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A tethered niacin-derived pincer complex with a nickel-carbon bond in lactate racemase

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Science  03 Jul 2015:
Vol. 349, Issue 6243, pp. 66-69
DOI: 10.1126/science.aab2272

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Nickel pincers as enzyme cofactors

Organometallic nickel complexes long synthesized in the laboratory exist naturally in enzymes as well. Desguin et al. determined the structure and metal-binding residues of the Ni-containing active site in bacterial lactate racemase (see the Perspective by Zamble). A dithiodinicotinic acid mononucleotide derivative cofactor binds Ni through sulfur and carbon bonds, resembling synthetic nickel pincer complexes. Genes encoding accessory proteins involved in the synthesis of this cofactor are widely distributed in other bacteria, suggesting its involvement in other enzymes.

Science, this issue p. 66; see also p. 35

Abstract

Lactic acid racemization is involved in lactate metabolism and cell wall assembly of many microorganisms. Lactate racemase (Lar) requires nickel, but the nickel-binding site and the role of three accessory proteins required for its activation remain enigmatic. We combined mass spectrometry and x-ray crystallography to show that Lar from Lactobacillus plantarum possesses an organometallic nickel-containing prosthetic group. A nicotinic acid mononucleotide derivative is tethered to Lys184 and forms a tridentate pincer complex that coordinates nickel through one metal-carbon and two metal-sulfur bonds, with His200 as another ligand. Although similar complexes have been previously synthesized, there was no prior evidence for the existence of pincer cofactors in enzymes. The wide distribution of the accessory proteins without Lar suggests that it may play a role in other enzymes.

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