Peroxidase distal histidine

DET calculations on the hyperfine coupling constants of ethyl imidazole as a model for histidine support experimental results that the preferred histidine radical is formed by OH addition at the C5 position [00JPC(A)9144]. The reaction mechanism of compound I formation in heme peroxidases has been investigated at the B3-LYP level [99JA10178]. The reaction starts with a proton transfer from the peroxide to the distal histidine and a subsequent proton back donation from the histidine to the second oxygen of the peroxide (Scheme 8). 

However, the distal histidine apparently has no effect on lignin and Mn-dependent peroxidase compound I formation. Although all active site amino acid residues that are proposed to participate in compound I formation of peroxidase (37) are conserved in lignin and Mn-dependent pa oxidases, the lack of pH dependence may be a result of some inherent structural and conformational differences between lignin and Mn-dependent peroxidases and other peroxidases. 

Figure 2. Comparison of P. chrysosporium MnP-1 and other peroxidases at regions near the proximal and distal histidines. The peroxidase sequences used were manganese peroxidase (MnP) (20), cytochrome c peroxidase (CCP) QS), horseradish peroxidase (HRP)...

In the case of beef liver catalase, distal histidine (His74) is believed to serve as a general acid-base catalyst to facilitate the heterolytic 0—0 bond of hydroperoxide bound to the heme (Scheme 2) (21). The Asnl47 residue located near the heme could assist the heterolysis by making the distal site into a polar atmosphere. The same acid-base mechanism has been attributed to peroxidases... 

As discussed in Scheme 2, the distal histidine in peroxidase and catalase is a crucial amino acid residue for the formation of compound I. In fact, the... 

While most of peroxidases utilize the distal histidine as a general acid-base catalyst, chloroperoxidese (CPO) uniquely uses glutamate as the terminal acid-base catalyst (Scheme 8) (56). 

Figure 6.2 The mechanism of cytochrome-c-peroxidase complex formation, (a) Native enzyme, (b) Activated complex with the acid-base catalytic function of distal histidine (His) and stabilization of negative charge by arginine (Arg) residue of the active site, (c) Hypothetic intermediate oxene complex, (d) Complex I after intramolecular electron regrouping of oxene complex with Fe4+ and free radical X fragment formation.

As in the other members of the superfamily, the heme pocket of ligninolytic peroxidases includes two conserved histidine residues disposed above and below the heme plane (Fig. 3.4a). The second histidine acts as the fifth ligand of the heme iron, occupying a proximal position, while the first one is at a higher distance being, therefore, called distal histidine (by extension, the regions located below and above the heme plane are also called proximal and distal regions). 

The first step of peroxidase catalysis involves binding of the peroxide, usually H2C>2, to the heme iron atom to produce a ferric hydroperoxide intermediate [Fe(IE)-OOH]. Kinetic data identify an intermediate prior to Compound I whose formation can be saturated at higher peroxide concentrations. This elusive intermediate, labeled Compound 0, was first observed by Back and Van Wart in the reaction of HRP with H2O2 [14]. They reported that it had absorption maxima at 330 and 410 nm and assigned these spectral properties to the ferric hydroperoxide species [Fe(III)-OOH]. They subsequently detected transient intermediates with similar spectra in the reactions of HRP with alkyl and acyl peroxides [15]. However, other studies questioned whether the species with a split Soret absorption detected by Back and Van Wart was actually the ferric hydroperoxide [16-18], Computational prediction of the spectrum expected for Compound 0 supported the structure proposed by Baek and Van Wart for their intermediate, whereas intermediates observed by others with a conventional, unsplit Soret band may be complexes of ferric HRP with undeprotonated H2O2, that is [Fe(III)-HOOH] [19]. Furthermore, computational analysis of the peroxidase catalytic sequence suggests that the formation of Compound 0 is preceded by formation of an intermediate in which the undeprotonated peroxide is coordinated to the heme iron [20], Indeed, formation of the [Fe(III)-HOOH] complex may be required to make the peroxide sufficiently acidic to be deprotonated by the distal histidine residue in the peroxidase active site [21],... 

Fig. 5.4 Schematic representation of the Poulos-Kraut peroxidase mechanism in which the conserved distal histidine serves as an acid-base catalyst that transfers a proton from the H202 to the terminal oxygen after formation of the [Fe(III)-OOH] intermediate. The proximal histidine iron ligand and the catalytic histidine and arginine are shown. In HRP, these residues are His 170,...

Fig. 4.78. Role of the distal histidine in the "pull" mechanism for the cleavage of the dioxygen bond and creation of the high-valent iron-oxo porphyrin it cation radical (Compound I) in peroxidases.

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