 plays key roles in reprogramming cancer cells, scientists are now looking for drugs and therapies to block it or devise other ways to activate aerobic metabolism.