Catalase catalytic cycle

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

Figure 16-14 The catalytic cycles and other reactions of peroxidases and catalases. The principal cycle for peroxidases is given by the colored arrows. That of catalases is smaller, making use of step a, b, and c , which is marked by a light green line.

A closer look at the catalytic cycle, as shown in Fig. 4.12, shows how zinc plays a largely structural role while copper continually changes oxidation state and coordination environment. It should be immediately apparent that one of the by-products is the equally toxic reactive species, hydrogen peroxide. Fortunately a second enzyme, catalase, is present to scavenge the peroxide and convert it to oxygen and water. 

Various porphyrin compounds such as NFevOEP are known.62 The i Fe=N) stretching frequency of 853 cm-1 is lower than that of the Cr and Mn analogues the nitrido Fev complex is stable only at very low temperature ( 30 K). It is widely accepted that one of the intermediate species in the catalytic cycle of cytochrome P-450 (see later) contains pentavalent iron. The reduction of an R3CFev moiety is an accepted part of the mechanism of dismutation of hydrogen peroxide to water and oxygen catalyzed by catalase. 

Four-valent iron centers are well known as catalytic intermediates in enzymatic heme catalysis. The catalytic cycles of, for example, catalase, peroxidase, and cytochrome P450, all have in common an intermediate that is comprised of a ferryl Fe(lV) and a further oxidation equivalent located either on the heme ring or on the protein moiety. This reaction intermediate is called compound 1 (cpd 1) and has a system spin of 5 = 1/2 (e.g., in chloroperoxidase) or 5 = 3/2 (e.g.. 

During its catalytic cycle, the Mn catalases shuttle between the Mn and the Mn oxidation states in 2 two-electron reactions (137). Overall this process converts two hydrogen peroxide molecules into one molecule of dioxygen and two water molecules. A proposal for the catalytic cycle of catalase is presented in Scheme 8 (23). When the... 

This was followed by an article in 1997 (445), wherein Naruta et al. prepared a dimer with Mnwporphyrin(OMe)(OMe or OH). The analysis of this system indicated that catalase activity occurred via approach of hydrogen peroxide to the pocket of the dimer, followed by deprotonation and concomitant reduction of the dimer and oxidation of the hydrogen peroxide to dioxygen. Their proposed catalytic cycle is shown in Scheme 22. Similar systems have been used to generate dioxygen from water (discussed later). 

III). As a byproduct of these two le equivalent oxidation steps, electron donors (designated as AH) are converted to free radicals (A ). The peroxidative cycle of catalase is about 1000-times slower than the catalytic cycle [120]. Since SA can act as a substrate for both of the steps constituting the peroxidative cycle (steps 3 and 4), it shunts the enzyme into this slower peroxidative cycle and thereby causes an inhibition of catalase activity. 

Fig. 2. The reaction cycles of catalase. The oxidation states of the heme iron are shown in parentheses. The native enzyme is in the ferric form (Fe III). AH represents an electron donor, while A denotes a free radical. The first step (step 1) in the catalytic cycle is the 2e equivalent reduction of HjOj to HjO with the accompanying oxidation of the ferric enzyme (Fe HI) to compound I (Fe V) [112]. Compound I is converted back to the native enzyme by a 2e " equivalent reduction and the corresponding oxidation of a second molecule of HjOj to O2 (step...

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