18 Fatty Acid Hydroxylations using P450
Monooxygenases
Ulrich Schwaneberg and Uwe T. Bornscheuer
18.1 Introduction
Cytochrome P450 enzymes belong to the class of monooxygenases (E.C. 1.14.x.y).
They are widely distributed in nature (Ortiz de Montellano, 1995; Lewis, 1996) and
play a key role in various steps of primary and secondary metabolism as well as in
detoxification of xenobiotic compounds (Stegeman and Lech, 1991; Goldstein and
Faletto, 1993; Bolwell et al., 1994). Structural similarities of P450 enzymes reside in
a heme system in the catalytic center which contains – in contrast to other enzymes –
a fifth cysteine ligand co-ordinated to the iron atom. This feature is responsible for
the characteristic spectral properties of P450 enzymes: the maximum absorption at
450 nm in the differential spectra of carbon monoxide, which gave P450 systems
their name (Omura and Sato, 1964).
From a functional point of view, all P450 enzymes catalyze the transfer of mo-
lecular oxygen to usually nonactivated aliphatic or aromatic X-H bonds (X: -C, -N, -
S) (Goldstein and Faletto, 1993). Furthermore, a remarkable number of P450 en-
zymes are capable of epoxidizing -C¼C- double bonds (Ruettinger and Fulco,
1981; Lewis, 1996). For these oxygenation reactions most P450 enzymes require
cofactors such as NADPH or NADH as reduction equivalents.
P450 enzymes play a pivotal role in metabolic pathways such as ergosterol bio-
synthesis (Berg et al., 1988), the biosynthesis of insect and plant hormones (Durst
and O’Keefe, 1995; Feyereisen, 1999), the formation of colors and odors of plants
(Holton et al., 1993; Holton, 1995), in aromatisation of androgen to estrogen, and in
the metabolism of arachidonic acid to prostaglandins, leukotrienes and thrombox-
anes (Kupfer and Holm, 1989; Simpson et al., 1997). Furthermore, they are involved
in activation and detoxification of xenobiotic compounds. As phase I enzymes
(Goldstein and Faletto, 1993), P450 enzymes generate water-insoluble or barely
soluble compounds for further metabolism by oxygenation. Subsequent reactions
are performed by phase II enzymes such as glutathione transferases,N-acetyltrans-
ferases or sulfotransferases, which add a further polar group to the initially hydro-
xylated or epoxidized compounds and thereby make these metabolites water-soluble
and bioavailable (Goldstein and Faletto, 1993).
The diversity of P450-catalyzed conversions has a tremendous biotechnological,
pharmacological and toxicological potential. This is especially important because
many reactions are difficult to perform by chemical means. In the following sections
an overview regarding this potential with special emphasis on P450-mediated lipid
modifications is provided. Limitations, as well as actual developments and future
prospects, are also discussed.
Enzymes in Lipid Modification.Edited by Uwe T. Bornscheuer
Copyright2000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. ISBN: 3-527-30176-3
