Metalloproteins geometry

Fig. 10. Examples of coordination geometry in the entatic state (34) where the Lewis acidity of Zn(II) in its metalloprotein is considerably altered as compared with that for (a) the Zn2t aquo ion (30), (b) Zn(II) in alcohol dehydrogenase, (c) Zn(II) in carbonic anhydrase, and (d) Zn(II) in carboxypeptidase. Redrawn after Ref. (22).

As Ti is incorporated in the silicate lattice, the volume of the unit cell expands (consistent with the flexible geometry of the ZSM-5 lattice) (75), but beyond a certain limit, it cannot expand further, and Ti is ejected from the framework, forming extraframework Ti species. Although no theoretical value exists for such a maximum limit in such small crystals, it depends on the type of silicate structure (MFI, beta, MCM, mordenite, Y, etc.) and the extent of defects therein, the latter depending to a limited extent on the preparation procedure. Because of the metastable positions of Ti ions in such locations, they can expand their geometry and coordination number when required (for example, in the presence of adsorbates such as H20, NH3, H2O2, etc.). Such an expansion in coordination number has, indeed, been observed recently (see Section II.B.2). The strain imposed on such 5- and 6-fold coordinated Ti ions by the demand of the framework for four bonds with tetrahedral orientation may possibly account for their remarkable catalytic properties. In fact, the protein moiety in certain metalloproteins imposes such a strain on the active metal center leading to their extraordinary catalytic properties (76). 

The first article in this volume, by Jenny P. dusker, treats general aspects of metal liganding to functional groups in proteins. This article presents a detailed summary of the geometry of interaction of metals with the various chemical groups of proteins. It also presents, in Sections I through VIII, a lucid development of the principles and terminology of the field of metal-protein interactions. It is with these sections that the newcomer to the field of metalloproteins should start. 

The particular values and power dependence for the d-s mixing term are also not too critical although a certain threshold must be achieved. Tetragonally elongated Jahn-Teller distortions of d9 CuNe species (36) and the trigonal geometry of the oxidized copper center in Type 1 metalloproteins (37) can be achieved with an inverse sixth order power dependence and an associated a6 parameter of at least 300,000 kcal mol-1 A6. However, since eds also depends on symmetry—e.g., it makes no contribution for octahedral complexes—there are many systems where d-s mixing has a minimal effect. 

NMR-active " Cd ions were employed as a surrogate probe for Zn Indeed, " Cd NMR has been widely employed in the spectroscopic study of metalloproteins which bear Zn(II) in their native state. The adaptable ligand coordination, number and geometry of Cd(II) are rather similar to Zn(II) and in many cases where Cd(II) has replaced Zn(II), the catalytic activity of the metaUoenzymes has been retained even to a low extent . 

The solution structures of a number of metalloproteins with paramagnetic metal centers were determined with molecular mechanics and dynamics in combination with NMR spectroscopy (see also Chapter 9)1124-126,189]. Due to the complexity of the molecules, for metalloproteins a crystal structure of the compound or a derivative is often needed for the definition of the starting geometry. Molecular dynamics is then used to find low-eneigy conformers. The dynamics calculations also allow the visualization of areas of large flexibility, and this may lead to some understanding of the enzyme mechanism11891. 

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