Turnip Peroxidases

Although many of the cDNAs did not reveal significant homology to any sequence in the database, several interesting observations were made. Of the Cu-inducible clones, two showed protein homology to known sequences one appeared to be related to horseradish and turnip peroxidase, which are stress-inducible proteins the other had features characteristic of cytochrome oxidase, a Cu-containing enzyme. 

Turnip peroxidase (TuP) A, 49000d Ferriprotoporphyrin IX - Fine needles... 

Turnip peroxidase 8 -233 to -160 Lignification, cell wall formation [60]... 

Ricard J, Mazza G, William RJP (1972) Oxidation reduction potentials and ionization states of two turnip peroxidases. Eur J Biochem 28 566-578... 

Quintanilla-Guerrero F, Duarte-Vazquez MA, Garcia-Almendarez BE et al (2008) Polyethy lene glycol improves phenol removal by immobilized turnip peroxidase. Bioresour Technol 99 8605-8611... 

Quintanilla-Guerrero F, Duarte-Vaquez M, Tinoco R et al (2008) Chemical modification of turnip peroxidase with methoxypolyethylene glycol enhances activity and stability for phenols removal using the immobilized enzyme. J Agric Food Chem 56 8058-8065... 

Matto M, Husain Q (2009) Decolorization of direct dyes by immobilized turnip peroxidase in batch and continuous processes. Ecotoxicol Environ Saf 72 965-971... 

The debate on whether the pJCa of 8.6 for compound II of horseradish peroxidase (HRP-II) represents the ionization of an iron-bound water molecule continues. Dunford adduces further kinetic evidence that it does while Schejter provides additional n.m.r. evidence that there is no water molecule in the inner co-ordination sphere of the iron(m) atom however, the latter interpretation of the data has been questioned by Vuk-Pavlovi6 and Benko, who suggest that the sixth ligand site of the metal contains what they term a sedentary water molecule. The formation of HRP-I from the reaction of native horseradish peroxidase with HgOa h a rate constant of 2.5 x 10 1 mol s at 25 °C (pH 7.10), with an activation energy of 3.5 1.0 kcal mol". The kinetics of cyanide and fluoride binding to turnip peroxidases Pi and P7 have also been measured. "... 

The tyrosyl radical was also identified in a study of radical intermediates in turnip peroxidase isozymes. Reaction of turnip isoperoxidases 1, 3 and 7 with hydrogen peroxide causes the formation of an exchange-coupled oxoferryl-porphyrinyl radical species but, unexpectedly, at basic pH it was shown from high-field measurements that a hydrogen-bonded tyrosyl radical (gx = 2.0066)... 

The reaction of a crystalline turnip peroxidase has been shown to proceed with hydrogen peroxide and any of a group of electron acceptors, cytochrome C, methylene blue, or ferric iron in place of oxygen (Yamazaki and Souzi, 1960). A mechanism was proposed in which both hydrogen peroxide and the other oxidant participate in sequential steps of the oxidation of indoleacetic acid. When oxygen is the second oxidant, it is reduced to hydrogen peroxide, which, therefore, plays a catalytic role. 

Horse-radish peroxidase, HRP turnip peroxidase, TP dihydroxyfumaric acid (previously misnamed dioxymaleic acid), DHF peroxidase donors, AH and BH free radicals, AH, DHF, etc. indoleacetic acid, lAA peroxidase peroxide compounds I, II, and III, Compounds I, II, and III, respectively. 

Another example, even more complex, is the dual action of turnip peroxidase. Here again Mn++ is an effector. 

Peroxidase does not catalyze lAA peroxidation under anaerobic conditions in the absence of a H acceptor. This can probably be explained by enzyme inactivation. The free radicals °IAA cannot enter into reaction (83) and will destroy the enzyme. The lack of inhibition by CO and the promoting action of Mn++ and phenols is explained in Fig. 23. The manganous ions catalyze the autoxidation of the redogenic substances (YHj). It produces H20g, starting thereby the turnip peroxidase reaction (TP). This is quite similar to the situation seen in reaction (68). BH... 

Fig. 23. The relation between oxidogenic substrates (XHj), redogenic substrates (YHj), and acceptors (B) in the oxidase activity of turnip peroxidase. Free-radical intermediates are formed YH and XH. ...

The possible occurrence of free radicals in biochemical processes has been refuted by several authors because of their socalled high reactivity (Laidler, 1958). If free, the radicals would destroy in no time cellular components, especially the enzyme proteins themselves. We have dealt with this matter in our discussion of turnip peroxidases activity. We would like therefore to examine the chemical properties of the free radicals in a more general way to pro" e that these substances have a graded reactivity. 

---