Enzymes alkali metal ions

PEDERSEN, CHARLES JOHN (1904-1989). An American bom in Korea, Pedersen received a M.S. from M.I.T. in 1927. In 1987 he was awarded the Nobel Prize for Chemistry for his work in elucidating mechanisms of molecular recognition, which are fundamental to the enzymic catalysis, regulation and transport. He reported that alkali metal ions could be bonnd by crown ethers into a more rigid, layered structure, in which the alkali metal ion was bound into the center of the ring. This field of study is called host-guest chemistry,... 

The structural role of iron to be discussed here is, in general, somewhat broader than that already described in the literature for Ca2+, Mg2+, and the light alkali metal ions (9—11). These cations appear to function mainly as effectors of enzyme activity, either by modulating conformational changes of small amplitude that regulate the affinity of the substrate for the active site or, more directly, by bridging the substrate to the enzyme. These proteins possibly are already in a native state, even in the absence of the metal ion. By contrast, and with the possible exception of Mn2+, in most cases heavy metal ions are necessary constituents to hold the structure of the metalloprotein in a conformation that is close, if not identical, to that of the active species. This role is in addition to whatever action the heavy ion may play in the catalytic event itself. 

Whittam (1962a) and Whittam and Ager (1962) have shown that ATP hydrolysis within the erythrocyte ghost is stimulated by ions in the medium and by Na+ ions inside the ghost. The effect of Na" " on ATPase depends on the sodium localization in relation to the inner and outer surface of the membrane, since, at low external potassium concentrations, external sodium decreases the activity of the enzyme. Thus, the competitive inhibition of K+ by Na" externally opposes the synergistic effect of internal Na and external upon the activity of ATPase. Also, some alkali metal ions, such as lithium, rubidium, and cesium, show synergistic and directional stimulation of ATP breakdown i.e., internal lithium and sodium, as well as external lithium, potassium, rubidium, and cesium, stimulate ATP hydrolysis, while internal potassium, rubidium, and cesium or external sodium do not (Whittam, 1962b). 

The use of alkali and alkaline earth group metal ions, especially those of sodium, potassium, magnesium, and calcium, for maintenance of electrolyte balance and for signaling and promotion of enzyme activity and protein function are not discussed in this text. Many of these ions, used for signaling purposes in the exciting area of neuroscience, are of great interest. In ribozymes, RNAs with catalytic activity, solvated magnesium ions stabilize complex secondary and tertiary molecular structure. Telomeres, sequences of DNA at the ends of chromosomes that are implicated in cell death or immortalization, require potassium ions for structural stabilization. 

Among potentiometric enzyme sensors, the urea enzyme electrode is the oldest (and the most important). The original version consisted of an enzyme layer immobilized in a polyacrylamide hydrophilic gel and fixed in a nylon netting attached to a Beckman 39137 glass electrode, sensitive to the alkali metal and NHj ions [19, 2A Because of the poor selectivity of this glass electrode, later versions contained a nonactin electrode [20,22] (cf. p. 187) and especially an ammonia gas probe [25] (cf. p. 72). This type of urea electrode is suitable for the determination of urea in blood and serum, at concentrations from 5 to 0.05 mM. Figure 8.2 shows the dependence of the electrode response... 

Interaction of alkali metals with naturally-occurring compounds is of interest in connection with the active transport of these ions and of small molecules across cell walls, and with the functioning of sodium and/or potassium activated enzymes. 

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