Chemistry Reference
In-Depth Information
FIGURE 14.5
Active site of PHM with coordinated O
2
. The Cu
A
and Cu
B
sites are linked by a water molecule and the substrate analog IYT.
The position of the hydroxylated C
a
is denoted by an asterisk.
(From
Rosenzweig & Sazinsky, 2006
.
Copyright 2006, with permission
from Elsevier.)
trapped by freezing crystals of the enzyme which had been soaked with a slowly reacting substrate, N-acetyl-
diiodo-tyrosyl-D-threonine (IYT), in the presence of oxygen and ascorbate. Electron density was observed that
was best modeled as O
2
within coordinating distance of the catalytic Cu in the precatalytic complex (
Figure 14.5
)
,
replacing the solvent molecule observed in all other PHM structures. Bound substrate has been proposed to
mediate electron transfer between the two Cu centres, each of which contributes one electron for O
2
reduction.
Figure 14.6
presents a possible mechanism for PHM and D
H. Substrate and O
2
bind to the reduced enzyme,
triggering initial O
2
activation involving electron transfer from the type 2 Cu atom, to form the Cu-superoxo
intermediate. A second electron is then transferred from the other Cu site, followed by-product release and
reduction of the two Cu sites by ascorbate. The question of the exact mechanism of long-range electron transfer
between the two Cu sites remains to be established.
(i)
b
Dinuclear Type 3 copper proteins
Haemocyanin, tyrosinase, and catechol oxidase all belong to the type 3 Cu protein family which are characterised
by two closely spaced antiferromagnetically coupled copper ions. However, while haemocyanin is an O
2
carrier
protein, catechol oxidase, which converts catechols to the corresponding o-quinones, and tyrosinase, which, in
addition to converting catechols to quinones, also hydroxylates monophenols (e.g., tyrosine), are both enzymes.
They have very different structures and sequences (
Figure 14.7
)
. Yet, as we will see shortly, despite this, they have
very similar active sites.
Figure 14.7
presents the arrangements of the domains within the subunit structures of two
haemocyanins from the haemolymph of the horse shoe crab (Limulus polyphemus) and the North Pacific giant
octopus (Octopus dofleini), a streptococcal tyrosinase and catechol oxidase from sweet potato. The location of the
copper centres are shown as are the amino acid residues which block access to the catalytic site (blocking residues).
The structures of the oxy forms at high resolution confirms, as predicted from model compounds, that the
dioxygen molecule is bound in a peroxo-dicopper(II) complex, corresponding to the
2
:
2
-peroxo, illustrated
mh
h
