Adenosylcobalamin-dependent diol dehydratase (DD) undergoes suicide
inactivation by glycerol, one of its physiological substrates, resulting in the
irreversible cleavage of the coenzyme Co–C bond. The damaged cofactor
remains tightly bound to the active site.
The X-ray structure of the diol dehydratase–adeninylpentylcobalamin com-plex revealed that the adenine moiety of adenosylcobalamin is anchored in
the adenine-binding pocket of the enzyme by hydrogen bonding of N3
with the side chain OH group of Sera224, and of 6-NH2, N1 and N7 with
main chain amide groups of other residues.
Adenosylcobalamin-dependent diol and glycerol dehydratases are isofunc-tional enzymes and undergo mechanism-based inactivation by a physiologi-cal substrate glycerol during catalysis. Inactivated holoenzymes are
reactivated by their own reactivating factors that mediate the ATP-depen-dent exchange of an enzyme-bound, damaged cofactor for free adenosylco-balamin through intermediary formation of apoenzyme.
ThehadBC and hadI genes from Clostridium difficilewere functionally
expressed inEscherichia coliand shown to encode the novel 2-hydroxyiso-caproyl-CoA dehydratase HadBC and its activator HadI. The activated
enzyme catalyses the dehydration of (R)-2-hydroxyisocaproyl-CoA to
isocaprenoyl-CoA in the pathway of leucine fermentation. The extremely
oxygen-sensitive homodimeric activator as well as the heterodimeric dehy-dratase, contain iron and inorganic sulfur; besides varying amounts of zinc,
other metal ions, particularly molybdenum, were not detected in the dehy-dratase....
[x-(Adenosyl)alkyl]cobalamins (homoadenosylcobalamins) are useful ana-logues of adenosylcobalamin to get information about the distance between
Co and C5¢, which is critical for Co-C bond activation. In order to use
them as probes for exploring the active sites of enzymes, the coenzymic
properties of homoadenosylcobalamins for diol dehydratase and ethanol-amine ammonia-lyase were investigated. The kcat and kcat
adenosylmethylcobalamin were about 0.27% and 0.15% that for the regu-lar coenzyme with diol dehydratase, respectively. ...
The three genespduCDEencoding the diol dehydratase of
Lactobacillus collinoides, have been cloned for overexpres-sion in the pQE30 vector. Although the three subunits of the
protein were highly induced, no activity was detected in cell
extracts. The enzyme was therefore purified to near homo-geneity by ammoniumsulfate precipitation and gel filtration
chromatography. In fractions showing diol dehydratase
activity, three main bands were present after SDS/PAGE
with molecular masses of 63, 28 and 22 kDa, respectively....
Recombinant glycerol dehydratase ofKlebsiella pneumoniae
was purified to homogeneity. The subunit composition of
the enzyme was most probablya2b2c2
.When(R)- and (S)-propane-1,2-diolswere used independently as substrates, the
rate with the (R)-enantiomer was 2.5 times faster than that
with the (S)-isomer. In contrast to diol dehydratase,an iso-functional enzyme,the affinity of the enzyme for the (S)-isomer was essentially the same or only slightly higher than
that for the (R)-isomer (Km(R)
/Km(S) ¼1.5). ...
The pathway of the oxidation of propionate to pyruvate in
Escherichia coliinvolves five enzymes, only two of which,
methylcitrate synthase and 2-methylisocitrate lyase, have
been thoroughly characterized. Here we report that the
isomerization of (2S,3S)-methylcitrate to (2R,3S)-2-methyl-isocitrate requires anovel enzyme,methylcitratedehydratase
(PrpD), and the well-known enzyme, aconitase (AcnB), of
the tricarboxylic acid cycle.
The rare 6-deoxysugard-rhamnose is a component of bacterial cell sur-face glycans, including the d-rhamnose homopolymer produced by Pseu-domonas aeruginosa, called A-band O polysaccharide. GDP-d-rhamnose
synthesis from GDP-d-mannose is catalyzed by two enzymes. The first is
a GDP-d-mannose-4,6-dehydratase (GMD). The second enzyme, RMD,
reduces the GMD product (GDP-6-deoxy-d-lyxo-hexos-4-ulose) to GDP-d-rhamnose.
The serotype a-specific polysaccharide antigen ofActinoba-cillus actinomycetemcomitansis an unusual sugar, 6-deoxy-D-talose.Guanosinediphosphate (GDP)-6-deoxy-D-talose is
the activated sugar nucleotide form of 6-deoxy-D-talose,
which has been identified as a constituent of only a few
microbial polysaccharides. In this paper, we identify two
genes encoding GDP-6-deoxy-D-talose synthetic enzymes,
GDP-a-D-mannose 4,6-dehydratase and GDP-4-keto-6-deoxy-D-mannose reductase, in the gene cluster required for
the biosynthesis of serotype a-specific polysaccharide anti-gen from A.