Front Matter

18.3 P450 fundamentals

18.3.1 Classification and nomenclature of Monooxygenases

The P450 superfamily is one of the largest and oldest gene families (Nelson and

Strobel, 1988; Nelson et al., 1993). Based on a list of P450 enzymes published

by Nelson et al. in 1996, 481 P450 sequences from 74 gene families were re-

ported. Since then, the number of P450s increased tremendously, and currently

more than 1000 putative P450 sequences are described (http://drnel-

son.utmem.edu/CytochromeP450.html). For example, the genome project of the

plantArabidopsis thalianahas led until the end of December 1999 to the identifica-

tion of more than 270 new putative P450 genes (http://drnelson.utmem.edu/Ara-

blinks.html) (Nelson, 1999). The classification of P450 genes is based on primary

sequence homologies. All P450 genes with a protein sequence homology>40 %

belong to the same gene family. If the protein sequence homology is>55 %,

the corresponding P450 genes belong to the same subfamily. To describe a cyto-

chrome P450 gene, it is recommended to use the italicized abbreviation ‘CYP’

for all P450 genes except for mouse andDrosophila, which are represented by ita-

licized ‘Cyp’. The CYP abbreviation is followed by an arabic number denoting the

family, a letter designating the subfamily (when two or more exist), and an arabic

numeral representing the individual gene within the subfamily. For example,

CYP4A1 represents thefirstgene in the P450 subfamilyAof the P450 gene family

4. To illustrate the evolutionary relationships, it was suggested to merge P450 fa-

milies to so-called P450 clans (Nelson, 1998). According to this classification,

theCYP51 family was identified to be the likely common ancestor of all P450 en-

zymes (Nelson, 1999).

Depending on the realization of the electron transfer system (reductase system),

P450 enzymes are divided into four classes (Degtyarenko, 1995; Peterson and Gra-

ham, 1998):

* Class I: these mainly occur in mitochondrial systems and most bacteria. The elec-

tron systems consist of a FAD-domain as reductase and a further iron – sulfur

protein.

* Class II: are often located in the endoplasmic reticulum and require only a single

protein for the electron transfer, a FAD/FMN-reductase.

* Class III: do not require reduction equivalents. The P450 enzymes directly convert

peroxygenated substrates which have already “incorporated” the oxygen.

* Class IV: only one enzyme is known, which receives its electrons directly from

NADH.

396 18 Fatty Acid Hydroxylations using P450 Monooxygenases