16.3 Substrate and product specificities of lipoxygenases
16.3.1 Soybean lipoxygenase-1 (SBLOX-1)
Substrate specificity of SBLOX-1
The publication in 1967 by Hamberg and Samuelsson was a landmark in the field of
LOXs (Hamberg and Samuelsson, 1967). Indeed, this work led to a number of im-
portant conclusions being made on SBLOX-1 substrate and product specificities, and
on the stereochemical outcome of the reaction catalyzed by this enzyme. By testing
different natural PUFAs as well as various synthesized unnatural analogs with vary-
ing chain lengths and double bond positions, Hamberg and Samuelsson came to the
conclusion that “the structural requirement for substrates of SBLOX-1 is the pre-
sence of acis,cis-1,4-pentadiene group with its methylene group located in position
x-8”. This study indicated that such substrates would be oxygenated if the chain
length were between C 18 and C 22 , and that substrates with an additional double
bound located either inx-3 (LnA) or inx-9 (c-linolenic acid) position would be
oxygenated by SBLOX-1 as well as heavily unsaturated fatty acids such as AA
(C20 : 4), eicosapentaenoic acid (EPA,C20 : 5) and docosahexaenoic acid
(DHA,C22 : 6). If a methyl group was branched inx-6 position, or if the pentadiene
system was located either inx-8 orx-9 position, then no oxygenation occurred. The
authors showed the enzyme to be n-6-specific, and that the resulting asymmetric
carbon, bearing the hydroperoxide function, was of theSabsolute configuration.
By using tritium labeling, they also established that an antarafacial relationship ex-
isted between the attack of oxygen and the stereospecific removal of the proShy-
drogen atom of the methylene group of the pentadienyl system of the substrate.
The oxygenation by SBLOX-1 of various positional isomers of LA, in which the
methylene interruptedZ,Z-pentadienyl system occurs from the 2,5- to the 14,17-po-
sition (Figure 4) was studied two years later by Holman et al. (1969). These workers
noticed two maxima of activity corresponding to the natural substrate, LA and to the
13 Z,16Zisomer (50 % of LA activity). It was later shown that the formed 17-
HPOD was of theSabsolute configuration (Egmond et al., 1975). Hence, some
flexibility in the position of the pentadienyl system was tolerated by SBLOX-1,
a fact confirmed later by Ku ̈hn et al. (1990) using a series of AA isomers that pos-
sessed a 1Z,4Z,7Z,10Z-undecatetraenyl system located at eitherx-3,x-4,x-5,x-6 or
x-7 position (Figure 5).
344 16 Properties and Applications of Lipoxygenases
Figure 4. Oxygenation of various positional isomers of linoleic acid catalyzed by SBLOX-1 (Holman
et al., 1969).
Figure 5. Oxygenation of various positional isomers of AA catalyzed by SBLOX-1 (Ku ̈hn et al., 1990).