2. • Assumptions: The drug-protein binding is
reversible.
• On the protein molecule one binding site is
present
• Under this condition the protein binding of
drug may be described as follows:
• Protein (P) + Drug (D) Drug-Protein
complex (DP ) .
• or, P + D P D
3. PD: protein binding complex
P: unbounded protein
D: Free drug
Ka: Association constant
PD = Ka{P}{D} eq(1)
Ka = {PD}/{P}{D}
4. Pt : Total amount protein
{Pt} = {P} + {PD} (2)
{P} = {Pt} – {PD} (3)
Substituting the value of {P} in equation (1)
{PD} = Ka({Pt} – {PD}){D}
{PD} = Ka{D}{Pt} – Ka{D}{PD}
Ka{D}{Pt} ={PD} + Ka{D}{PD}
Ka{D}{Pt} = {PD}(1+Ka{D})
{PD} = Ka{D}{Pt} / 1+ Ka{D}
5. {PD} / {Pt} = Ka{D} / 1 + Ka{D}
r = {PD} / {Pt} = Ka{D} / 1 + Ka{D} (4)
r describes the situation where 1 mole of drug
binds to one mole of protein
Equation 4 is one of the form of the langmuir
If N is the number of the independent binding
site available then eq 4 written as
r = N .( Ka{D} / 1 + Ka{D} (5)
1/r = 1/ N Ka{D} + 1 / N (6)
6. Rearrange eq nu (5)
NKa{D} = r + rKa{D}
r = Nka{D} – rKa{D}
r/{D} = NKa – rKa (7)
7. • Direct Plot:
• It is made by plotting r versus [D] as shown
Note that when all the binding sites are
occupied by the drug, the protein is saturated
and plateau is reached.
