Chemistry Reference
In-Depth Information
with interdomain motion. The two lobes of the active site cleft swing together from an open to a closed
conformation by about 8
˚
. This also has the consequence of excluding water from the active site, which may
explain why phosphoryl transfer to glucose is 4
10
4
times faster than to water.
Another characteristic of this kinase family, as has been shown by Jeremy Knowles (
Knowles, 1980
)
using
ATP-made chiral in its
-phosphoryl group, is that phosphoryl group transfer occurs with inversion of configu-
ration. This is taken to be indicative of a direct, in-line transfer of the phosphoryl group from substrate to product
by the addition of a nucleophile to the phosphorus atom yielding a trigonal bipyramidal intermediate, the apices of
which are occupied by the attacking and leaving groups (
Figure 10.3
)
.
g
CH
2
OH
H
H
O
ADP
H
+
H
OH
P
18
17
-
O
O
-
16
O
OH
OH
H
OH
Glucose
16
O ADP
-
O
Trigonal
bipyramid
intermediate
18
O
-
P
-
O
Glucose
17
16
18
17
-
O
O
-
O
P
H
2
C
O
H
H
H
+
ADP
H
OH
OH
OH
H
OH
Glucose-6-phosphate
FIGURE 10.3
In the phosphoryl transfer reaction catalysed by hexokinase, the
g
-phosphoryl group of ATP inversion of configuration.
(Adapted from
Voet & Voet, 2004
: pp. 1591.)
Hexokinase forms a ternary complex with glucose and Mg
2
þ
e
ATP before the reaction takes place, which, as
a result of the domain closure, places ATP in close proximity to the C6 hydroxyl group of glucose (
Figure 10.4
).
By complexing the phosphate groups of ATP, Mg
2
þ
is thought to shield their negative charges, making the
g
OH group of the glucose molecule.
However, it also seems that, as in many of the other members of the superfamily, the Mg
2
þ
ion not only binds
-phosphorus atom more accessible to nucleophilic attack by the C6
e
