Bromoperoxidase mechanism

Abbreviated proposed mechanism of vanadium bromoperoxidase-catalyzed bromination 2123, 2124, 2175, 2261, 2263, 2269, 2280, 2328). 

Figure 1 Simplified model for the reaction mechanism of the vanadium bromoperoxidases. As we will see later this mechanism may be slightly different for the vanadium chloroperoxidases...

Steady-state kinetic studies showed that the kinetics of the enzyme resemble those of the vanadium bromoperoxidases. The chloroperoxidase exhibits a pH profile similar to vanadium bromoperoxidases although the optimal pH of 4.5-5.0 is at a lower value. At low pH the enzyme is inhibited by chloride in a competitive way whereas at higher pH values the activity displays normal Michaelis-Menten type of behavior (see Michaelis Constant). The log Km for chloride increases linearly with pH whereas that for hydrogen peroxide decreases with pH demonstrating that in the catalytic mechanism protons are involved. These observations have led to a simplified ping-pong type of mechanism for the chloroperoxidase similar to that shown in (Figure 1). 

Scheme 1. Simplified mechanism of action of vanadium bromoperoxidase. The species in boxes represent inhibited forms of the enzyme.

In contrast, the fungal heme-containing chloroperoxidase from Cal-dariomyces fumago is rapidly inactivated by HOBr (31). As early as 1926 Sauvageau (30) concluded that certain species of seaweeds are able to produce free bromine in solution. Scheme I gives the tentative reaction mechanism of bromoperoxidase with its substrates (32,34). 

Recent papers describe models for molybdenum-containing enzymes [106], Certain vanadium complexes have been described to mimic the binding site reactions of vanadium haloperoxidases [62, 107], Scheme XI.20 demonstrates the mechanism of active species formation in bromoperoxidase proposed on the basis of the investigation of model reactions [108],... 

Littlechild J, Rodriguez EG, Isupov M. Vanadium containing bromoperoxidase-insights into the enzymatic mechanism using X-ray crystallography. 7. Inorg. Biochem. 2009 103 617-621. 

Butler, A. and Tschirret-Guth, R.A. (1996) On the selectivity of vanadium bromoperoxidase, in Mechanisms of Biohalogenation and Dehalogenation, vol. 25F (eds D.B. Janssen, K. Soda, and R. Wever), Royal Netherlands Academy of Arts and Sciences, Amsterdam, pp. 55-68. 

The biosynthesis of long-chain acids with even numbers of carbon atoms having been demonstrated (see above), there is every reason to suppose that the introduction of bromine takes place in the final stage, from the corresponding acid by the action of a bromoperoxidase, and it is reasonable to suppose that the same is true for adds with odd numbers of carbon (Lam et al, 1989). The same mechanism applies to both sets of acids by elongation, desaturation and halogenation, from palmitic acid for even acids and pentadecanoic acid for odd acids, as summarized in Figiue 19.13 (Lam et al., 1989 Carballeira, 1997 Dembitsky and Srebnik, 2002). 

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