ACID–BASE PROPERTIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 52

ACID-BASE PROPERTIES
Acid-base chemistry of amino acids and reactive intermediates is prerequisite
to understand oxidative mechanisms in proteins and enzymes. For example,
metal binding to the biological ligand and its subsequent oxidation by reactive
species is determined by whether functional groups of amino acids are proton-
ated or unprotonated. This is the case in the redox activity of glutathione,
which is exclusively associated with the SH groups of cysteine under physio-
logical conditions [1, 2]. The sulfhydryl group of cysteine also plays a crucial
role in both the structure and function of proteins [1, 2]. Moreover, the oxida-
tive chemistry of proteins is influenced by many factors such as hydrogen
bonds, noncovalent interactions, aromatic-aromatic interactions, and metal-
protein binding, which vary with protonation of amino acid side-chain groups.
Reaction rates of amino acid chains are thus affected by the pH of the reaction
medium. This is described in Chapters 3-6. Besides protonation of amino acids,
the acid-base equilibria of reactive intermediates (halogens, oxygen, nitrogen,
sulfur, and phosphate species) also influence the rates of the reactions. For
example, protonated halogen species (e.g., HOCl) is more reactive than unpro-
tonated species (e.g., OCl − ). More on the role of protonated/unprotonated
species of reactive intermediates is discussed in subsequent chapters.
Basically, the pH dependence of reaction rates can be understood by con-
sidering equilibrium species of both amino acids and reactive intermediates
in the system (see Chapter 6). The distributions of different forms of amino