Chemistry Reference
In-Depth Information
O
Closed
Closed (Active)
C
H
2
O
2
O
-
H
2
O
O
Mn
+3
Mn
+3
H
O
O
C
C
O
-
O
O
OH
H
O
O
Mn
+2
Mn
+2
Mn
+3
Mn
+3
O
OH
2
H
O
O
2
C
O
-
O
O
Mn
+2
Mn
+2
H
2
O
2
H
2
FIGURE 16.7
Catalytic reaction cycle for manganese catalase turnover.
(Adapted from
Whittaker, Barynin, Igarashi, & Whittaker, 2003
.
)
In mammals, hepatic arginase is the terminal enzyme of the urea cycle, which represents the major end-product
of nitrogen metabolism
the average adult human excretes some 10 kg of urea per year. The enzyme is not
restricted to the liver, since ornithine is a precursor of the nonessential amino acid proline, and a biosynthetic
precursor of polyamines, required for rapidly dividing tissues. Arginine is also the precursor of the important
messenger in many vertebrate signal-transduction pathways nitric oxide, NO (
Scheme 16.1
)
, of which more shortly.
A common feature of arginases, whether eukaryoyic or prokaryotic, is the requirement of divalent cations for
activity, and in almost all arginases, they have 2 spin-coupled Mn(II)/subunit, which are some 3.3
˚
apart.
Figure 16.8
a
presents a ribbon plot of the arginase trimer. The dinuclear Mn(II) centre is located at the bottom of
a15
˚
-deep active site cleft.
Figure 16.8
b shows the dinuclear cluster of arginase. Mn
2
A
is coordinated with
square pyramidal geometry, leaving a vacant coordination site that permits octahedral coordination geometry as
a means of transition state stabilisation in catalysis. Mn
2
B
is coordinated with octahedral geometry. An interesting
feature is the hydrogen bond donated by a metal-bridging hydroxide to the noncoordinating oxygen of Asp 128:
residues analogous to Asp 128 are found in the active sites of a large number of other dimetallic hydrolases.
A mechanism which is consistent with biochemical, enzymological, and structural data is outlined in
Figure 16.9
and the steps are detailed here. (a) The first step involves binding of the substrate L-arginine to the
enzyme in which the side chain of Glu 277 plays an important role; the substrate guanidium group does not
coordinate to the manganese ions, (b) Nucleophilic attack of metal-bridging hydroxide at the substrate gua-
nidinium group leads to formation of a neutral, tetrahedral intermediate which is stabilised by the dinuclear
e
