Structural role

Leucine residues 2, 5, 7, 12, 20, and 24 of the motif are invariant in both type A and type B repeats of the ribonuclease inhibitor. An examination of more than 500 tandem repeats from 68 different proteins has shown that residues 20 and 24 can be other hydrophobic residues, whereas the remaining four leucine residues are present in all repeats. On the basis of the crystal structure of the ribonuclease inhibitor and the important structural role of these leucine residues, it has been possible to construct plausible structural models of several other proteins with leucine-rich motifs, such as the extracellular domains of the thyrotropin and gonadotropin receptors. 

Although exopolysaccharides do not normally have a structural role, they do form structures that can be detected by either light or electron microscopy. Exopolysaccharides may form part of a capsule closely attached to the microbial cell surface, or appear as loose slime secreted by the cell but not directly attached to it mucoid Exopolysaccharide producing cells usually form mucoid colonies on solid media and colonies liquid cultures of these cells may become very viscous. However, growth conditions can... 

The concept of silicates as inorganic polymers was implicit in the ideas developed by W. H. Zacheriasen in the early 1930s. He conceived of silicates as consisting of macromolecular structures held together by covalent bonds but including network-dwelling cations. These cations were not assumed to have a structural role but merely to be present in order to balance the charges on the anionic polymer network. 

Hyaluronic acid is a linear polysaccharide found in the highest concentrations in soft connective tissues where it fills an important structural role in the organization of the extracellular matrix (23,24). It has been used in ophthalmic preparations to enhance ocular absorption of timolol, a beta blocker used for the treatment of glaucoma (25), and in a viscoelastic tear formulation for conjunctivitis (26). The covalent binding of adriamycin and daunomycin to sodium hy-aluronate to produce water-soluble conjugates was recently reported (27). 

In some species, onion (2), tomato, and sugar beet (13), the interface regions between cells, ie the middle lamella and the cell corners, are rich in relatively unesterified pectins which may function in cell-cell adhesion and play an important structural role in tissue integrity. Cell corners, in particular, may act as joists in the scaffolding function of the wall, bearing much of the mechanical load of the tissue (Jeronomidis, pers. comm.). In Zinnia leaves, although all of the cell-walls contain methyl-esterified pectin. 

The precise structural role played by the water molecules in these cements is not clear. In the zinc oxychloride cement, water is known to be thermally labile. The 1 1 2 phase will lose half of its constituent water at about 230 °C, and the 4 1 5 phase will lose water at approximately 160 C to yield a mixture of zinc oxide and the 1 1 2 phase. Water clearly occurs in these cements as discrete molecules, which presumably coordinate to the metal ions in the cements in the way described previously. However, the possible complexities of structure for these systems, which may include chlorine atoms in bridging positions between pairs of metal atoms, make it impossible to suggest with any degree of confidence which chemical species or what structural units are likely to be present in such cements. One is left with the rather inadequate chemical descriptions of the phases used in even the relatively recent original literature on these materials, from which no clear information on the role of water can be deduced. 

Rabinovich, E. M. (1983). On the structural role of fluorine in silicate glasses. Journal of Physical Chemistry, 24, 54-6. 

The recruitment of zinc for a structural role, or to activate an enzyme, has been observed. The zinc ion induces the dimerization of human growth hormone (hGH), with two Zn ions associated per dimer of hGH. This is confirmed by replacement of possible zinc binding residues resulting in weakened binding of the zinc ion. Formation of a zinc-hGH dimeric complex may be important for storage of hGH in secretory granules.975 In a toxic role, anthrax lethal factor is one of the three components of the secreted toxin and is a zinc-dependent protease that cleaves a protein kinase and causes lysis of macrophages.976... 

Zinc is a constituent of over 300 enzymes with much research into the coordination of zinc to the protein backbone, and how its chemistry is modulated by the donor set and environment.2 As well as the Lewis acid catalysis properties in enzymes, the structural role in zinc finger proteins has been a major area of research since the late 1990s. A number of reviews on zinc physiology, enzymology, and proteins in general have been published.978-981 There is extensive analysis available to classify the mononuclear sites in zinc proteins and identification of structural relationships of the extended environment.982,983... 

As mentioned at the end of Section 6.9.3, reviews of the crystal chemistry of mercury minerals with lower oxidation states and with oxo-centered building blocks have been published72,73 and the structural role of Hg22+ and Hg34+ groups discussed.407... 

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