Enzyme metal-substrate bridge complex

Considering the role of metals in the mechanism of enzyme-catalyzed elimination and enolization reactions, so that we might understand the role of the metal, we will restrict ourselves to those cases in which both the substrate and the enzyme donate ligands to the metal, in the ternary complex i,e, in which an enzyme-metal-substrate bridge complex (E-M-S) has been established. 

Figure 1. Distances in established enzyme-metal-substrate bridge complexes from longitudinal nuclear relaxation rates...

In ternary enzyme-metal-substrate bridge complexes, metals might activate enolization and elimination reactions by coordination of the electronegative atom (X) attached to the a carbon atom ( a activation ). 

Divalent cations, univalent cations, or both are essential cofactors for a large number of enzymes. Kinases as a class, for example, share the requirement for such cations, while in other instances, other metal ions are inhibitors of metal-activated enzymes, e.g., activation by Mg " and inhibition by Ca for many kinases and synthetases. Mildvan (1970) has reviewed the models that have been proposed to account for activation (or inhibition) of enzymes by metal ions. The "substrate bridge" and "metal bridge" models conceive of the metal ions either combining with the substrate to form a chelate or interacting with the enzyme to complete the required binding site. These complexes usually involve the active site, but Schramm (1974) has demonstrated activation of AMP nucleosidase by MgATP at a modifier site instead. 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

Metal bridges are formed from enzymes or hormones to the substrates. These intermediate complexes are able to enhance the reaction in a number of metabolic processes, 3. Some metal ions are known to act as a cofactor or coenzyme being involved in redox reactions directly. Further, metalloproteins may serve as a pool for metal storage. Examples for this are displayed by ferritin, transferrin or ferrichrome, to mention just a few. 

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