Enzymes Pharmacodynamics😊

Definition : Specialized proteins that act as biological catalyst

Exception : Ribozymes (RNA)

GRPS

Ribozyme	Location	Function
Group II introns	-	RNA splicing
Ribonuclease P	Nucleus	Post-transcriptional modification of tRNA
Peptidyl transferase	28S rRNA	Translation
snRNA	Spliceosome	RNA splicing

Enzyme Mechanism of Action

Binding Sites:

Active site:

Substrate binding.

Allosteric site:

Site for regulators/modifiers.

Free Energy Change (ΔG):

Formula:

ΔG = Energy of reactants − Energy of products

Effect of Enzyme:

Lowers activation energy.
No change in ΔG.

Enzyme–Substrate Complex Theories:

Fish (Emil Fischer) Locked with a key and brought to land by koshy (Koshland)

1. Emil-Fischer’s Template Theory

Lock and Key Mechanism

Substrate fits perfectly into the enzyme’s active site.
Lock and key cannot explain dynamic changes

2.Koshland’s Induced Fit Theory:

Active site undergoes conformational change

upon substrate binding.

Ensures a more precise fit.

Kinetics of Elimination

Half-Life (t½)

Definition:

Time for plasma concentration to reduce by half

t1/2 = 0.693 / K

(K = elimination rate constant)
Since K = CL / Vd,

t1/2 = (0.693 × Vd) / CL

First-order kinetics:

t½ is constant regardless of dose
Example:

If t½ = 6 hours, after 24 hours (4 t½):

Drug remaining = 6.25%
Drug eliminated = 93.75%

Clinical Use:

Guides dosing interval/frequency

Mnemonic: HalfLife = Learnt To make ViDeo CLear

t1/2 = ln(2) Vd/CL = 0.7 Vd/CL
E.g. In the above case, if we need to calculate t1/2 –
t1/2 = 0.7 X Vd/CL or 0.7/kE = 0.7/0.015 = 46.7 hr

Equilibrium Constant (Keq):

A + B → C + D (→ K1; ←K2)

Keq = K1/K2 ;

Keq = [Products]/[Substrates]

Independent of enzyme action.

Order of Kinetics

Mnemonic:

First Order → depend on first concentration

Zero Order → Zero equation → same amount is reduced

Feature	Zero-Order Kinetics	First-Order Kinetics
Elimination Rate	Amount is constant	Fraction is constant
Rate	Rate = Constant	Rate ∝ Plasma Conc. (PC)
Clearance (CL)	CL ∝ 1/PC	Constant
Half-Life (t1/2)	t1/2 ∝ PC	Constant
ie, Response to ↑↑ [drug]	Clearance ↓↓ & t1/2 → ↑↑	Rate ↑↑
Limit	Capacity-limited ie Liver/kidney are saturated	Flow-limited
Zero-Order Drugs	Mnemonic: Zero WATT Power • Warfarin • Alcohol/Aspirin • Theophylline • Tolbutamide • Phenytoin	Most of the drugs first = • flow-limited, • fraction constant • first = fixed t1/2 • first = fixed CL
Serum drug concentration (Cp) vs time graph	Straight line Zero - amount and rate constant	Curved line

Important Example:

After 270 minutes (third 90-minute cycle):

78.125 mg × 0.375 = 29.297 mg eliminated,
78.125 - 29.297 = 48.828 mg remains.

After 360 minutes (fourth 90-minute cycle):

48.828 × 0.375 = 18.310 mg eliminated,
48.828 - 18.310 = 30.518 mg remains after 6 hours.

Michaelis-Menten kinetics

Graph: Hyperbola curve

Michaelis-Menten Equation:

V1 = (Vmax × [S])
(Km+[S])

Michaelis Constant (Kₘ):

[S] at Vₘₐₓ/2

Inversely proportional to enzyme’s affinity for substrate

High Km → Bad ❌

↓ Affinity
More substrate is needed to achieve half-maximal velocity

Ideal substrate: Low Km

If Km is 100 micromol:

Substrate concentration of 100 micromol is needed.
For the enzyme to achieve ½ maximal velocity.

If Km is 1000 micromol:

Substrate concentration of 1000 micromols is needed.
For the enzyme to achieve the same ½ Vmax.

Allosteric Enzyme

Factors Affecting Rate of Reaction:

1. Substrate Concentration [S][S]:

Initial stage:

1st order kinetics: Rate ∝ [S]

Later stage:

0 order kinetics: Active sites saturated, Rate ≠ [S]

2. Enzyme Concentration [E][E]:

Directly proportional

Enzyme concentration doubles,

rate of reaction/velocity doubles

In a reaction, the substrate is available in a concentration that is 1000 times the Km value of the enzyme. After 9 minutes of reaction, 1% of substrate is converted to product (12 µg/ml). If the concentration of the enzyme is changed to 1/3 and concentration of substrate is doubled, what is the time taken to convert the substrate into the same amount of product (12 µg/ml)?

Options:

9 minutes

4.5 minutes

27 minutes

13.5 minutes

ANS

27 minutes

Applied Aspect:

In a reaction mixture, if S = 1000 x Km

In such a mixture, it follows zero order kinetics
ie, velocity is independent of further added substrates

3. Temperature and pH:

Graph shows enzyme activity rises, peaks, then falls due to denaturation

Optimum conditions:

At Vmax

Optimum Temperature:

35–40°C

Optimum pH: 5–9

Applied Qs

If temperature is increased 10 times, what is rate of reaction

💡

Q₁₀ Effect:

10°C ↑ = 2× increase in rate of reaction

Mnemonic: Kuttan likes temperature

Pharmacodynamics

Types of Enzyme Inhibition

A. Reversible (Pharmacological agents).

Competitive.

Uncompetitive.

Mixed.

Noncompetitive is a special type of mixed inhibition.

B. Irreversible.

Lineweaver-Burke Plot/Double Reciprocal Plot

Double reciprocal plot

1/S on X-axis → 1/Km
1/V on Y-axis → 1/Vmax
Straight line

Type	Km	Vmax	Lineweaver-Burke Plot
Competitive	↑	-	Lines intersect at Y-axis (same Vmax)
Non-Competitive	-	↓	Lines intersect at X-axis (same Km)

Catalytic Constant:

AKA turnover number = Enzyme efficiency

Kcat = [Vmax]/ [Et]

Et = Total enzyme concentration.

Catalytic efficiency = Kcat/Km

Kcat α Efficiency of enzyme

Km α 1 / Efficiency of enzyme

Feature	Competitive (Reversible)	Uncompetitive inhibition	Non -competitive	Mixed inhibition	Competitive (Irreversible)
Mnemonic	Competetive → V E (WE are) constant	U → Parallel shift → both ↓	V shape Non comp → Normal Km	Mixed → reversal of normal → Vmax ↓, Kmax ↑	ㅤ
Shape of line	X shape	Parallelly shifted line.	V shaped	X shaped	ㅤ
Vmax	N	↓↓	↓↓	↓↓	ㅤ
Km	↑↑	↓↓	N	↑↑	ㅤ
Substrate Resemblance	Yes Reduces the probability of ES complex and Product formation.	No	No special type of mixed inhibitor.	No	Yes
Reversibility	Reversible	ㅤ	Reversible or irreversible (context-dependent)	ㅤ	Irreversible (or high affinity)
Overcome by ↑[S]	Can be overcome	Cannot be overcome → more ESI complex	Cannot be overcome	Cannot be overcome	Cannot be overcome
Binding Site	Active site	ㅤ	Distinct (allosteric) site	Both free enzymes and enzyme substrate complexes.	Active site
Bind E-S Complex	No	Yes	Yes	Yes → Binds both ES complex and an ESI complex	No

Competition → We Become first
No competition → We become less — **Competition** **→ We Become** **first**
**No competition** **→ We become** **less**

Mneumonic

Competitive

Kilometer (Km) increases
But WE (Vmax, Efficacy) r constant
we can OVERCOME by increasing effort (↑ substrate → reverses)
Competitive - X graph

Non-competitive inhibitor

Non-Kmpitivie → No Km change
If no competition → No Victory (Vmax ↓↓)

Q. Identify the type of inhibition.

A. Competitive inhibition.

B. Uncompetitive inhibition.

C. Mixed inhibition.

D. Non-competitive inhibition.