International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F, ch. 12.1, pp. 254-255   | 1 | 2 |

Section 12.1.6. Metal-ion replacement in metalloproteins

D. Carvin,a S. A. Islam,b M. J. E. Sternbergb and T. L. Blundellc*

aBiomolecular Modelling Laboratory, Imperial Cancer Research Fund, 44 Lincoln's Inn Field, London WC2A 3PX, England,bInstitute of Cancer Research, 44 Lincoln's Inn Fields, London WC2A 3PX, England, and cDepartment of Biochemistry, University of Cambridge, 80 Tennis Court Road, Cambridge, CB2 1GA, England
Correspondence e-mail:  tom@cryst.bioc.cam.ac.uk

12.1.6. Metal-ion replacement in metalloproteins

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The metal-ion cofactor can sometimes be displaced by dialysis or diffusion by a heavy-atom solution, but usually the cofactor is first removed by a chelating agent (e.g. EDTA) or by acidification. These are best carried out on the crystals. Alternatively, the metal can be substituted by biosynthesis of the metalloprotein under enriched conditions of the substituting metal, an approach which has been successful in displacing zinc with cobalt and other lighter metals.

The metal ions are best substituted by a metal of similar character and radius. Thus, calcium is an A-group metal which prefers ligands containing oxygen atoms that may originate from carboxylic, carboxyamide, hydroxyl, main-chain carbonyl groups and water molecules. Divalent alkaline earth metal ions (e.g. Sr2+, Ba2+) or trivalent lanthanide ions can bind at calcium sites but can give very different coordination geometry and stability. Nd3+ and Sm3+ can displace some Ca2+ ions with negligible change in structure (Fig. 12.1.6.1[link]). On the other hand, zinc has a relatively small ionic radius and is more polarizing. Structural zinc atoms are often tetrahedrally coordinated by cysteine residues, while those at active sites frequently bind histidine, often in association with a water molecule and/or carboxylate ligands. Cadmium or mercury can replace zinc, but often with a conformational change leading to lack of isomorphism.

[Figure 12.1.6.1]

Figure 12.1.6.1 | top | pdf |

The displacement of calcium by samarium in thermolysin. The samarium of the heavy-atom derivative is shown superposed on the parent crystal structure.








































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