Biology Reference
In-Depth Information
account that in biology, as in any other field, “all models are wrong, but some are
useful” (
Box & Draper, 1987
).
18.1.4
Thermodynamic profile of binding (meaning of enthalpy
and entropy signs)
Interactions can occur only if the variation of free Gibbs energy of the process is neg-
ative (
0
). This is a fundamental thermodynamic law, which is valid for all inter-
acting systems. Gibbs energy has two components, enthalpic and entropic:
D
D
G
<
H
of binding
and association constant
K
a
. Using the above equation and standard thermodynamic
relationship (
G
¼D
H
T
D
S
. Fitting a binding isotherm allows us to determine
D
RT ln
K
a
), entropy of binding could be easily calculated
(
Ladbury & Doyle, 2004
). Thus, contrary to other methods, ITC allows one to de-
termine both components of Gibbs energy after one single experiment, providing
us with information about the nature of the interaction. The values of enthalpy
and entropy can be either positive or negative. They constitute the energetic signature
of the interaction, also referred to as the thermodynamic profile of binding. Interac-
tions are going to be favored by negative
D
G
¼
H
is negative
(exothermic reaction), the entropic component of free Gibbs energy could be either
favorable (
D
H
and/or positive
D
S
.If
D
D
S
>
0
) or unfavorable (
D
S
<
0), as long as
D
G
stays negative. Otherwise,
if
D
H
is positive (endothermic reaction), then entropy of binding should be favorable
(
0
). In the last case, it can be typically concluded that binding is driven by hy-
drophobic interactions. For example, during binding, there is a burying of hydropho-
bic areas in the interface of interaction or conformational changes in one interacting
molecule that lead to hiding of hydrophobic surfaces. Otherwise, a highly favorable
enthalpy and an unfavorable entropy of binding are usually associated with a high
degree of hydrogen bonding formed upon interaction, in addition to conformational
changes (
Ladbury & Doyle, 2004; Ross & Subramanian, 1981
). In addition to pro-
viding information about driving forces, the thermodynamic parameters of interac-
tion of tubulin with different ligands can sometimes be correlated with the
differences in biological activity between these ligands (
Buey et al., 2004, 2005
).
D
S
>
18.1.5
Temperature dependence of
DH
It should be noted that
H
of binding depends on temperature. In the temperature
range where interacting molecules are not denatured, this dependence is linear
and usually has a negative slope. This slope corresponds to heat capacity change
of binding (
D
C
p
) which is generally correlated with the surface of the area buried
upon complex formation (
Ladbury & Doyle, 2004
). A consequence of this temper-
ature dependence is that at certain temperatures, the enthalpy of binding could be
equal to zero, making such binding undetectable by ITC. In other words, the absence
of signal during an ITC experiment does not necessarily mean that there is no inter-
action between molecules, but could signify that
D
D
H
of binding is equal to zero at the
chosen experimental temperature. In this case, entropy is the driving force of the
