Calcium structural distribution

This chapter will examine the properties (structure, distribution), pharmacology (openers and blockers as tools to study K+ channels) of calcium and potassium channels. Emphasis will be on integration of the information to develop a picture of the physiological roles of the different types of ion channels. Since potassium channels regulate arterial smooth muscle function by controlling membrane potential, the membrane potential and its relationship to smooth muscle function will be discussed first, as well as the role that K" channels play in controlling membrane potential. 

In striated muscle, there are two other proteins that are minor in terms of their mass but important in terms of their function. Tropomyosin is a fibrous molecule that consists of two chains, alpha and beta, that attach to F-actin in the groove between its filaments (Figure 49-3). Tropomyosin is present in all muscular and muscle-fike structures. The troponin complex is unique to striated muscle and consists of three polypeptides. Troponin T (TpT) binds to tropomyosin as well as to the other two troponin components. Troponin I (Tpl) inhibits the F-actin-myosin interaction and also binds to the other components of troponin. Troponin C (TpC) is a calcium-binding polypeptide that is structurally and functionally analogous to calmodulin, an important calcium-binding protein widely distributed in nature. Four molecules of calcium ion are bound per molecule of troponin C or calmodulin, and both molecules have a molecular mass of 17 kDa. 

Fig. 31. Fractal structures obtained experimentally at different stages of aggregation of a colloidal monolayer of 1 /im sulfonated polystyrene particles on the surface of an aqueous calcium chloride solution, initially uniformly distributed (Robinson and Earnshaw, 1992).

Ion channels are large proteins which form pores through the neuronal membrane. The precise structure and function of the ion channels depend on their physiological function and distribution along the dendrites and cell body. These include specialized neurotransmitter-sensitive receptor channels. In addition, some ion channels are activated by specific metal ions such as sodium or calcium. The structure of the voltage-dependent sodium channel has been shown to consist of a complex protein with both a hydrophilic and a hydrophobic domain, the former domain occurring within the neuronal membrane while the latter domain occurs both inside and outside the neuronal membrane. 

The interaction of Ca2+ with pectins is discussed. The role of carboxylic acid salt formation and the degree of polymerization are first considered in terms of electrostatic and/or cooperative specific interactions. Then the effect of the degree of esterification and that of the pattern of carboxylic group distribution are discussed pectin esterase forms blocks which behave as fully hydrolyzed polymers and favor aggregation. Finally, the role of the calcium addition on the degree of aggregation was established. All the data show the important role of molecular structure of the pectins on calcium interactions. 

As stated above, in the stoichiometric FA structure, calcium ions are distributed into two crystallographic sites. Ca(l) sites, often referred to as columnar sites, are spaced by one half of the c-axis parameter, along the three-fold axes. Each Ca (l)2+ ion is nine-fold coordinated to oxygen ions, three oxygen atoms being slightly... 

Compounds of this homologous series with n > 1 exist and have been analyzed by neutron diffraction methods. More specifically, precise structural determinations of La gCa OujOg (27)(28) and La19Sr11Cu2Oe (27) have been carried out and have shown that the sequence of layers, their composition, and the general distribution of the atoms in the unit cell are entirely consistent with the scheme of Figure 9. The structural parameters of the calcium compound are given in Table 8. We may write the sequence of layers for these two compounds as... 

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