Aldolase metal binding

Cooper, S. J., Leonard, G. A., McSweeney, S. M., Thompson, A. W., Naismith, J. H., Qamar, S., et al. The Crystal Structure of a Class II Fructose-1,6-Bisphosphate Aldolase Shows a Novel Binuclear Metal-Binding Active Site Embedded in a Familiar Fold. Structure 1996, 4, 1303-1315. 

Muscle aldolase is strongly inhibited by traces of heavy metals and its activity is not decreased by metal binding reagents such as cysteine and a,a -dipyridyl. Yeast aldolase, which has been extensively purified by Warburg, is inactivated by cysteine and reactivated by ferrous, zinc, or cobaltous ion. Aldolase of Clostridium perfringens, on the other hand, is reactivated by ferrous or cobaltous ions in the presence of cysteine. Pea aldolase is not inhibited by heavy metals nor by cysteine and is not activated by ferrous or cobaltous ions. It is puzzling that although each of these aldolases catalyzes the same thermodynamic reaction, they still possess markedly different activation requirements. 

Evidence for the binding of protective molecules to important cellular macromolecules continues to appear. The protection of GED against inactivation of trypsin, lysozyme, and aldolase was considered not due to radical scavenging, but to mixed disulfide formation. Protection of DM by diamino disulfides was attributed to bound disulfide and protection of lactate dehydrogenase and catalase by serotonin was also attributed to complex formation, possibly with the metal ions in the enzymes. 

Type n aldolases are found predominantly in bacteria and fungi, and are Zn " -dependent enzymes (Scheme 2.182) [1378]. Their mechanism of action was recently affirmed to proceed through a metal-enolate [1379] an essential Zn " atom in the active site (coordinated by three nitrogen atoms of histidine residues [1380]) binds the donor via the hydroxyl and carbonyl groups. This facilitates pro-(/ )-proton abstraction from the donor (presumably by a glutamic acid residue acting as base), rendering an enolate, which launches a nucleophilic attack onto the aldehydic acceptor. 

Its metal chelating binding mode has been determined by liganded structures of native protein and active-site mutants. In concert, these studies have enabled derivation of a conclusive blueprint for the catalytic cycle of metal-dependent aldolases that successfully rationalizes all key stereochemical issues [50]. 

Aldolases are a class of lyases that catalyze the reversible, stereoselective addition of an aldol donor component (nucleophile) to an acceptor component (electrophile) [9,75-81]. Typical products are 3-hydroxy ketones, a structural element that is frequendy incorporated in the framework of complex natural products. Mechanistically, aldolases promote the abstraction of the aldol donor a-proton, thereby generating a carbon nucleophile bound at the active site. In type I aldolases, an enamine is produced by covalent binding to a conserved lysine residue while in the type 11 aldolases, an enediol is formed by chelating coordination to an essential transition metal cation (mostly Zn +), which is acting as a Lewis acid promotor. Aldolases are often highly selective for the donor substrate, tolerating... 

Mechanistic classification of aldolases, (a) Class I, an enamine is produced by covalent binding to a conserved lysine residue, (b) Class II, an enolate is formed by chelating coordination to a transition metal cation (mostly Zn ), which acts as a Lewis acid promotor. 

Mechanistically, the activation of the aldol donor substrates is achieved by stereospecific deprotonation along two different pathways (Fig. 2) [28] Class I aldolases bind their substrates covalently via imine/enamine formation to an active site lysine residue to initiate bond cleavage or formation (Fig. 2a) in contrast, class II aldolases utilize transition metal ions as a Lewis acid cofactor (usually Zn " ) which facilitates (Fig. 2b) deprotonation by a bidentate coordination of the donor to give the enediolate nucleophile [29]. Usually, the approach of the aldol acceptor to the enzyme-bound nucleophile occurs stereospecif-ically following an overall retention mechanism, while the facial differentiation of the aldehyde carbonyl is responsible for the relative stereoselectivity. In this manner, the stereochemistry of the C—C bond formation is completely controlled by the enzyme, in... 

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