Nickel pincer model of the active site of lactate racemase involves ligand participation in hydride transfer

Abstract

Lactate racemase is the first enzyme known to possess a metal pincer active site. The enzyme interconverts D-and L-lactic acid, which is important for the assembly of cell walls in many microorganisms. Here, we report a synthetic model of the active site of lactate racemase, which features a pyridinium-based SCS pincer ligand framework bound to nickel. The model complex mediates the dehydrogenation of alcohols, a reaction relevant to lactate racemization. Experimental and computational data indicate ligand participation in the dehydrogenation reaction.biomimetic chemistry | lactate racemase | nickel | pincer ligands | hydride transfer P incer complexes are widely applied in homogeneous catalysis thanks to their stability, diversity, and tunability (1-4). However, only very recently has the first pincer complex been discovered in the active site of a metalloenzyme (5-7). It was reported that lactate racemase, an enzyme responsible for the racemization of lactic acid and hence important for cell wall assembly in many microorganisms, hosts a nickel pincer cofactor (Fig. 1). The nickel center is coordinated by an SCS pincer ligand derived from a nicotinic acid mononucleotide, in addition to histidine (200) (5, 6). Based on the structure of this active site, it was proposed that the pincer ligand could reversibly capture a hydride from lactate at the carbon atom coordinated to nickel, in a manner similar to nicotinamide adenine dinucleotide in hydride transfer enzymes (Fig. 1). Nevertheless, evidence for this mechanism is still lacking.Although several nickel SCS pincer complexes have previously been reported (8-10), none exhibited an essential feature present in the active site of lactate racemase, namely a pyridinium-based pincer backbone. According to the proposed enzyme mechanism, the pyridinium group enables ligand participation in the hydride transfer reaction, which is likely impossible for a more conventional pyridine group. This hypothesis might be tested using model complexes containing either a pyridinium or a pyridine backbone. A further motivation for synthetic models of this unusual active site is the perspective of developing a new, bioinspired pincer ligand platform that enables metal ligand cooperation in catalysis (11,12). With these considerations in mind, we began investigating the biomimetic chemistry of lactate racemase. Here, we report the synthesis and characterization of a synthetic mimic of lactate racemase. Using dehydrogenation of alcohols as a model reaction, we show that the pyridinium functionality indeed facilitates hydride transfer by ligand participation. ResultsSynthesis. The pyridine-based ligand precursor (4) was synthesized from 4-chloro-3,5-dimethyl pyridine (1, Fig. 2). Oxidation of 1 by KMnO 4 gave 4-chloropyridine-3,5-dicarboxylic acid (2), which was converted to a dicarboxamide (3) by amidation. Reaction of 3 with Lawesson's reagent [2,4-bis(4-methoxyphenyl)-2,4-dithioxo-1,3,2,4-dithiadiphosphetane] yielded the ligand precursor 4 containing thioamides (13). Upon treatment o...

Functional Models of the Nickel Pincer Nucleotide Cofactor of Lactate Racemase

“…TheNicenter retains the expected square-planar coordination environment. Compared to the previous model 1, [7] the NiÀSbonds in 5-7 are slightly longer (by about 0.05 )a sar esult of replacing thioamides with thiocarboylates in the pincer ligands (see Table S2). Judging from the CÀOb ond length (about 1.22 ), the CÀOm oiety is best represented by adouble bond.…”

“…An alternative pathway involving dissociation of His200 and reversible hydride transfer to Ni (path II) seems less favorable but cannot be excluded. [7] Recently,wereported aNi II model supported by ap yridinium-3,5-bisthioamide SCS pincer ligand (1;F igure 1c). [7] Recently,wereported aNi II model supported by ap yridinium-3,5-bisthioamide SCS pincer ligand (1;F igure 1c).…”

“…Small-molecule models of enzymes provide an important tool to probe the mechanism underlying enzyme reactions.A lthough metal pincer complexes are ubiquitous in synthetic chemistry, [6] the structure of the NPN cofactor is unique.M odels of NPN have only begun to be developed. [7] This model mediated dehydrogenation of an alcohol, but the reaction was irreversible such that no racemization activity was observed. [7] This model mediated dehydrogenation of an alcohol, but the reaction was irreversible such that no racemization activity was observed.…”

Functional Models of the Nickel Pincer Nucleotide Cofactor of Lactate Racemase

“…Although metal pincer complexes are ubiquitous in synthetic chemistry, the structure of the NPN cofactor is unique. Models of NPN have only begun to be developed . Recently, we reported a Ni II model supported by a pyridinium‐3,5‐bisthioamide SCS pincer ligand ( 1 ; Figure c) .…”

“…Models of NPN have only begun to be developed . Recently, we reported a Ni II model supported by a pyridinium‐3,5‐bisthioamide SCS pincer ligand ( 1 ; Figure c) . This model mediated dehydrogenation of an alcohol, but the reaction was irreversible such that no racemization activity was observed.…”