212 an x ray study of the cyanate and cyanide

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212. An x-ray study of the cyanate and cyanide derivatives of the native enzyme has shown that the anions sit in the cavity without binding to the metal ion (M. Lindahl, L.A. Svensson, and A. Liljas, Proteins 15 (1993), 177). Since NCO- has been shown to interact with the paramagnetic cobalt(II) center, and l3C-enriched cyanide has been shown to interact with 67Zn-substituted CA (see Reference 67), it appears that the structures in the solid state and solution are strikingly different. 213. Recent x-ray data on the adduct of 1,2,4-triazole with HCA II confirm H-bonding with Thr-200 (S. Mangani and A. Liljas, 1. Mol. Bioi. 232 (1993), 9). 214. An HCOj-complex of the His-200 mutant of HCA II has been studied by x-ray methods. The data are consistent with the coordinated oxygen being protonated and H-bonded to Thr-199 (Y. Xue et al., Proteins 15 (1993), 80).
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3 Calcium in Biological Systems STURE FORSEN AND JOHAN KORDEL Physical Chemistry 2, Chemical Centre, University of Lund I. INTRODUCTION Calcium, like many other "inorganic elements" in biological systems, has dur- ing the last decade become the subject of much attention both by scientists and by the general public. 1 The presence and central role of calcium in mammalian bones and other mineralized tissues were recognized soon after its discovery as an element by Davy in 1808. Much later, the insight arrived that Ca 2 + ions could play an important role in other tissues as well. Experiments of great his- torical influence were performed by the British physiologist Sidney Ringer a little over a century ago. 2 He was interested in the effects of various cations on frog-heart muscle and somewhat serendipitously discovered that Ca 2+ ions, ev- erpresent in the tap water distributed in central London, in millimolar concen- trations were necessary for muscle contraction and tissue survival. Today it is widely recognized that Ca 2 + ions are central to a complex intra- cellular messenger system that is mediating a wide range of biological pro- cesses: muscle contraction, secretion, glycolysis and gluconeogenesis, ion transport, cell division and growth (for definitions of terms in boldface, see Appendix A in Section IX). The detailed organization of this messenger system is presently the subject of considerable scientific activity, and some details are already known. One of the links in the system is a class of highly homologous Ca 2+ -binding proteins, to be discussed later on in this chapter, that undergo Ca 2+ -dependent conformational changes and respond to transitory increases in intracellular Ca 2+ -ion concentrations. A prerequisite for the proper function of the calcium messenger system in higher organisms is that the cytosolic Ca 2+ concentration in a "resting" cell be kept very low, on the order of 100 to 200 nM. Transitory increases in the Ca 2 + concentration that may result from hormonal action on a membrane receptor must rapidly be reduced. Several transport proteins, driven either by ATP hydrolysis or by gradients of some other ion like Na +, are involved in this activity. 107
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108 3 / CALCIUM IN BIOLOGICAL SYSTEMS Ca 2+ ions are also known to play various roles outside cells. In the plant
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