• The quantitative study of energy relationships
and energy conversions in living system.
• All biological transformations obey laws of
• part of universe in which observations are
It includes all the reactants and products
present, the solvent that contains them and the
• universe apart from system.SURROUNDINGS:
• System and surrounding together.UNIVERSE:
• System that neither exchanges matter nor
its energy from surroundings.
• System that exchanges energy but does not
matter with surroundings.
• System that exchanges both energy and
matter with surroundings.OPEN SYSTEM
• If two systems are separately in thermal
equilibrium with third, they will also be in
ZEROTH LAW OF
• In any physical or chemical change, the
total amount of energy in the universe
remains constant, although the form of the
energy may change.
FIRST LAW OF
• The total entropy of the universe is
SECOND LAW OF
• Heat content of the system. Denoted by
• Reflects the no. and kinds of chemical
bonds in the reactants and products.
• Quantitative expression for randomness or
disorder in the system.
• Denoted by ‘S’.
• Free energy content of the system.
Denoted by ‘G’.
• Expresses the amount of energy capable
of doing workduring a reaction at constant
temperature and pressure
G = H – TS.
The free energy change,
ΔG = ΔH – TΔS.
where, ΔH = the enthalpy change, reflecting the kinds
and numbers of chemical bonds and non-covalent interactions
broken and formed.
ΔS = the entropy change, describing the
change in the system’s randomness.
For a process to occur spontaneously, ΔG must be
• The process having negative value of
• Spontaneous in nature.
• ΔG = ΔH – TΔS < 0
• The process having positive value of
• Are not spontaneous.
• ΔG = ΔH – TΔS > 0
•Cellular functions depend largely on macromolecules such as proteins
and nucleic acids.
•ΔG for there formation have positive values: the molecules are less
stable and more highly ordered then the mixture of their monomeric
•These thermodynamically unfavorable, endergonic reactions are
carried out by coupling them with exergonic reactions (usually
hydrolysis of ATP): the sum of free energy change is negative , eg,
Amino acids --------------> polymer ΔG1 is positive.
--Ᵽ--Ᵽ ----------------> --Ᵽ + Ᵽ ΔG2 is
•For such coupled reactions sum of ΔG1 and ΔG2 is negative, ie the
overall process is exergonic.
Drug-receptor interaction can be written as :
L:Sl + R:Sr + Sbulk <=> L*:R*:Slr + S*bulk
where, L:Sl = free ligand in solution surrounded by perturbed
R:Sr = free receptor together with a perturbed solvation shell,
Sbulk = bulk solvent,
L*:R*:Slr = complex ofligand and receptor and a perturbed
solution shell, and,
S*bulk = bulk solvent.
The position of equilibrium and affinity of ligand are defined by ‘Free
ΔG = RT ln Kd
this can be used to determine binding affinity by ΔG.
ΔG itself can be calculated from,
ΔG = ΔH – TΔS
The overall free energy can be calculated by formula such as,
ΔG = ΔGt+r + ΔGr + ΔGx + Δgconf
where, ΔGt+r = cost of binding the ligand to the receptor,
ΔGr = cost of restricting internal rotations,
ΔGx = sum of contributions of individual functional
groups X, including weak intermolecular bonds such as hydrogen
bonds and free energy change associated with solvent
reorganization(the hydrophobic effect), and
Δgconf = energy penalty for binding high energy
•Nelson D L, Cox M M, ‘Lehninger’s Principles of Biochemistry’
edition 6th 2007.
•Murray R K, Granner D K, Rodwell V W, ‘Harper’s Illustrated
Biochemistry’ edition 27th Mc Graw Hill, 2006.
•Larsen P K, Liljefors T, & Madsen U ‘Textbook of Drug Design and
Discovery’ edition 3rd, Tylor and Fransics, 2002.