Chemistry Reference
In-Depth Information
In Cu-containing proteins, three types of Cu centres are found, classified on the basis of their visible, UV, and
EPR spectra, as originally proposed by one of the pioneers of Cu biochemistry, Bo Malmstr
¨
m(
Malkin and
Malmstr
¨
m, 1970
)
. Types 1 and 2 centres have a single Cu atom which has an intense blue colour in Type 1
centres, but the single Cu atom in Type 2 centres is almost colourless. In contrast, Type 3 centres have a di-Cu
centre which is EPR-silent. More details of the three types are given below:
3,000 M
1
cm
1
); EPR spectrum
Type 1 Cu(II): intense blue optical absorption band (
l
max
w
600 nm;
3
>
with an uncommonly small hyperfine splitting in g
Q
region
Type 2 Cu(II): weak absorption spectrum; EPR spectrum characteristic of square-planar Cu(II) complexes
Type 3 Cu(II): di-copper centre; strong absorption in the near UV (
l
max
w
330 nm); no EPR spectra, the two
coppers are antiferromagnetically coupled.
The Type 1 copper ions are normally coordinated in a distorted tetrahedral centre (
Figure 14.1
) by three strong
ligands, a cysteine and two histidines, and one weaker ligand such as methionine sulfur or a nitrogen or oxygen
L
L
L
Cys
L
L
L
L
His
His
Cu
Cu
L
L
Cu
Cu
L
L
O
R
R
L
L
FIGURE 14.1
Classification of Cu sites. From left to right: Type 1; Type 2, Type 3.
donor. Type 2 centres have typically a square-planar or tetragonal geometry around the Cu with nitrogen or oxygen
ligands. Type 3 coppers are usually each coordinated by three histidines, with a bridging ligand such as oxygen or
hydroxyl anion.
Type 1 Blue Copper Proteins
e
Electron Transport
The blue copper proteins are so called on account of their intense blue colour which is derived from the strong
Cys
Cu
2
þ
charge transfer band at around 620 nm in the electronic absorption spectrum. The type 1 Cu centres,
function, like cytochromes, exclusively as electron transport proteins. They are found in mobile electron transfer
proteins like azurin and plastocyanin, as well as in more complex enzymes which contain multiple functional
sites, where they serve to deliver to or take up electrons from the catalytic site. An intriguing question is how Cu
can function in rapid electron transfer reactions
1
when Cu(I) and Cu(II) have such drastically different prefer-
ences in coordination geometry. As we pointed out above, four-coordinate Cu(II) complexes are square-planar,
while the corresponding Cu(I) complexes tend to be more tetrahedral. When the type I copper centre in plas-
tocyanin was first characterised by X-ray crystallography (
Figure 14.2
(
a),(b)), it revealed a copper-binding site
which was virtually the same in the apoprotein and in the copper-containing protein, whether the copper was
Cu(I) or Cu(II). In other words, the protein imposes a binding site geometry on the metal, which is in reality
closer to that of Cu(I) than of Cu(II), such that Cu(II) has no possibility to rearrange
2
toward its preferred
geometry. The copper-coordination site (
Figure 14.2
(b)) is highly distorted with two His nitrogen and one Cys
e
1. An important concept in enzymology is that while catalysis involving bond cleavage and formation requires conformational change, and is
relatively slow (maximum
w
10
8
s
1
), electron transfer is much more rapid (10
12
s
1
), which does not allow much time for conformational
change!
2. Whereas enzyme catalysis invariably involves movement and conformational change, in electron transfer, which is orders of magnitude
faster, there is no time for movement.
