S-Homocysteinylated Proteins - Homocysteine in Protein Structure/Function and Human Disease - page 117

(NH 2 )-COO , respectively. The equilibrium between these forms involves the
hydrogen cation transfer that is governed by microscopic pK a values for the
dissociation of each species:

NH 3 þ Þ COO $ S NH 3 þ Þ COO p

ð A Þ HS CH 2 CH ð CH 2 CH ð K a ¼ 8 : 21

NH 3 þ Þ COO $ COO pK a ¼

ð B Þ HS CH 2 CH ð HS CH 2 CH ð NH 2 Þ 8 : 65

Þ S NH 3 þ Þ COO $ S COO pK a ¼ ð C CH 2 CH ð CH 2 CH ð NH 2 Þ 10 : 0

COO $ S COO p ð D Þ HS CH 2 CH ð NH 2 Þ CH 2 CH ð NH 2 Þ K a ¼ 9 : 56

GluCys show that half of each thiol is
dissociated at pH 8.8, 10.0, 9.2, and 9.9, respectively. At physiological pH

Titrations of Cys, Hcy, GSH, and γ

7.4,
the extent of ionization of a thiol group in CysGly, cysteine, Hcy, GSH, and
γ

¼

GluCys is calculated to be 11 %, 6 %, 1 %, 1 %, and 1 %, respectively [191].
Subsequent studies of Reuben and Bruice [192] show that experimental data for
Hcy titration fit a model with four ionizations:

NH 3 þ Þ COO ð A Þ HS CH 2 CH 2 CH ð

$ S NH 3 þ Þ COO p CH 2 CH ð K a ¼ 9 : 02

NH 3 þ Þ COO ð B Þ HS CH 2 CH 2 CH ð

COO $ HS CH 2 CH ð NH 2 Þ pK a ¼ 9 : 04

Þ S NH 3 þ Þ COO ð C CH 2 CH 2 CH ð

$ S COO CH 2 CH ð NH 2 Þ pK a ¼ 9 : 71

COO ðDÞ HS CH 2 CH 2 CH ðNH 2 Þ

$ S COO p CH 2 CH ð NH 2 Þ K a ¼ 9 : 69

In the case of GSH, the data fit best a single ionization [192]:

NH 3 þ Þγ NH 3 þ Þγ ð S ð A Þð Glu ð HS Þ Cys Gly $ð Glu Þ Cys Gly p K a ¼ 8 : 72

NH 3 þ Þγ ð B Þð Glu ð HS Þ Cys Gly $ð NH 2 Þγ Glu ð HS Þ Cys Gly pK a ¼ 9 : 47

These pK a values indicate that the cysteine thiol is more reactive than the Hcy
thiol, which in turn exhibits reactivity similar to the GSH thiol.
A search for potentially redox-active cysteine disulfides by scanning the Protein
Data Bank (for structures of proteins in alternate redox states) reveals over 1,134
pairs of proteins, many of which exhibit conformational difference between alter-
nate redox states [409]. The search identifies classes of proteins that exhibit
disulfide oxidation following expulsion of metals such as zinc, major reorganiza-
tion of the polypeptide chains in association with disulfide-redox activity, order/
disorder transitions, and changes in quaternary structure. As discussed in a greater
detail in the following sections, similar changes in proteins can be experimentally
induced by binding of Hcy via a disulfide bond with a protein cysteine residue.