Potassium Pyruvic kinase

Facilitated diffusion within organisms takes place when carriers or proteins residing within membranes—ion channels, for instance—organize the movement of ions from one location to another. This diffusion type is a kinetic, not thermodynamic, effect in which a for the transfer is lowered and the rate of diffusion is accelerated. Facilitated diffusion channels organize ion movements in both directions, and the process can be inhibited both competitively and noncompetitively. It is known that most cells maintain open channels for K+ most of the time and closed channels for other ions. Potassium-ion-dependent enzymes include NaVK+ ATPases (to be discussed in Section 5.4.1), pyruvate kinases, and dioldehydratases (not to be discussed further). 

Figure 12-20 Equilibria in pyruvate kinase reaction as studied by 31P NMR at 40.3 MHz, pH 8.0,15°C. (A-C) Equilibria with low enzyme in levels 15% 2H20. (A) nP NMR spectrum of 1.5 ml of reaction mixture PEP, 13.3 mM ADP, 14.1 mM MgCl2, 20 mM potassium Hepes buffer, 100 mM KC, 50 mM without enzyme. (B) Equilibrium mixture after the addition of 1 mg of pyruvate kinase to the reaction mixture. (C) Equilibrium after the addition of potassium pyruvate (final concentration of 200 mM) to the sample of the spectrum in (B). (D,E) Equilibrium with enzyme concentrations in excess of the substrates. Sample volumes 1.1 ml with 10% 2H20. (D) Equilibrium mixture set up with enzyme (2.8 mM active sites) 2.8 mM PEP 2.4 mM ADP 5.7 mM MgCl2 100 mM potassium Hepes 100 mM KC1. (E) Spectrum after the addition of 50 pi of 400 mM EDTA (pH readjusted to 8.0) to the sample of spectrum D. The EDTA removes metal ions, stopping the catalytic reactions and sharpening the resonances. From Nageswara Rao et al.685...

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). 

Cation-activated enzymes are those that require a cation for maximal activity but for which that cation is generally not a direct component of the enzyme reaction mechanism. They were originally identified by Evans and Sorger in a study of activation of enzymes by monovalent metal cations. These activating cations convert the enzyme to a form with enhanced catalytic activity. When a cation is part of the enzyme mechanism, its requirement is essential and no reaction occurs in its absence. On the other hand, if the enzyme is cation-activated, the presence of the cation merely increases the activity of the enzyme from a finite value, that is, the enzyme generally works (but not as well) in the absence of the activating cation. A simple example, to be described here, is provided by the activation of pyruvate kinase by potassium ions that bind 6-8 A from the active site of the enzyme. Reviews on aspects of the subject of cation-activated enzymes by Suelter, Woehl and Dunn," and Larsen and Reed are highly recommended. 

D-7) Pyruvate kinase deficiency. The cell can not produce the ATP that normally is pr uced during this reaction and is needed for the cell s sodium/potassium pump. There is a hemolytic anemia. The diagnosis can be made by assay for pyruvate kinase in red blood cells. The inheritance is autosomal recessive. 

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 is a cofactor and activates a large variety of enzymes, including glycerol dehydrogenase, pyruvate kinase, L-threonine dehydrase, and ATPase. Its acute toxicity is primarily due to its action as an electrolyte. Excessive or diminished potassium levels can disrupt membrane excitability and influence muscle cell contractility and neuronal excitability. 

Sodium and lithium are potent inhibitors of pyruvate kinase, for which potassium is an obligatory activator. 

The function of magnesium in enzyme activity may either be to form a complex with the substrate, as in the magnesium-ATP complex formed in creatine kinase and phosphofructokinase, or to bind to the enzyme and either produce an allosteric activation or play a direct role in catalysis. If an enzyme is known to utilize a nucleotide as one of its substrates, it can be assumed that magnesium is also required for catalysis. The magnesium ion possibly acts as an electrostatic shield. The enzyme pyruvate kinase, described earlier, and shown in Figure 1, requires both magnesium and potassium ions for maximal activity. 

One of the best studied enzymes involving phosphoryl group transfer is pyruvate kinase, which catalyzes the transfer of a phosphoryl group from phosphoenolpyruvate to ADP. Pyruvate kinase requires a mono- and a di-valent cation for activation, and will be discussed in more detail in Section 62.1.3. It is well known that is the most effective monovalent activator. Figure 7 shows how the activity of the enzyme varies with the ionic radius of the monovalent cation. It is clear that NH4 (1.43 A), Rb (1.47 A) and Tl (1.47 A) are able to replace (1.33 A), while (0.68 A), Na" (0.97 A) and Cs" (1.67 A) are ineffective. Studies in which has been replaced by Tl have been most informative. Tl has an affinity some 50 times greater than for the potassium site, and so its binding could be detected more readily by equilibrium dialysis techniques. This showed that four TT cations were bound per molecule of enzyme, which has four subunits. 

The use of thallium-205 as a probe for potassium has been suggested, - and Kayne and Reuben have used the broadening between the signals of the two paramagnetic ions thallium(i) and manganese(n) to estimate the distance between the monovalent and bivalent activators in rabbit muscle pyruvate kinase. Several cases of the inhibition of Ca + transport processes by lanthanides have been reported, and Williams and co-workers have proposed the use of the lanthanides as probes for calcium. 

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. 

A reversible reaction catalyzes the conversion of pyruvate to phosphopyruvate, and the enzyme involved is pyruvic kinase. The equilibrium of that reaction is on the side of the formation of ATP. Thus, pyruvate kinase is the enzyme responsible for the conversion of phosphoenolpyruvate to pyruvate. The enzyme has been crystallized from muscle it requires ADP, potassium, and magnesium and is noncompetitively inhibited by some estrogenic steroids. Steroids alter the enzyme s viscosity and electrophoretic properties. From this observation, it was assumed that steroids act by modifying the protein molecule. 

An increase in red cell membrane permeability to sodium and potassium has been described in a number of red cell disorders, including hereditary spherocytosis and pyruvic kinase deficiency. In hereditary spherocytosis, the rate of entry of radioactive sodium into red cells is greater than normal, but potassium... 

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. 

R)- and (S)-3,3,3-trifluoro-2-hydroxy-2-methylpropionic acid (Fig. 4) are intermediates for the synthesis of a number of potential pharmaceuticals, which include ATP-sensitive potassium channel openers for the treatment of incontinence [11], and inhibitors of pyruvate dehydrogenase kinase for the treatment of diabetes [12]. 

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