Homocysteine-Thiolactone - Homocysteine in Protein Structure/Function and Human Disease - page 25

Table 3.3 Second-order rate constants, k, for reactions of Hcy-thiolactone with proteins and
lysine derivatives (pH 7.4)
Protein (kDa) or lysine derivative k at 25 C(M 1 h 1 ) k at 37 C(M 1 h 1 )

α 2 -Macroglobulin (725) 400

Low-density lipoprotein (500) 150

Fibrinogen (340) 101

γ -Globulin (140) 112

Transferrin (80) 150 560

Albumin (68) 128 466

Hemoglobin (64) 84 600

MetRS (64) 60

α-Crystallin (36) 10

DNase I (37) 9

Trypsin (24) 9

Myoglobin (16) 40

Cytochrome c (12.5) 36 150

RNase A (12.5) 3

Poly-Lys (150) 6,700

LysLys 26

LysAla 3

Lysine 1 5

α -N-acetyl-lysine 3.8

ε -N-acetyl-lysine 1.2

Linear kinetics observed in a wide range of reagent concentrations (compiled from [78, 84])

Reaction 3.4 Chemical modification of a protein lysine residue by Hcy-thiolactone (Reprinted
from [68])

temperature 37 C, Hcy-thiolactone modifies proteins by forming N-Hcy-protein
adducts, in which Hcy is N-linked to the

-amino group of protein lysine residues as
shown in Reaction 3.4 [68, 73, 78]. Other amino acid side chain groups in protein do
not appreciably react with Hcy-thiolactone. Indeed, N-linked Hcy is found in
albumin [78, 96, 212, 213], hemoglobin [214], fibrinogen [116, 215], and cyto-
chrome c [136] only on lysine residues. In proteins, the reactivity of side chain
lysine ε-amino residues is much greater than the reactivity of the N-terminal amino
group [78]. In free lysine, the ε-amino group exhibits threefold greater reactivity
with Hcy-thiolactone than the

ε

-amino group of lysine (second-order rate constants,
k obs /[Lys], are 3.8 M 1 h 1 and 1.2 M 1 h 1 , respectively, at pH 7.4; Table 3.3 ).
These studies were facilitated by the availability of radiolabeled [ 35 S]Hcy-
thiolactone of high specific activity [78], prepared as illustrated in Fig. 3.2 .

α