BLBS102-c07 BLBS102-Simpson March 21, 2012 11:12 Trim: 276mm X 219mm Printer Name: Yet to Come
136 Part 2: Biotechnology and Enzymology
Figure 7.9.(A) A plot of the initial rate (v) against initial substrate
concentration ([S]) for a reaction obeying the Michaelis-Menten
kinetics. The substrate concentration, which gives a rate of half the
maximum reaction velocity, is equal to theKm.(B)The
Lineweaver-Burk plot. The intercept on the 1/vaxis is 1/Vmax,the
intercept on the 1/[S] axis is− 1 /Kmand the slope isKm/Vmax.
(Eblin 1964) suggests that molecules are in constant motion,
which obey the same laws of mechanics as macroscopic objects.
Since enzymes are biomolecules, which catalyse chemical reac-
tions, they contribute to the energetic background of a chemical
system. As a result, enzymes are directly related to thermody-
namic parameters.
In this section, we will focus on the most representative ther-
modynamic parameters that are useful, in order to describe ex-
perimental data. The fundamental thermodynamic parameters
are the Helmholtz free energy (F), the entropy (S) and the en-
thalpy (H). Free energy (F) correlates the energy (E)ofasystem
with its entropy (S):F=E−TS,whereTis the temperature.
The entropy (S) of a system is the range of its disorder and the
thermodynamic definition is (Rakintzis 1994) (the symbolqis
used to represent the amount of heat absorbed by a system):
S=
∫
q
T
dT⇔S=qln(T)
Furthermore, enthalpy is a constitutive function of pressure
(P) and temperature (T), and energy (E) is a constitutive func-
Figure 7.10.Relationship of the activity–pH and
activity–temperature for a putative enzyme.
tion of volume (V) and temperature (T), respectively (Rakintzis
1994):
H=f(P,T)
dH=
[
∂H
∂P
]
T
dP+
[
∂H
∂T
]
P
dT
E=f(V,T)
dE=
[
∂E
∂V
]
T
dV+
[
∂E
∂T
]
V
dT
At this point, we have to emphasise that in biological systems,
a normal experiment deals with a constant pressure (atmospheric
pressure) and not with a constant volume. This means, that we
determine not a change of energy (E) of a studied body (e.g.
enzyme), but a change in its enthalpy:H=E+PV.However,
the quantity (PV) is considered insignificant in case that we study
a protein for instance, because the volume per molecule is too
small. Hence, there is no difference betweenHandE, and we can
refer both of them simply as “energy’ (Finkelstein and Ptitsyn
2002). Likewise, there is no difference between Helmholtz free
energy (F=E−TS) and Gibb’s free energy (G=H−TS),
since we consider thatHis equivalent toE. As a result, we