Thomas Klabunde

Crystal structure of a plant catechol oxidase containing a dicopper center

Catechol oxidases are ubiquitous plant enzymes containing a dinuclear copper center. In the wound-response mechanism of the plant they catalyze the oxidation of a broad range of ortho-diphenols to the corresponding o-quinones coupled with the reduction of oxygen to water. The crystal structures of the enzyme from sweet potato in the resting dicupric Cu(II)-Cu(II) state, the reduced dicuprous Cu(I)-Cu(I) form, and in complex with the inhibitor phenylthiourea were analyzed. The catalytic copper center is accommodated in a central four-helix-bundle located in a hydrophobic pocket close to the surface. Both metal binding sites are composed of three histidine ligands. His 109, ligated to the CuA site, is covalently linked to Cys 92 by an unusual thioether bond. Based on biochemical, spectroscopic and the presented structural data, a catalytical mechanism is proposed in which one of the oxygen atoms of the diphenolic substrate binds to CuB of the oxygenated enzyme.

Crystal structure of the secreted form of antigen 85C reveals potential targets for mycobacterial drugs and vaccines.

The antigen 85 (ag85) complex, composed of three proteins (ag85A, B and C), is a major protein component of the Mycobacterium tuberculosis cell wall. Each protein possesses a mycolyltransferase activity required for the biogenesis of trehalose dimycolate (cord factor), a dominant structure necessary for maintaining cell wall integrity. The crystal structure of recombinant ag85C from M. tuberculosis, refined to a resolution of 1.5 Å, reveals an α/β-hydrolase polypeptide fold, and a catalytic triad formed by Ser 124, Glu 228 and His 260. ag85C complexed with a covalent inhibitor implicates residues Leu 40 and Met 125 as components of the oxyanion hole. A hydrophobic pocket and tunnel extending 21 Å into the core of the protein indicates the location of a probable trehalose monomycolate binding site. Also, a large region of conserved surface residues among ag85A, B and C is a probable site for the interaction of ag85 proteins with human fibronectin.

STRUCTURAL RELATIONSHIP BETWEEN THE MAMMALIAN FE(III)-FE(II) AND THE FE(III)-ZN(II) PLANT PURPLE ACID PHOSPHATASES

The primary structure of uteroferrin (Uf), a 35 kDa monomeric mammalian purple acid phosphatase (PAP) containing a Fe(III)‐Fe(II) center, has been compared with the sequence of the homodimeric 111 kDa Fe(III)‐Zn(II) kidney bean purple acid phosphatase (KBPAP). The alignment suggests that the amino acid residues ligating the dimetal center are identical in Uf and KBPAP, although the geometry of the coordination sphere might slightly differ. Secondary structure predictions indicate that Uf contains two βαβαβ motifs thus resembling the folding topology of the plant enzyme. Guided by the recently determined X‐ray structure of KBPAP a tentative model for the mammalian PAP can be constructed.

The Dimetal Center in purple acid phosphatases

Purple acid phosphatases (PAPs) contain a dinuclear metal center in their active site and hydrolyze phosphoric acid esters at low pH. Characteristic of this group of acid phosphatases is their resistance to inhibition by tartrate and their purple color, due to the presence of a tyrosine residue ligated to a ferric iron. The mammalian enzymes all contain a mixed-valent di-iron unit in their catalytic active form, first identified in the bovine spleen and porcine uterus enzymes, while a heterodinuclear Fe(III)Zn(II) unit has been characterized for the most studied plant enzyme from kidney bean. The enzymes from porcine uterus and bovine spleen can be converted into active FeZn forms and the plant enzyme can be transformed into an active FeFe form. In recent years the dimetal center of PAPs has been studied using numerous spectroscopic methods such as Mossbauer spectroscopy, EPR, NMR, EXAFS, magnetic, electrochemical and resonance Raman studies characterizing most of the metal coordinating residues, the metal-metal separation and providing evidence of the similarity between enzymes from different sources. Analysis of the products of hydrolysis of a substrate containing a chiral phosphorus by 31P NMR, stopped-flow measurements and kinetic studies all support a reaction path involving nucleophilic attack of a Fe(III)-bound hydroxide ligand on the phosphate ester. The recently solved crystal structure of the plant enzyme provides the structural basis for the understanding of the two-metal ion mechanism of this class of enzymes.

X-ray absorption studies of the purple acid phosphatase from red kidney beans (native enzyme, metal exchanged form)

Abstract Purple acid phosphatase from red kidney beans (KBP) catalyzes the hydrolysis of activated phosphoric acid monoesters and contains a heterodinuclear Fe(III)Zn(II) core in its active site. Iron K-edge X-ray absorption data have been obtained for the native enzyme and for a metal exchanged derivative, where Zn(II) was substituted by Fe(III). The environment of the native enzyme consists of 2.5 O/N at 1.91 A, 3 O/N at 2.09 A, and 1 Zn at 4.05 A. For the metal exchanged form we obtained 2.5 O/N at 1.94 A, 2.5 O/N at 2.09 A, and 1 Fe at 3.79 A.