Pyruvate kinase sodium ions

Potassium is required for enzyme activity in a few special cases, the most widely studied example of which is the enzyme pyruvate kinase. In plants it is required for protein and starch synthesis. Potassium is also involved in water and nutrient transport within and into the plant, and has a role in photosynthesis. Although sodium and potassium are similar in their inoiganic chemical behavior, these ions are different in their physiological activities. In fact, their functions are often mutually antagonistic. For example, K+ increases both the respiration rate in muscle tissue and the rate of protein synthesis, whereas Na+ inhibits both processes (42). 

Potassium ions are required by a variety of enzymes (Suelter, 1970). These enzymes are activated by K to a greater extent than by Na. Pyruvate kinase, an enzyme of the glycolytic pathway, is the most well known K-requiring enz5one. It is activated by a variety of monovalent cations, as indicated in Table 10.7. Ammonium ions at 100 mM support catalytic activity however, the concentration of ammonium ions in the cell is under 1.0 mM. Sodium at 100 mM weakly supports activity. The concentration of Na in the ceU is only about 10 mM, indicating that the importance of Na in supporting enzyme activity in vivo is nil. Rubidium, which is chemically similar to K, supports activity however, Rb is not a physiological cation. 

Potassium ions activate pyruvate kinase its activity therefore reflects the potassium concentration. Interference by sodium is eliminated by binding to a kryptand, and that from ammonium ions by the addition of glutamate dehydrogenase. 

Whereas sodium participates in metabolism mainly by its cationic properties, potassium is more directly involved in metabolism. Potassium stimulates the activity of a specific enzyme— pyruvic kinase—and is required for the phosphorylation of fructose-1-phosphate to fructose-1,6-diphosphate. Similarly, potassium stimulates acetyl kinase activity. Many alterations in the bioenergetic pathways of the cell are accompanied by changes in the intracellular concentration of potassium. After insulin administration, some of the potassium of the extracellular fluid is transferred inside the cells. During oxidative phosphorylation, potassium accumulates inside the mitochondria, and dinitrophenol uncouples the ion penetration and the oxidation. 

---