Synthetic Models for Vanadium Haloperoxidases - American Chemical

Chapter 12

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 28, 2016 | http://pubs.acs.org Publication Date: December 10, 1998 | doi: 10.1021/bk-1998-0711.ch012

Synthetic Models for Vanadium Haloperoxidases Vincent L. Pecoraro, Carla Slebodnick, and Brent Hamstra Department of Chemistry, The Willard H. Dow Laboratories, The University of Michigan, Ann Arbor, MI 48109-1055

Vanadium halperoxidase (VHPO) model compounds have been extremely useful in helping to illucidate details of the mechanism of activity of the enzymes. This paper summarized mechanistic studies of peroxide binding and halide oxidation by VHPO functional models and how the results of these studies relate to the enzyme. In addition, EPR and ESEEM spectroscopic models for the reduced and inactive V(IV) form of VHPO have been used to predict the vanadium coordinations environment in the enzyme and a mechanism of inactiviation has been proposed. Vanadium is considered a trace element in biology and has garnered little attention from scientists concerned with its biological roles until the past two decades (1-3). The introductory chapter by Kustin and the other articles in this symposium series have presented a testament to the intense studies of scientists and clinicians since the discovery of the vanadium haloperoxidase and the recognition that vanadate and vanadyl complexes exhibit insulin mimetic properties. In this contribution, we present an overview of the synthetic modeling approach that our laboratory has undertaken over the past decade in order to understand the structure, spectroscopy and reactivity of vanadium in biologically relevant oxidation levels. We will focus our attention on studies that provide insight specifically for the vanadium haloperoxidases (VHPOs). These enzymes, found in marine algae, lichens and fungi, are responsible for the oxidation of halides by hydrogen peroxide according to equation 1 (4-7).

H X + H 0 ~> ΧΟΗ + H 0 2

2

2

RX + 2 H 0 2

(1)

Our first efforts for modeling the reactivity of this enzyme centered on identifying V(V) chelates that might be capable of binding peroxide in a stable and reversible manner (8,9). Since the x-ray structure of the vanadium chloroperoxidase had not yet appeared, it seemed reasonable to employ ligands of moderate denticity which utilized functional groups that are biologically relevant. We selected the class of substituted tripodal amines represented by nitrilotriacetic acid (nta*) and functionalized iminodiacetic acid derivatives (ida) such as hydroxyethyliminodiacetic acid (heida), aminoethyliminodiacetic acid (aeida), pyridylmethyliminodiacetic acid (pmida). These and related ligands are shown in Figure 1. The cis-

©1998 American Chemical Society Tracey and Crans; Vanadium Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

157

158 dioxovanadium(V) complexes of these ligands are easily prepared in aqueous solution and adopt the predicted structure which is shown in Figure 2 for the [V0 (ada)]" derivative (Slebodnick, C. Pecoraro, V . L . unpublished data). These molecules are water soluble and stable. In addition, they may be solubilized in non-aqueous solvents such as acetonitrile by the addition of crown ethers which bind the countercations of the salt. Reaction of [V0 (ada)]" or any of the other cis-dioxovanadium(V) derivatives with hydrogen peroxide in aqueous or acetonitrile solutions leads to new complexes which have a side-on coordinated peroxide as illustrated in Figure 3(8). We have recently investigated the details of this binding event and have shown that complexation can occur under neutral or acidic conditions. Under neutral conditions, peroxide binding curves saturate with added peroxide indicating that a reversible intermediate is formed. In contrast, if one equivalent of acid is added to the solution this saturation behavior is eliminated and one observes clean first order kinetics for the binding of peroxide to the dioxo dimer to form the oxo,peroxo derivative V O ( 0 ) L . The reaction sequence shown in Figure 4 illustrates our proposed mechanism for peroxide binding under these two solution conditions (10). We have observed that V O ( 0 ) L derivatives will not react efficiently to oxidize any halide in water or in acetonitrile. However, dramatic rate enhancements for halide oxidation are obtained in acetonitrile if one equivalent of a strong acid is added to the solution (8,9). We have proposed that the V O ( 0 ) L is activated by protonation of the coordinated peroxide to give V O ( H 0 ) L which is similar to the highly reactive alkylperoxides that have been reported by Mimoun (11-13). The pK^ for the coordinated peroxide can be deduced from the hydrogen ion concentration dependence of the reaction. Table I provides both the rates for bromide oxidation and the calculated p K for the complex. One should notice that the p K for hydrogen bromide in acetonitrile is similar to that for V O ( H 0 ) L . Therefore, excess HBr must be added to obtain a maximum rate. We do not see chloride oxidation because the chloride anion is simply too basic. Therefore, rather than attacking V O ( H 0 ) L as a nucleophile at the protonated peroxo oxygen, the far more rapid and thermodynamically driven proton transfer reaction occurs to generate V O ( 0 ) L and HC1. Our proposed mechanism for bromide oxidation by a protonated peroxovanadium(V) complex is illustrated in Figure 5 (8). A t this point, we can not distinguish whether the halide binds to vanadium prior to oxidation, as has been proposed previously for the enzyme (14), or if it attacks the peroxide directly. However, we consider halide coordination unlikely because this would reduce the halides nucleophilicity, thus deactivating the system. Halide coordination to vanadium may be possible if the entropie factors resulting from orienting the halide directly next to the peroxide are large enough to compensate for the reduced nucleophilicity of the vanadium-coordinated halide. 2


2

2

2

2

2

a

a

2

2

2

Table I. Kinetic Data for Bromide Oxidation by Peroxovanadium Complexes Complex pK k , bromide ( M - V ) [VO(0 )Hheida]280 ± 4 0 6.0 ±0.3 170 ± 3 0 6.0 ±0.3 [V0(0 )nta]25.8 ± 0 . 4 220 ± 3 0 [VO(0 )ada][VO(02)bpg] 5.4 ±0.3 21±3 H 0 6.0 ± 0 . 4 3.7 ±0.9 1

2

a

2

2

2

2

2

It is instructive to contrast the differing roles of protons in the formation versus activation of peroxo vanadium complexes. Protons enhance the rate of binding of peroxide to the cis dioxovanadium complex by protonating the leaving hydroxide in the intermediate complex VO(OH)(OOH)L. In the absence of acid, hydroxide can compete with hydroperoxide for binding to vanadium. The more basic Tracey and Crans; Vanadium Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

159

E^nta

0

0

%heij ^

H

^aeida

H3ada


H

0

V-N^ .0 xV NH,

OH Hjpmida

QH

0.yOH

r r ^

Hbpg

ο γ

0

/ F

(His496)

N

V (Lys353)

11. Schematics comparisons of VC1PO: (a) x-ray structure of native enzyme (VOeooaori* = 1-65 Â; V - O , ^ = 1.93 Â; V - N = 1.96 Â) (14); (b) the azide inhibited x-ray structure ( V - O , ^ = 1.65 Â; V - N . ^ = 1.98 A , V - N , , , = 2.25 A (26); (c) the proposed peroxide structure, based on the azide structure and model compounds; and (d) the observed peroxide bound x-ray structure ( V - O ^ = 1.60 A i V - O ^ ^ = 1.93 A ; V - O = 1.87 A ; V - N = 2.19 A) (14). peroriae

Tracey and Crans; Vanadium Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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of peroxide to the enzyme. Our structural and spectroscopic studies on V(IV) model compounds have allowed us to deduce a reasonable chemical model for the reduced, inactive enzyme and propose a mechanism for enzyme inactivation. While these accomplishments are gratifying, we are still working to achieve the Holy Grail of this field: chloride oxidation. We are happy to report that we now have systems from which we can extract measurable rates of chloride oxidation by vanadium peroxo catalysts and it appears that protonation chemistry is again important in these more demanding reactions (Slebodnick, C ; Colpas, G. J.; Pecoraro, V. L., unpublished data). We hope to present details of this very important new development in small molecule modeling chemistry in the near future. Acknowledgments The authors acknowledge Dr. Gerard Colpas for many useful discussions and our collaboration with Professor Wayne Frasch and Dr. Russell LoBruto in our ESEEM studies. This work was supported by the NIH (GM42703 to VLP) and by a postdoctoral fellowship to CS (GM18370). References 1. Chasteen, N. D., Ed. Vanadium in Biological Systems, Vol. (Kluwer, Dordrecht, The Netherlands) 1990, pp. 222. 2. Sigel, H.; Sigel, Α., Ed. Vanadium and Its Role in Life, Metal Ions in Biological Systems, Vol. 31 (Marcel Dekker, Inc., New York) 1995. 3. Slebodnick, C.; Hamstra, B. J.; Pecoraro, V. L. Structure and Bonding 1997, 89, 51-108. 4. Butler, A.; Baldwin, A. H. Structure and Bonding 1997, 89, 109-32. 5. Vilter, H. Metal Ions in Biological Systems 1995, 31, 325-62. 6. Butler, Α.; Walker, J. V. Chem. Rev. 1993, 93, 1937-44. 7. Wever, R.; Krenn, Β. E. "Vanadium Haloperoxidases" In Vanadium in Biological Systems, Vol. ; N. D. Chasteen, Ed. (Kluwer Academic Publishers, Dordrecht) 1990, pp. 81-97. 8. Colpas, G. J.; Hamstra, B. J.; Kampf, J. W.; Pecoraro, V. L. J. Am. Chem. Soc. 1996,118,3469-78. 9. Colpas, G. J.; Hamstra, Β. H.; Kampf, J. W.; Pecoraro, V. L. J. Am. Chem. Soc. 1994,116,3627-8. 10. Hamstra, B. J.; Pecoraro, V. L. Inorg. Chem. submitted. 11. Mimoun, H.; Chaumette, P.; Mignard, M.; Saussine, L.; Fischer, J.; Weiss, R. Nouv. J. Chem. 1983, 7, 467-75. 12. Mimoun, H.; Saussine, L.; Daire, E.; Postel, M.; Fischer, J.; Weiss, R. J. Am. Chem. Soc. 1983, 105, 3101-10. 13. Mimoun, H.; Mignard, M.; Brechot, P.; Saussine, L. J. Am. Chem. Soc. 1986, 108, 3711-8. 14. Messerschmidt, Α.; Prade, L.; Wever, R. Biol. Chem. 1997, 378, 309-315. 15. van Schijndel, J. W. P. M.; Barnett, P.; Roelse, J.; Vollenbroek, E. G. M.; Wever, R. Eur. J. Biochem.1994, 225, 151-7. 16. Soedjak, H. S.; Butler, A. Biochim. et Biophys. Acta 1991, 1079, 1-7. 17. Everett, R. R.; Soedjak, H. S.; Butler, A. J.Biol.Chem. 1990, 265, 15671-9. 18. de Boer, E.; Wever, R. J. J. Biol. Chem.. 1988, 263, 12326-32. 19. Everett, R. R.; Kanofsky, J. R.; Butler, A. J. Biol. Chem.. 1990, 265, 4908-14. 20. de Boer, E.; Boon, K.; Wever, R. Biochemistry 1988, 27, 1629-35. 21. Hamstra, B. J.; Houseman, A. L. P.; Colpas, G. J.; Kampf, J. W.; LoBrutto, R.; Frasch, W. D.; Pecoraro, V. L. Inorg. Chem. 1997, 36, 4866-74. 22. LoBrutto, R.; Hamstra, B. J.; Colpas, G. J.; Pecoraro, V. L.; Frasch, W. D. J. Am. Chem. Soc. in press.


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23. Chasteen, N. D. In Biological Magnetic Resonance, Vol. 3; L. J. Berliner and J. Reuben, Ed. (Plenum Press, New York) 1981, pp. 53-119. 24. Cornman, C., R.; Zovinka, E. P.; Boyajian, Y. D.; Geiser-Bush, Κ. M.; Boyle, P. D.; Singh, P. Inorg. Chem. 1995, 34, 4213-19. 25. de Boer, E.; Keijzers, C. P.; Klaassen, A. A. K.; Reijerse, E. J.; Collison, D.; Garner, C. D.; Wever, R. FEBS Letters 1988, 235, 93-97. 26. Messerschmidt, Α.; Wever, R. Proc. Nat. Acad. Sci. U. S. A. 1996, 93, 392-6.


Synthetic Models for Vanadium Haloperoxidases - American Chemical

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