Biology Reference
In-Depth Information
Gould (1959, p. 319) describes PMR in another way:
if a given sequence of steps constitutes the favoured mechanism for the forward
reaction, the reverse sequence of these steps constitutes the favoured mechanism for the
reverse reaction.
...
.
(3.10)
...
Enzymologists often write a generalized enzymic reaction thus:
þ
$
$
þ
S
E
S
E
P
E
(3.11)
a
b
c
where E is the enzyme, S the substrate, and P the product. Clearly, Scheme
3.11
is
not microscopically reversible, since the sequence of events followed in the direc-
tion from left to right is not the same as that from right to left. There is no P
E in the
scheme. In order to modify Scheme
3.11
, so as to make it microscopically revers-
E
z
,
E
z
$
S
þ
E
$
S
þ
E
$
S
E
$½
S
P
P
E
$
P
þ
E
$
P
þ
E
a
b
e
d
e
f
g
h
(3.12)
where the two superscripted Es represent the so-called Franck-Condon states,
which are conformationally strained high-energy states that are in thermal equilib-
rium with their associated ground states (Reynolds and Lumry 1966). Of the two
Franck-Condon states, E* is long-lived (with lifetimes thought to be much longer
than ~10
12
s, the typical time required for electronic transitions) and E
{
is short-
lived, lasting long enough for electronic transitions to take place as a part of a
chemical reaction, that is, covalent rearrangements. Hence, we may refer to E* and
E
{
as “stable” and “unstable” Franck-Condon states, the latter often symbolized by
square brackets, [
] (Ji 1974a, 1979). Evidently, Scheme
3.12
, which is a species
of Eq. 2.26, is microscopically reversible, that is, the scheme is mechanistically
symmetric
with respect to the inversion around the symbol
...
.
There are several unusual features about Scheme
3.12
that require special
attention:
,
1. Enzymes are postulated to undergo thermal fluctuations between their ground
state, E, and energized states, E* (called “stable Franck-Condon states”) in the
absence of its substrate.
2. The substrates bind only to the stable Franck-Condon states of enzymes, E*, and
not to its ground state, E. This contrasts with the traditional induced-fit hypothe-
sis of Koshland (1958). To highlight this difference, the Franck-Condon princi-
ple-based mechanism of ligand binding is referred to as the “pre-fit” hypothesis.
3. Enzyme-catalyzed chemical reactions can occur only at the unstable
Franck-Condon state, denoted as E
{
and enclosed within the square brackets, [
].
4. The energy stored in E* at state
b
is thermally derived and hence cannot be
utilized to do any work lest the Second Law of Thermodynamics is violated
...
