The Mechanism of Class II, Metal‐Dependent Aldolases

Abstract

The four distinct dihydroxyacetone phosphate dependent aldolases". ' I enjoy increasing interest for preparative asymmetric synthesis because of their capacity to build up two new stereogenic centers with high chiral induction.[31 While all DHAP aldolases have a very broad substrate tolerance for the aldol acceptor substrate, they appear to have a high substrate specificity for dihydroxyacetone phosphate (DHAP) as the aldo1 donor, and only few isosteric replacements of the phosphate ester moiety are t~l e r a t e d .~~] Diastereoselectivity may be limited for certain cases in the control of the stereocenter at C-4, which points to occasional inverse binding of the aldehyde carbonyl group.r2ck Certainly, a detailed understanding of the contributions of active site residues in substrate recognition and in the catalytic event is highly desirable to further improve the predictive value of the method.Aldolases have been divided into two classes according t o their mode of donor activation.[51 Class I aldolases achieve stereospecific deprotonation of the substrate by means of covalent linkage to an active site lysine residue (imine/enamine formation), while class I1 aldolases utilize transition metal ions (usually Zn2 +) as essential Lewis acid cofactors to facilitate deprotonation (Fig. 1). For the class I FruA,"] despite extensive efforts using modern techniques of enzymology, site-directed mutagenesis. and protein crystallography,[61 a conclusive model that accounts for the function of the active site residues in the individual steps of catalysis is advancing very slowly.[71 In particular, no structure with bound substrates or inhibitors is yet available.Based on work with the class I1 FruA from yeast, several early hypotheses for the mechanism of Zn-dependent aldolases have been developed. According to ESR and NMR relaxation rate measurements on the Mn'+-substituted holoenzyme, DHAP is bound through its phosphate group,[*] and the carbonyl is polarized by Zn2+ through an intervening imidazole ring (Fig. l , A).'91 Subsequent FT-IR and deuterium exchange studies with native yeast aldolasellO1 led to the conclusion that aldehyde activation occurs by an additional direct coordination of the carbonyl (Fig. 1, B). Recently, Dreyer and Schulz[""I reported the X-ray structure for FucA (2.13 A resolution), the first of a class I1 aldolase. The active site, which is located at the interface between two subunits of the homotetramer, contains the catalytically active Zn2+ ion tightly coordinated by three Ne atoms of histidine residues (His92, His94, His155; Fig. 1, C). Thus, all previous mechanistic hypotheses must be rejected in light of the steric restraints imposed on the Zn2+ ion. which precludes coordination of more than a single substrate, and on its histidine ligands, which cannot act as a proton relay between bound substrates. Under the slightly alkaline conditions required for FucA crystallization, the natural substrate L-fuculose 1-phosphate 1 (Scheme 1) cannot be used for X-ray analysis of the enzyme-ligand comple...

The polyketide cyclase RemF from Streptomyces resistomycificus contains an unusual octahedral zinc binding site

“…suggest potential mechanistic similarities of RemF to class II aldolases where a zinc ion acts as a Lewis acid in aldol condensation reactions [32].…”

Lanthanide (III) salt complexes: Arrayed acid‐base networks for enantioselective catalysis. The nitroaldol reaction upon aldehydes and trifluoromethylketones

“…1) where the phenolic OH of tyrosine 113 0 acts as Brönsted acid to activate an aldehyde, whereas the divalent zinc ion coordinated by three histidine residues works as Lewis acid to activate a dihydroxy acetone monophosphate, and the terminal carboxylate anion of glutamic acid 73 acts as a Brönsted base to generate the reactive enolate. 5,6 RESULTS AND DISCUSSION A key decision to be taken to access complexes endowed with such an arrayed network of acid and base sites was that of selecting chiral ligands having rigid conformations as a result of the existence of intramolecular hydrogen bonds, or otherwise. In this regard, we decided to employ C 2 -symmetric binolam 7,8 1 and their (N)-oxide derivative binolamo 9 2 (illustrated in Fig.…”

“…Unfortunately, working at this temperature implied long reaction times. Eventually, we found that the addition of an external strong base (5% molar) such as 1,8-diazabicyclo [5,4,0]undec-7-ene (DBU) or 1,8-bis(dimethylamino)naphthalene (proton sponge 1 ) significantly accelerate the reaction, even at 2408C.…”

Direct Catalytic Asymmetric Aldol Reactions Assisted by Zinc Complex in the Presence of Water

“…Using stronger coordinating elements such as nitrogen was envisioned by analogy to type II aldolases in which the zinc ion is tightly coordinated by three histidine residues in the reaction active site. [15] After many trials we found that chiral bis(prolinamides) 2 and 3, readily synthesized in two steps from protected proline and chiral diphenylethylenediamines, [16] meet these design criteria. The ligands with a two-carbon linker between both amino acid moieties were found as the most attractive candidates in the first stage of the optimization.…”