REACTIVE OXYGEN SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 179

O

O

O N O

N

N

HO •

O 2 N

o

m
p

OH

OH

HOO •

•

OH

H • OO

H

H

H

(and other sites of the ring) +16 Da

Figure 4.32. Oxidation of Phe by • OH radical (adapted from xu and chance [261] with
the permission of the American chemical Society).

In the presence of O 2 , the addition of oxygen also takes place. The final
product, DOPA, was formed with the elimination of HOO • , which on further
hydroxylation yielded trihydroxyphenylalanine (TOPA). In the oxidation of
peptides (Tyr-Leu and Leu-Tyr) by • OH, produced under the Fenton reaction,
a wide variety of oxidation products were obtained, which include 1, 2, 3, and
4 oxygen atoms containing peptides [355]. Formation of the peroxy group
occurred preferentially in the c-terminal residue. Other oxidation products
with double bonds or keto groups and dimers were also identified. The oxida-
tive damage to poly(glu, tyr) (4 : 1) peptides by • OH was also detected using
the biosensor [319]. This biosensor thus may be used to study damage to the
protein by • OH.
The oxidation of Phe by • OH is presented in Figure 4.32. The rapid addition
of • OH to the aromatic ring with little selectivity yielded a hydroxycyclohexa-
dienyl radical. In the presence of O 2 , the fast reaction of the radical with O 2
and subsequent elimination of HOO • formed a mixture of stereoisomers of
Tyr with a ratio of o -/ m -/ p -Tyr being 2.0 : 1.0 : 1.5 or 1.3 : 1.0 : 2.1 under different
pH and oxygen concentrations [14]. Further hydroxylation yielded DOPA and
TOPA. The cross-link formation and less overall yield of these isomers occurred
in the absence of O 2 . In the oxidation of angiotension by • OH, generated by
Fenton reaction, products related to the attack at side chains of Phe and Try
were formed [356].

4.5 CONCLUSIONS

cys, Met, Trp, Tyr, His, and Phe are among several amino acid residues of
proteins, determined to be more likely prone to the attack of ROS. A com-
parison of their reactivity as free amino acids with different ROS is presented
in Figure 4.33. The reactivity of O •− was orders of magnitude lower than other
ROS. 1 O 2 and O 3 had significant reactivities (∼10 4 -10 8 /M/s), while the • OH
radical was the most reactive oxidant with nearly diffusion-controlled rate
constants. However, the modification of proteins and the inactivation of
enzymes must be interpreted by considering other factors such as the half-lives