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Radicals in Enzyme Catalysis 249
The same combination of SAM and an [4Fe4S] cluster is found in the
activating enzymes for pyruvate formate lyase and anaerobic ribonucleotide
reductase, and, as we shall see in the next section, in the enzyme biotin synthase.
Thus, although at Wrst it was termed ‘the poor man’s vitamin B 12 ’, the involvement
of SAM in radical chemistry is proving to be at least as great as its more
elaborate cousin.
11.5 Biotin synthase and sulphur insertion reactions
The coenzyme biotin is an essential cofactor in enzymes such as acetyl CoA
carboxylase responsible for a-carboxylation reactions, described in Section 7.5.
Mammals must consume biotin in their diet, but bacteria are able to biosynthesise
biotin using a remarkable pathway. The Wnal step in the pathway, catalysed
by biotin synthase, is a sulphur insertion reaction shown in Figure 11.14. This
type of reaction has little chemical precedent, but is also used for the biosynthesis
of the cofactor lipoic acid, as shown in Figure 11.14.
The mechanism of biotin synthase is still a topic of active research, but
several parallels have emerged with the reactions described in Section 11.4.
The enzyme requires SAM for activity, and a reduced [4Fe4S] cluster, which is
assembled from two [2Fe2S] clusters. Enzyme activity also requires several
other redox proteins and redox cofactors for activation, whose precise role
has yet to be established; however, it seems likely that, as described in Section
11.4, the [4Fe4S] is somehow able to activate SAM to generate a 5 0 -deoxyadenosine
radical. It is thought that this radical abstracts a hydrogen atom at C-9
to generate a radical intermediate, as shown in Figure 11.15, which abstracts a
sulphur atom from the [4Fe4S] cluster. Abstraction of a second hydrogen atom
at C-6 generates a second radical intermediate, which cyclises to form the Wvemembered
ring. Abstraction of a sulphur atom from the [4Fe4S] cluster means
O
O
HN
9
H 3 C
NH
6
CO 2 H
biotin synthase
S-adenosyl Met
[4Fe4S] red
HN
H
S
NH
H
H
CO 2 H
CO 2 H
COSCoA
lipoate synthase
S-adenosyl Met
[4Fe4S] red
S
S
H
COSCoA
Figure 11.14 Enzymatic sulphur insertion reactions.