Catalase compounds

Figure 5. Comparison of the optical absorption spectra of cobalt(III) porphyrin ir-cation radical species with those of catalase compound / and horseradish peroxidase compound L The ground states of the bromide and perchlorate species are 2A lu and 2Agu, respectively.

Fig. 4. Visible spectra of catalase, compound I, and compound II 5 [xM (heme) beef liver catalase (Boehringer-Mannheim) in 0.1 M potassium phosphate buffer pH 7.4, 30°C. Compound I was formed by addition of a slight excess of peroxoacetic acid. Compound II was formed from peroxoacetic acid compound I by addition of a small excess of potassium ferrocyanide. Absorbance values are converted to extinction coefficients using 120 mM for the coefficient at 405 nm for the ferric enzyme (confirmed by alkaline pyridine hemochromogen formation). Spectra are corrected to 100% from occupancies of f 90% compound I, 10% ferric enzyme (steady state compound I) and 88% compound II, 12% compound I (steady state compound II). The extinction coefficients for the 500 to 720 nm range have been multiplied by 10. Unpublished experiments (P.N., 1999).

The catalase compound I appears to be converted in a two-electron reduction by H202 directly to free ferricatalase without intervention of compound II (Fig. 16-14, step c ). The catalytic histidine probably donates a proton to help form water from one of the oxygen... 

Many other species also contain catalases. In bacteria these can contain either haem or dihydroporphyrin (chlorin) prosthetic groups [52], However, the presence of a weak tyrosine-ligation to the iron appears to be present in all cases. This, combined with the lack (present in peroxidases) of an H-bond between an arginine residue and the ferryl oxygen, may explain why catalase compound I is uniquely reactive to H2O2. 

Rates of Formation of Catalase Compound I (Mammalian Catalases at pH 7, 25°)... 

Fia. 8. Reduction of horse erythrocyte catalase Compound I by methyl hydrogen peroxide at pH 7, 4°. Note also the ready oxidation of Compound II to Compound III by HaOj where k, 3 X 10 M" sec ... 

Apparent Thermodynamic Activation Parameters for Some Catalase Compound I-Mediated Oxidations ... 

The marked changes in spectra accompanying changes in spin state may be used as a simple method for estimating spin state populations. When attempting to relate this property to optical spectra or to the chemical behaviour of haemoproteins it is important to realise that spin state transitions may be temperature dependent. An example of this is catalase compound II at 77°K it is a mixture of low and high spin forms, at 20°K it is predominately high spin, whilst at room temperature optical spectra show it to be mainly low spin [11]. 

Green, M.T. (2001). The structure and spin coupling of catalase compound 1 A study of noncova-lent effects. J. Am. Chem. Soc. 123, 9218-9219. 

The reaction cycle of the catalases (Fig. 6), like that of the peroxidases, begiris with the high-spin ferric state (7i) which reacts with a molecule of hydrogen peroxide to form the Compound I intermediate (14). Next, however, oxidation of a second hydrogen peroxide molecule yields dioxygen, with the concomitant return of catalase Compound I to the native resting state. Catalases can be made to produce a Compound II intermediate that is generally described as an Fe =0 complex like Compound II of the peroxidases. 

The absorption spectrum of the Compound I form of chloroperoxidase is different from that of horseradish peroxidase Compound I, and is more closely analogous to that of catalase Compound I. This suggests that it may also have a Aiu ground state [22, 50]. However, the EPR spectrum of chloroperoxidase Compound I indicates that there is electron density at the meso carbons this finding is inconsistent with a Ai ground electronic state [50, 93]. Thus, the different absorption spectral properties of the Compound I intermediates of peroxidases may not derive solely from differences in orbital symmetry. Rather, other factors such as the nature of the axial ligand or the macrocycle stereochemistry may be responsible for these spectral differences. 

As mentioned in Sect. C.II.2, above, inhaled mercury vapor persists in the blood stream sufficiently long to reach the blood-brain barrier. Due to its high lipophilicity, mercury vapor rapidly diffuses across the blood-brain barrier, where it is oxidized to ionic mercury by the catalase-compound I system. Oxidized mercury is easily subjected to the reaction with SH-containing ligand in the central nervous system thus fixation of mercury occurs. Little is known concerning the precise causal relationship between the sites of mercury deposition in the brain and the characteristic neurotoxicity of mercury vapor. It is conceivable that essentially the same mechanism of toxicity as that of ionic mercury is operating at molecular level. 

Azide forms compounds with catalase (98), peroxidase (210), and ferrihemoglobin (95). The susceptibility of the catalase compound is 14,500 X 10- cc., indicating ionic bonding. The ferrihemoglobin compound is stated to be covalent (35). 

For the reasons mentioned above, differential titration and redox potential determinations cannot be used to determine the nature of the heme-linked groups. Under such conditions it seems impossible to solve the whole question of the heme-linked groups of catalases by the aid of the methods we have at our immediate disposal. Some amino acid analyses have been carried out, but these cannot yet contribute to our knowledge of the heme-linked groups. However, some information has been obtained from magnetometric observations, from spectrophotometric studies of different catalase compounds with inhibitors, and from activity determinations in solutions of different pH values and varying concentrations of inhibiting anions. 

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