|
Abstract
|
The
glycyl radical enzyme-catalyzed addition of n-alkanes to fumarate creates a
C-C-bond between two concomitantly formed stereogenic carbon centers.
Researchers assigned the configurations of the two diastereoisomers of the product
resulting from n-hexane activation by the n-alkane-utilizing denitrifying
bacterium strain HxN1, i.e. (1-methylpentyl)succinate, as (2S,1′R) and
(2R,1′R). Experiments with stereospecifically deuterated n-(2,5-2H2)hexanes
revealed that exclusively the pro-S hydrogen atom is abstracted from C2 of
the n-alkane by the enzyme and later transferred back to C3 of the
alkylsuccinate formed. These results indicate that the alkylsuccinate-forming
reaction proceeds with an inversion of configuration at the carbon atom (C2)
of the n-alkane forming the new C-C-bond, and thus stereochemically resembles
a SN2-type reaction.
Accordingly, the reaction may occur in a concerted manner, which
may avoid the highly energetic hex-2-yl radical as an intermediate. The
reaction is associated with a significant primary kinetic isotope effect
(kH/kD ≥3) for hydrogen, indicating that the homolytic C-H-bond cleavage is
involved in the first irreversible step of the reaction mechanism. The
(1-methylalkyl)succinate synthases of n-alkane-utilizing anaerobic bacteria
apparently have very broad substrate ranges enabling them to activate not
only aliphatic but also alkyl-aromatic hydrocarbons. Two denitrifiers and one
sulfate reducer were shown to convert the nongrowth substrate toluene to
benzylsuccinate and further to the dead-end product benzoyl-CoA. For this
purpose, however, the modified β-oxidation pathway known from
alkylbenzene-utilizing bacteria was not employed, but rather the pathway used
for n-alkane degradation involving CoA ligation, carbon skeleton
rearrangement and decarboxylation.
|
