Food Biochemistry and Food Processing (2 edition)

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