Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

246 Chapter 11
H
N
H
O
734
H
N
H
PFL activating
enzyme
[4Fe4S] red
S-adenosyl Met
H 3 C
O
O−
O
O
H
N 734
N
H
O
+ CoASH +
pyruvate formate lyase H 3 C SCoA
H
O
O−
Figure 11.9 Pyruvate formate lyase.
The second protein radical to be discovered was in the enzyme pyruvate
formate lyase, which catalyses the reversible interconversion of pyruvate and
CoA with formate and acetyl CoA, as shown in Figure 11.9. This is a key step in
the anaerobic fermentation of glucose, converting pyruvate into acetyl CoA. In
1989 a stable radical species was detected in E. coli pyruvate formate lyase,
which was subsequently shown to be located on the a-carbon of a glycine
residue, at Gly-734. The glycine radical is generated by a separate activating
enzyme.
The glycine radical is present at the active site of the enzyme, which also
contains two cysteine residues, Cys-418 and Cys-419. The catalytic mechanism
is thought to involve the formation of a thiyl radical at Cys-419, followed by
attack on the keto group of pyruvate. C2C fragmentation then generates a
formate radical, which abstracts a hydrogen atom from Gly-734 to complete
the catalytic cycle, as shown in Figure 11.10.
Several other examples of protein radicals in enzyme-catalysed reactions
have since been discovered. A stable tyrosyl radical is involved in the reaction
catalysed by prostaglandin H synthase, and a glycine radical is implicated in the
anaerobic ribonucleotide reductase. A stable tryptophan radical has been implicated
in cytochrome C peroxidase, in proximity to a haem cofactor. Thus, it
has been Wrmly established that protein radicals can be involved directly in
enzyme catalysis.
11.4 S-adenosyl methionine-dependent radical reactions
Shortly after the discovery of stable protein radicals described above, several
observations emerged that implicated the coenzyme S-adenosyl methionine
(SAM) in radical-mediated enzyme reactions. The activating enzyme for pyruvate
formate lyase was found to require SAM for activity (see Figure 11.9).
Similarly, the activating enzyme for anaerobic ribonucleotide reductase