Entatic state

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).

Anyhow, our study has demonstrated the benefit of "strained" polymeric catalyst-substrate complexes, a phenomenon well-known in enzymology (26) and once indicated by the term "entatic state" (16). 

A hypothetical conformational state defined by a geometrically and electronically strained site within an enzyme thought to facilitate the conversion of an enzyme-substrate complex to the transition state. Vallee and Williams defined the entatic state as an abnormal condition of localized strain transmitted by relief of compression or other steric clashes elsewhere in the enzyme. They suggested that catalytic rate enhancements arise from the heightened reactivity of catalytic group(s) that have experienced unimolecular activation. Williams ... 

Ensemble average fluctuation with time, CORRELATION FUNCTION ENTATIC STATE ENTERING GROUP ENTEROPEPTIDASE ENTEROTOXIN ENTHALPY... 

Table I lists isomorphous replacements for various metalloproteins. Consider zinc enzymes, most of which contain the metal ion firmly bound. The diamagnetic, colorless zinc atom contributes very little to those physical properties that can be used to study the enzymes. Thus it has become conventional to replace this metal by a different metal that has the required physical properties (see below) and as far as is possible maintains the same activity. Although this aim may be achieved to a high degree of approximation [e.g., replacement of zinc by cobalt(II)], no such replacement is ever exact. This stresses the extreme degree of biological specificity. The action of an enzyme depends precisely on the exact metal it uses, stressing again the peculiarity of biological action associated with the idiosyncratic nature of active sites. (The entatic state of the metal ion is an essential part of this peculiarity.) Despite this specificity, the replacement method has provided a wealth of information about proteins that could not have been obtained by other methods. Clearly, there will often be a compromise in the choice of replacement. Even isomorphous replacement that should retain structure will not necessarily retain activity at all. However, it has become clear that substitutions can be made for structural studies where the substituted protein is inactive (e.g., in the copper proteins and the iron-sulfur proteins). It is also possible to substitute into metal coenzymes. Many studies have been reported of the...

The mechanism of action of lysozyme has been suggested to involve strain in the substrate as a result of binding to the protein.31 The protein requires that the substrate binds to the protein in a distorted state that is close to the transition state of the reaction. The energy for this distortion can arise from the substrate binding energy. This example differs from the entatic state hypothesis because this strain arises from substrate binding and is not a feature of the protein structure. However, the entatic state... 

There is a third region of a protein that is neither on the surface nor in the interior but that is in a cleft. Such regions are often associated with enzyme action and examples show they have (1) intermediate mobility (e.g., tryptophan 62 of lysozyme or the tyrosine of carboxypeptidase) and (2) unfavorable energetics of exposed groups—the entatic state. 

Note that the entatic state differs from the situation where the substrate itself is strained by virtue of binding to the protein. Such a case is also found in lysozyme, where the energy of holding the substrate (a saccharide) is used partly to distort the substrate toward its transition state, thus lowering the activation energy. 

I have a question for Professor Williams regarding the concept of entasis [B. L. Vallee and R. J. P. Williams, Proc. Nat. Acad. Sci. (U.S.) 59, 498 (1968)]. Is it possible that many enzymes in structured regimes in vivo exist in an entatic state, and that such states collapse (e.g., the protein folds up) on extraction from the cell ... 

If our postulates are correct the most interesting feature of P-450 is the manner in which the protein has adjusted the coordination geometry of the iron and then provided near-neighbour reactive groups to take advantage of the activation generated by the curious coordination. Vallee and Williams (68) have observed this situation in zinc, copper and iron enzymes and referred to it as an entatic state of the protein. It is also apparent that some such adjustment of the coordination of cobalt occurs in the vitamin B12 dependent enzymes. As a final example we have looked at the absorption spectra of chlorophyll for its spectrum is in many respects very like that of a metal-porphyrin. This last note is intended to stress the features of chlorophyll chemistry which parallel those of P-450. 

Each molecule (molecular weight 30000) contains one zinc(II) atom, which is (approximately) tetrahedrally coordinated to two N atoms and one O atom from amino-acid residues plus a water molecule. The structures of both the enzyme and some enzyme-substrate complexes have been carefully studied and the detailed mechanism of the hydrolysis is now quite well understood. Without going into details, a crucial factor appears to be the pronounced distortion from regular tetrahedral coordination about the Zn(II), apparently imposed by the conformational requirements of the polypeptide chain. The conflict of interest between the needs of the Zn(II) atom - which, when four-coordinate, always assumes tetrahedral coordination - and the ligands induces an entatic state, a condition of strain and tension which enhances the reactivity at the active site. The Zn atom binds the substrate peptide via the O atom of the —CONH— peptide link, and the entatic state of the free enzyme facilitates formation of the enzyme-substrate complex. 

The Zn atom can be replaced by Co without much loss of enzymatic activity Co(II) shares with Zn(II) a liking for tetrahedral four-coordination. But if the Zn(II) is replaced by Ni(II) or Cu(II), the activity is virtually lost. Tetrahedral d8 and (especially) d9 complexes are subject to Jahn-Teller distortions, so that the imposition of a distortion from regular tetrahedral geometry does not lead to any strain there is no entatic state. 

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