The molybdenum cofactor isolated from sulfite oxidase (sulfite: ferricytochrome c oxidoreductase, EC 1.8.2.1) and xanthine dehydrogenase (xanthine:NAD+ oxidoreductase, EC 1.2.1.37) in the presence of iodine and KI (form A) has been shown to contain a pterin nucleus with an unidentified substituent in the 6 position [Johnson, J. L., Hainline, B. E. & Rajagopalan, K. V. (1980) J. Biol. Chem. 255, 1783-1786]. A second inactive form of the cofactor was isolated aerobically but in the absence of iodine and KI. The latter cofactor derivative (form B) is highly fluorescent, has a visible absorption band at 395 nm and, like form A, contains a phosphate group. Cleavage of the phosphate ester bond with alkaline phosphatase exposes a glycol function that is sensitive to periodate. Oxidation of form B with alkaline permanganate yields a highly polar compound with properties of a sulfonic acid, suggesting that the active molybdenum cofactor might contain sulfur. The sulfur-containing pterin urothione characterized by Goto et al. [Goto, M., Sakurai, A., Ohta, K. & Yamakami, H. (1969) J. Biochem. 65, 611-620] had been isolated from human urine. The permanganate oxidation product of urothione, characterized by Goto et al. as pterin-6-carboxylic-7-sulfonic acid, is identical to that obtained from form B. Because urothione also contains a periodate-sensitive glycol substituent, a structural relationship is suggested. The finding that urine samples from patients deficient in the molybdenum cofactor are devoid of urothione demonstrates a metabolic link between the two molecules.

https://www.pnas.org/content/pnas/79/22/6856.full.pdf

Structural and metabolic relationship between the molybdenum cofactor and urothione.

Publisher Summary This chapter describes the enzymes, xanthine oxidase, xanthine dehydrogenase, aldehyde oxidase, and sulfite oxidase. In addition to the molybdenum hydroxylases, sulfite oxidase is discussed in this chapter, though it contains heme rather than flavin and no iron-sulfur. This is because it contains molybdenum, apparently in a chemical environment within the active center, which is quite like that of the metal in the other enzymes. The term xanthine oxidase is to be taken to refer to the enzyme from milk. The enzymes to be considered are widely distributed. Thus, one or more molybdenum hydroxylases have been found in organisms as different in complexity as man and bacteria. In mammal's oxidation of hypoxanthine to xanthine and of xanthine to uric acid are steps in purine catabolism prior to excretion, uric acid being the end product in primates. Individual humans, genetically deficient in xanthine oxidase, are apparently little the worse for the deficiency. The reduced oxygen derivatives are supposed to be available for coupled oxidation reactions—for example, in drug catabolism. The most obvious objection to this hypothesis is the conclusion, discussed in the chapter, that the dehydrogenase form of the xanthine-oxidizing enzymes is more native than is the oxidase form. It must be conceded that the true role of the enzymes in humans and many other species remains somewhat problematical.

6 Molybdenum Iron-Sulfur Flauin Hydroxylases and Related Enzymes

Publisher Summary This chapter presents a discussion on the bioinorganic chemistry of pterin-containing molybdenum and tungsten enzymes. The chapter focuses on the current results and the interplay of model and enzyme chemistry. Attention is directed to sulfite oxidase and xanthine oxidase, the archetypal examples of molybdenum enzymes, containing, respectively, dioxo-Mo(VI) and oxo-thio-Mo(VI) oxidized centers. Biochemical and model studies of molybdopterin, Mo-co, and related species are described in this chapter. A brief survey on the physical and spectroscopic techniques employed in the study of the enzymes is presented and their impact on the current understanding of the coordination about the molybdenum atom in sulfite oxidase and xanthine oxidase is discussed. Structural and spectroscopic models are also presented in the chapter. The chapter also explains the xanthine oxidase cycle and facets of intramolecular electron transfer in molybdenum enzymes. The pterin-containing tungsten enzymes and the evolving model chemistry are discussed. Current descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon the comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The unexpected discovery that several hyperthermophilic organisms possess pterin-containing tungsten enzymes promises to stimulate the development of oxo-thio-tungsten chemistry.

Bioinorganic chemistry of pterin-containing molybdenum and tungsten enzymes

Molybednum-containing enzymes (Coughlan, 1980; Spiro, 1985) occupy a significant place in the development of the field now termed inorganic biochemistry. The importance of the metal as a biological trace element depends on its involvement in the known, and perhaps other as yet unknown, molybdoenzymes. That it plays a role in biological nitrogen fixation, the process whereby the enzyme nitrogenase in the root nodules of plants converts atmospheric nitrogen into ammonia, was recognized in the 1930s. The metal is also a constituent of a variety of other enzymes, having first been found in a mammalian enzyme, xanthine oxidase, in the 1950s.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0033583500004479

The inorganic biochemistry of molybdoenzymes.

Photochromism is defined as the reversible transformation of a chemical species, induced by the absorption of electromagnetic radiation, to a form with a different vibronic absorption spectra. Photochromic materials have attracted considerable interest owing to prospective real or potential applications in many fields, including protection (in spectacles, photobarriers, anti-fake, and camouflage), decoration, information storage, displays, optical switches, photomechanics, and so forth. Although numerous photochromic families have been reported to date, those based on an electron-transfer (redox) chemical process, especially for metal–organic complexes (MOCs), are rare. One of the biggest challenges in photochromic MOCs without photochromic organic ligands is to design photoinduced bistable systems based on different electron-transfer mechanisms, such as metal-centered electron transition (MC), ligand-tometal charge transfer (LMCT), metal-to-metal charge transfer (MMCT), intra ACHTUNGTRENNUNGli ACHTUNGTRENNUNGgand charge transfer (ILCT), and ligand-to-ligand charge transfer (LLCT). Structurally well-defined hybrids will give us better insight into the structure–photochromism relationships; nevertheless, few hybrids based on transition metals have been explored with simultaneous characterization of their structure and photochromism. Chopoorian and Loeffler have discovered the electron-attractive ability of a porous glass matrix indicated by the blue radical-ion species after the irradiation with UV light, in which an aqueous solution of p-phenylenediaminetetraacetic acid (p-PTDA) is absorbed. Thus, in the presence of electron-accepting components, organic ligands containing N ACHTUNGTRENNUNG(CH2CO2H)2 groups in MOCs may undergo a similar electron-transfer process. However, no coordination polymer constructed from ligands containing N ACHTUNGTRENNUNG(CH2CO2H)2 groups shows photochromism so far. Guo and coworkers have reported the only metal-assisted LLCT photochromic MOC, [Cd2(ic)(mc)(4,4’-bipy)3]·4H2O (ic = itaconate, mc=mesaconate, bipy= bipyridine), which undergoes an interesting photochromic transformation from yellow to blue upon UV irradiation. Herein, we report two photochromic MOCs with adjustable fluorescent intensities, [Zn3ACHTUNGTRENNUNG(TTHA)(4,4’bipy)1.5 ACHTUNGTRENNUNG(H2O)2]·6H2O (1 a) and [Zn3ACHTUNGTRENNUNG(TTHA)(4,4’-bipy)1.5ACHTUNGTRENNUNG(H2O)2]·3H2O (2 a), derived from a novel triamine-based polycarboxylate ligand containing the N ACHTUNGTRENNUNG(CH2CO2H)2 group, 1,3,5-triazine-2,4,6-triamine hexaacetic acid (H6TTHA), as electron donor, although the ligand H6TTHA itself does not exhibit photochromism, and 4,4’-bipy as electron capturer. The photochromic mechanisms based on electron-transfer chemical processes have been verified with direct and powerful ESR and X-ray photoelectron spectroscopy (XPS) measurements. Yellow needlelike crystals of 1 a and prismatic crystals of 2 a were obtained by the hydrothermal reactions of ZnACHTUNGTRENNUNG(NO3)2, H6TTHA, and 4,4’-bipy in a molar ratio of 3:1:2 at 140 8C and 120 8C for 72 h, respectively. The single-crystal X-ray diffraction data reveal that complexes 1 a and 2 a are 3D networks. As shown in Figure 1 and S3 in the Supporting Information, the 3D frameworks can be described as 2D layers constructed of Zn ions and TTHA pillared by the coordinated 4,4’-bipy groups. The biggest difference between the structures of complexes 1 a and 2 a is the coordination modes of the ligands (Scheme 1). Due to the flexibility, six of the arms show sig[a] Q.-L. Zhu, Prof. T.-L. Sheng, Dr. R.-B. Fu, S.-M. Hu, Prof. L. Chen, C.-J. Shen, X. Ma, Prof. X.-T. Wu State Key Lab of Structural Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Science, Fuzhou Fujian 350002 (P.R. China) Fax: (+86) 591-8371-9238 E-mail : wxt@fjirsm.ac.cn [b] Q.-L. Zhu, C.-J. Shen, X. Ma Graduate School of the Chinese Academy of Sciences Beijing, 100049 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201003274. Figure 1. 2D layer (left) and 3D framework along the c axis (right) in complex 1 a.

Redox-responsive photochromism and fluorescence modulation of two 3D metal-organic hybrids derived from a triamine-based polycarboxylate ligand.

Molybdenum-thiol complexes as models for molybdenum bound in enzymes.

THE compound p-phenylenediaminetetraacetic acid, p-PTDA (I), can be chemically oxidized to give a blue radical cation (II). We have now found that this reaction can be carried out in a novel photochemical manner. When an aqueous solution of the diamine is absorbed by porous glass and irradiated with ultra-violet light, it gives what appears to be the same blue radical-ion species. On interruption of irradiation, the photo-induced coloration rapidly bleaches. Since photochemical reactions leading to reversible colour changes (photochromism) are of fundamental and practical importance, we have undertaken the following investigation of this system.

W. F. Marzluff

Papers

Molybdenum-thiol complexes as models for molybdenum bound in enzymes.

Photochromism of p -Phenylenediaminetetraacetic Acid absorbed in Porous Glass