Chemistry Reference
In-Depth Information
Metallothionines, Zinc fingers
S
4
&S
3
N
Alcohol dehydrogenase
NOS
2
N
2
SS*
Blue copper proteins
SOD, Carbonic anhydrase
Carboxypeptidase A
N
2
O
N
2
O
2
Histidinol dehydrogenase
O
6
-O
8
Calmodulin, Troponin C, Lactalbumin
800
600
400
200
0
-200
(
113
Cd)/ ppm
δ
Fig. 3 Chemical shifts of
113
Cd for structurally characterized
113
Cd-substituted metalloproteins
relative to external 0.1 M [Cd(ClO
4
)]. The chemical shift positions are represented by
gray bars
and coordinating atoms are highlighted in
blue
with typical proteins listed besides. Here S
represents sulfur from cysteine, S* represents sulfur from methionine, O represents oxygen from
carboxylate or water, and N represents nitrogen from histidine [
71
]
However, the resonances from the
-domain (I, V, VI, and VII) are slightly shifted,
particularly for resonance VII, probably attributable to the hexa-peptide insertion in
this domain. The homonuclear 2D
113
Cd-
113
Cd, Fig.
4b
, clearly shows the correla-
tion of cadmium signals which confirms the existence of two Cd-thiolate clusters
[
81
]. The 2D
1
H-
113
Cd HMQC as shown in Fig.
4
c is normally used to obtain
detailed metal-thiolate connectivity within each of these clusters [
27
]. Recently,
metallothionines from different species, such as sea mussel
Mytilus gallopro-
vincialis
(Cd
7
MT10) [
81
] and blue crab
Callinectes sapidus
(MTc) [
82
], have
been studied by combined use of
113
Cd NMR,
113
Cd-
113
Cd COSY, and
1
H-
113
Cd
HMQC. Both proteins have 21 cysteine residues with the position of cysteines
distinct from mammalian MTs. The unique structure and dynamic features of the
metal-thiolate cluster in these proteins are obviously seen from their distinct NMR
parameters of Cd
7
MTC [
81
,
82
].
111
Cd NMR has also been applied in studies
of His-containing metallothionine, e.g., SmtA. The metal cluster of CdS
4
and
CdN
2
S
2
were identified [
83
], and
1
H-
111
Cd HMQC of Cd
7
SmtA unequivocally
demonstrated couplings of two Cd
2+
to both H
e
1
or H
d
2
protons of two histidine
residues [
41
]. Apart frommetallothionines, cadmium NMR has also been employed
to identify metal coordination environments in various metalloproteins, such as
zinc finger [
40
] and [NiFe] hydrogenase accessory protein HypA [
15
], where, in the
latter case, zinc ions (substituted by
113
Cd) are coordinated to four cysteine side-
chains tetrahedrally. Moreover, it was also used to investigate major zinc binding
sites on human albumin [
74
,
84
]. The chemical shifts of
111
Cd of human albumin
a
