Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

244 Chapter 11
− O 2 C
− O
CO − CO −
2 CO −
2 C
− O 2
2
2 C
CH 2
Ad
NH 3 +
H*
CH 2
Ad
NH
+ 3
CH 2
H*
Ad
NH3
+
CH 2
H*
Ad
Co III
Co II Co II Co II
− O 2 C
CO −
2
H*
NH
+ 3 CH 2
Ad
− O 2 C
CO −
2
H*
NH
+ 3 CH 2
Ad
− O 2 C
CO −
2
NH
*H
+ 3
CH 2
Ad
Co III
Co II
Co II
Figure 11.6 Mechanism for glutamate mutase.
proposed for this enzyme, involving the C2C fragmentation of the substrate
radical to give acrylic acid and a glycyl radical intermediate, which reattaches to
C-2 of acrylic acid to give the product radical. The mechanism is shown in
Figure 11.6.
The elucidation of X-ray crystal structures for vitamin B 12 -dependent
rearrangements is now permitting detailed studies of many unanswered questions:
how is the Co2C homolysis initiated How does the enzyme accelerate
radical-based rearrangements
11.3 The involvement of protein radicals in enzyme catalysis
The enzyme ribonucleotide reductase catalyses the conversion of ribonucleotides
(used for RNA biosynthesis) to 2 0 -deoxyribonucleotides (used for DNA
biosynthesis), shown in Figure 11.7. In 1973 it was discovered that incubation
of the R2 sub-unit of Escherichia coli ribonucleotide reductase with iron(II)
salts, oxygen and ascorbate led to the formation of a stable, long-lived radical
species. Detailed studies using electron paramagnetic resonance (EPR) spectroscopy
established that this species is a tyrosyl radical, formed in close
proximity to a binuclear iron cluster. This was the Wrst example of a stable
protein radical.
Reduction of the tyrosyl radical was shown to lead to the loss of enzyme
activity, thus it is required for catalysis; however, structural studies showed that
Tyr-122 bearing the protein radical was located approximately 35 Å away from
the active site of the enzyme, which is located on the R1 sub-unit of the enzyme.
It is now known that single electron transfers within the protein lead to the
formation of a cysteine radical on Cys-439 in the active site, which then
abstracts the C-3 0 hydrogen to initiate a rather complicated radical reaction,