Cresol Methylhydroxylase PCMH

PCMH is a flavocytochrome c localized in the periplasmic space of several types of Pseudomonad (Hopper and Taylor, 1977). It catalyzes the oxidation of / -cresol first to / -hydroxybenzyl alcohol and then to / -hydroxybenzaldehyde. Electrons are passed sequentially to the endogenous cytochrome subunit and then to an exogenous secondary electron acceptor protein, possibly an azurin or another cytochrome (Causer et al., 1984). 

The rate of intramolecular electron transfer from the flavin to the heme in PCMH has been measured by laser flash photolysis using 5-deazariboflavin semiquinone radical as a 1-electron reductant (Bhat- 

FIGURE 8. stereo diagram of p-cresol methylhydroxylase. The flavoprotein subunit is on the left and the cytochrome subunit is on the right. The flavin-binding domain of the flavoprotein subunit is on the bottom and the catalytic domain is on the top. Skeletal models of the heme and FAD prosthetic groups are also shown. 

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There are five classes of flavin-binding structural folds presented in Table 1 that are identified by the prototype protein in which they were first discovered. These are flavodoxin (FDX), ferredoxin reductase (FNR), triosephosphate isomerase (TIM), glutathione reductase (GR) and p-cresol methylhydroxylase (PCMH). The topologies of four of these five domains are shown in Figure 2. There are also four classes of primary acceptor/donor domain folds that are identified by the prototype protein where they were first discovered. They are cytochrome P450BMP (BMP), cytochrome b5 (CYTB5), cytochrome c (CYTC) and the 2Fe-2S plant-type ferredoxin (FDN). 

One of a few cases in which electron transfer of redox enzymes is expressed directly and reversibly at an electrode is concerned with p-cresol methylhydroxylase (PCMH). This is a flavocytochrome c enzyme of 115 kDa, which catalyzes the oxidative hydroxylation of p-cresol to p-hydroxybenzyl alcohol and subsequently to p-hydroxyben-zaldehyde. The structure of PCMH has recently been determined (56) at 3 A resolution. It is an a2 2 tetramer, with one subunit containing a covalently bound FAD and the other containing a c-type heme group. 

Fig. 2. Graph showing how voltammetric peak current is expected to diminish with increasing molecular weight in the case of redox-active spherical molecules diffusing to a planar electrode. PCMH = p-cresol-methylhydroxylase (see Sect 7.3)...

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