S-Homocysteinylated Proteins - Homocysteine in Protein Structure/Function and Human Disease - page 116

Chapter 7
S-Homocysteinylated Proteins

Human proteome includes 214,000 cysteine residues, which constitute 2.26 % of
the encoded amino acids [406]. Many proteins contain at least one cysteine residue
with a free -SH group as well as more numerous disulfide-bonded cysteine
residues. For example, of the 35 cysteine residues present in the structures of
albumin and

-globulin, the most abundant serum proteins, one has a free -SH
group and the other 34 are engaged in 17 disulfide bonds [407]. These cysteine
residues can engage in thiol-disulfide exchange reactions with low molecular
weight thiols or disulfides. Indeed, both albumin [91, 296, 408] and

γ

γ -globulin

[79, 106] carry S-linked cysteine, Hcy, CysGly, glutathione (GSH), and -GluCys.

Furthermore, specific cysteine residues, both free and disulfide-bonded, exhibit
different reactivities in the thiol-disulfide exchange reactions [409].
There are two different types of disulfides in protein structures that have
different functional roles: one type of disulfides stabilizes proteins by cross-linking
their polypeptide chains while other disulfides are redox reactive. For example,
while cysteamine is able to reduce only five disulfide bonds in

γ

-globulin, it reduces
only one disulfide bond in serum albumin [407]. Thus, biologically important
thiols, including Hcy, can potentially interact with specific protein cysteine residues
and affect protein's structure and function.
The extent of Hcy binding via disulfide bonds to protein thiols in vivo will
depend on their reactivity, the concentration of competing small-molecule thiols,
such as GSH and cysteine, and ionization constants (pK a values) of their sulfhydryl
groups. The concentration of major small-molecule thiols in healthy organisms
is several orders of magnitude higher than the concentration of Hcy. For example,
cellular concentrations of GSH are 5-10 mM, and plasma concentrations of
cysteine are about 0.3 mM, whereas total Hcy concentrations in plasma and tissues
are about 5-10

γ

M.
Classical studies of Benesh and Benesh [191] show that four different ionic
species of cysteine exist in solution at physiological pH: a form with both thiol and
amino groups protonated HS-CH 2 -CH(NH 3 + )-COO , a form with deprotonated
thiol and amino groups S-CH 2 -CH(NH 2 )-COO , and forms with either thiol or
amino group deprotonated, S-CH 2 -CH(NH 3 + )-COO and HS-CH 2 -CH

μ