Zinc-ligand interactions carbonyl

The free a-amino group of the dipeptide interacts with glutamate at position 270 the y-COOH group is involved and a water molecule is located between the amino and carboxyl groups. Interactions which directly involve the catalytic action of the enzyme are as follows (a) the carbonyl of the peptide bond ligands to a zinc atom, which itself is further bound to two histidines (positions 69,196) and a glutamate... 

The slow nucleophilic addition of dialkylzinc reagents to aldehydes can be accelerated by chiral amino alcohols, producing secondary alcohols of high enantiomeric purity. The catalysis and stereochemistry can be interpreted satisfactorily in terms of a six-membered cyclic transition state assembly [46,47], In the absence of amino alcohol, dialkylzincs and benzaldehyde have weak donor-acceptor-type interactions. When amino alcohol and dialkylzinc are mixed, the zinc atom acts as a Lewis acid and activates the carbonyl of the aldehyde. Zinc in this amino alcohol-zinc complex is regarded as a kind of chirally modified Lewis acid. Various kinds of polymer-supported chiral amino alcohol have recently been prepared and used as ligands in dialkylzinc alkylation of aldehydes. 

Extrapolation of observations obtained from model systems to enzymes must be treated with caution. Metalloenzymes do not operate with high concentrations of metal ions and for those cases that have been studied the substrate usually acts as a monodentate ligand when it is directly coordinated to a metal ion e.g. the carbonyl oxygen of the amide link to be cleaved coordinates to the zinc atom of carboxy-peptidase [21]. The protein itself is probably responsible for a large fraction of the binding energy resulting from the interaction of the substrate with the metalloen-zyme (Ch. 1). 

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