Puted and there is consequently no general agreement

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puted, and there is consequently no general agreement concerning its identity. However, EXAFS measurements of metal-metal separation and the strength of the magnetic coupling between the two metal centers provide evidence that a single atom bridges the two metals. 45 ,46 Another issue, which is of great impor- tance, is to find out how the energy released in the reduction of dioxygen is coupled to the synthesis of ATP. It is known that this occurs by coupling the electron-transfer steps to a proton-pumping process, but the molecular mecha- nism is unknown. 46 Future research should provide some interesting insights into the mechanism of this still mysterious process. V. OXYGENASES A. Background The oxygenase enzymes catalyze reactions of dioxygen with organic substrates in which oxygen atoms from dioxygen are incorporated into the final oxidized product. 2-4 These enzymes can be divided into dioxygenases, which direct both atoms of oxygen into the product (Reaction 5.53), and monooxygenases, where one atom of oxygen from dioxygen is found in the product and the other has been reduced to water (Reaction 5.54): Dioxygenase: substrate + *02-----'>substrate(*Oh Monooxygenase: substrate + *0 2 + 2H + + 2e - -----'>substrate(*O) + H 2 *0 B. Dioxygenases (5.53) (5.54) Dioxygenase enzymes are known that contain heme iron, nonheme iron, copper, or manganese. 66 ,67 The substrates whose oxygenations are catalyzed by these enzymes are very diverse, as are the metal-binding sites; so probably several, possibly unrelated, mechanisms operate in these different systems, For many of these enzymes, there is not yet much detailed mechanistic information. How- ever, some of the intradiol catechol dioxygenases isolated from bacterial sources have been studied in great detail, and both structural and mechanistic informa- tion is available. 66 ,67 These are the systems that will be described here. 1. Intradiol catechol dioxygenases The role of these nonheme iron-containing enzymes is to catalyze the deg- radation of catechol derivatives to give muconic acids (Reaction 5.55, for ex- ample). The enzymes are induced when the only carbon sources available to the bacteria are aromatic molecules. The two best-characterized members of this class are catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD),
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V. OXYGENASES 277 R := H, cis,cis-muGonic acid; Enz = catechol 1,2-dioxygenase O OH I ~ +0 2 R OH ~ {:eOOH ~ eOOH R R = COO-, p-carboxy cis,cis-muconic acid; Enz = protocatechuate 3,4-dioxygenase (5.55) a. Characterization of the Active Sites Even before the x-ray crystal struc- ture of PCD was obtained, a picture of the active site had been constructed by detailed spectroscopic work using a variety of methods. The success of the spec- troscopic analyses of these enzymes is a particularly good example of the im- portance and usefulness of such methods in the characterization of metallopro- teins. The two enzymes referred to in Reaction (5.55) have different molecular weights and subunit compositions,66 but apparently contain very similar active- site structures and function by very similar mechanisms. In both, the resting state
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