Cytochrome active site

Scheme 10.8 Biosynthesis of epothilone. Individual PKS domains are represented as circles and individual NRPS domains as hexagons. Acyl carrier proteins (ACPs) and thiola-tion domains (T) are posttranslationally modified by a phos-phopantetheinyl group to which the biosynthetic intermediates are covalently bound throughout the chain assembly. The thioesterase domain (TE) cyclizes the fully assembled carbon chain to give the 16-membered lactone. Following dehydration of Cl 2—Cl 3 to give epothilones C and D, the final step in epothilone biosynthesis is the epoxidation of the C12=C13 double bond by the cytochrome P450 enzyme P450epol<. KS ketosyn-thase KS(Y) active-site tyrosine mutant of KS AT acyltransfer-ase C condensation domain A adenylation domain ...

Ekins S, De Groot MJ, Jones JP. Pharmacophore and three-dimensional quantitative structure activity relationship methods for modeling cytochrome P450 active sites. Drug Metab Dispos 2001 29 936-44. 

Ribbon structures of two redox proteins, cytochrome c (a) and plastocyanin (b). The blowups show the active sites where transition metal atoms are located. 

Mutations at the active site of CYPlOl (cytochrome P450j,j jj) from a strain of Pseudomonas putida made possible the monooxygenation of chlorinated benzenes with less than three substituents to chlorophenols, with concomitant NIH shifts for 1,3-dichlorobenzene (Jones et al. 2001). Further mutations made it possible to oxidize even pentachlorobenzene and hexachlorobenzene to pentachlorophenol (Chen et al. 2002). Integration of the genes encoding cytochrome PTSO. into Sphingobium chlorophenolicum enabled this strain to partially transform hexachlorobenzene to pentachlorophenol (Yan et al. 2006). 

Loew GH, Harris DL. 2000. Role of the heme active site and protein environment in structure, spectra and function of the cytochrome P450s. Chem Rev 100 407. 

The improvement of its activity and stability has been approach by the use of GE tools (see Refs. [398] and [399], respectively). A process drawback is the fact that the oxidation of hydrophobic compounds in an organic solvent becomes limited by substrate partition between the active site of the enzyme and the bulk solvent [398], To provide the biocatalyst soluble with a hydrophobic active site access, keeping its solubility in organic solvents, a double chemical modification on horse heart cytochrome c has been performed [400,401], First, to increase the active-site hydrophobicity, a methyl esterification on the heme propionates was performed. Then, polyethylene glycol (PEG) was used for a surface modification of the protein, yielding a protein-polymer conjugates that are soluble in organic solvents. 

Fig. 6.9 The catalysts for denitrification. Nitrate is reduced by a molybdenum enzyme while nitrite and oxides of nitrogen are reduced today mainly by copper enzymes. However, there are alternatives, probably earlier iron enzymes. The electron transfer bct complex is common to that in oxidative phosphorylation and similar to the bf complex of photosynthesis, while cytochrome c2 is to be compared with cytochrome c of oxidative phosphorylation. These four processes are linked in energy capture via proton (H+) gradients see Figure 6.8(a) and (b) and the lower parts of Fig. 6.9 which show separately the active site of the all iron NO-reductase, and the active site of cytochrome oxidase (02 reductase).

Metalloproteins fall into three main structure categories depending on whether the active site consists of a single coordinated metal atom, a metal-porphyrin unit, or metal atoms in a cluster arrangement. In the context of electron-transfer metalloproteins, the blue Cu proteins, cytochromes, and ferre-doxins respectively are examples of these different structure types. Attention will be confined here mainly to a discussion of the reactivity of the blue Cu protein plastocyanin. Reactions of cytochrome c are also considered, with brief mention of the [2Fe-2S] ferredoxin, and high potential Fe/S protein [HIPIP]. 

Cytochrome c oxidase is an enzyme that couples the one-electron oxidation of cytochrome c to the four-electron reduction of 02 and is thus a crucial component of respiration. Cytochrome c contains the redox-active heme c, while cytochrome c oxidase contains a dinuclear Cua redox site in subunit II and three redox-active sites in subunit I heme a, heme a3, and Cur. It is believed that heme a is an electron-transfer site, while heme a3 and Cur function together at the 02 reduction site. 

Because many other chemicals can affect the enzymes responsible for n-hexane metabolism (see Section 2.3.3, Metabolism), the possibility of interactions is a significant concern. The initial step in n-hexane metabolism is oxidation to a hexanol by a cytochrome P-450 isozyme other chemicals can induce these enzymes, possibly increasing the rate of metabolism to the neurotoxic 2,5-hexanedione, or competing with M-hexanc and its metabolites at enzyme active sites, reducing the rate of metabolism. Interactive effects can be concentration and/or duration dependent. 

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