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...

Two bacterial aldolases, RhuA and FucA, which are readily accessible by overex‐pression, catalyze the asymmetric addition of dihydroxyacetone phosphate to various aldehydes. The high degree of enantio‐and diastereoselectivity for the L‐threo (RhuA) and D‐erythro (FucA) stereochemistry of the products and the high stability of the enzymes make it possible to prepare rare carbohydrates and their derivatives as well as other polyhydroxylated compounds.

No abstract

COMMUNICATIONS glycerol kinase H' H+ ATP 2: Procedure as for the synthesis of 1; [(Ph,P),)Pd(N,),] is used in place of [(dppe-Pd(N,)J to furnish deep red crystals stable in air and in about 10 % yield. FAB-MS: m/;(%):4673.8(15)[Mt -CI],4215.5(40)[Mi -CI -PPh, -Au],4177.5(100) [ M i -2C1-PPh3 -Au], 3916.8 (80) [M' -2CI -ZPPh, -Au], 3721.7 (50) [ M i -2CI -2PPh3 -~A u ] , 3685.3 (35) [M' -3CI -ZPPh, -~A u ] , 3421.2 (20) [ M + -3CI -3PPh, -2Au].107.85(2) , Z = 4, R = 0.058 for 6928 independent reflections with I > 3 4 1 ) and 383 refined parameters (Enraf-Nonius CAD4 diffractometer, SDP-Plus,

An O2-consuming side reaction of a-ribulose 1,5-bisphosphate carboxylase causes photorespiration in plants. This reaction may be an inevitable consequence of the enzyme's inability to protect its ene-diolate reaction intermediate from 02, a notion that is supported by the failure of persistent efforts to eliminate selectively its oxygenase activity by genetic manipulation. We have examined two aldolases with similar ene-diolate intermediates, L-rhamnulose l-phosphate aldolase and L-fuculose 1-phosphate aldolase. The former enzyme has an oxygenase activity, while the latter does not, suggesting that the reaction with 02 is not inevitable.