ANS 23- Enzymes as drug targets

what enzymes are ,specificity ,enzyme kinetics ,inhibitors vs antagonists,enzyme inhibition

Why we need enzymes ?

• Chemical reactions are too slow at body temperature

• as above 40 degrees proteins denature - membrane failure so we have enzymes to help

What is an enzyme ?

• a protein that speeds up a specific chemical reaction without being consumed

Key features :

• made of proteins ( amino acids in 3d shape)

• active site ( special pocket where reaction happens )

• specificity ( each enzyme only catalyses one type of reaction )

• not consumed ( used over , over )

How does an enzyme work?

• lowers activation energy

• reaction can now occur at body temperature

• does not change the equilibrium, just lowers where it lies

cycle:

• E + S ⇌ ES → E + P

  • E = Enzyme, S = Substrate, P = Product

  • Substrate binds → reaction happens → product released → enzyme ready again

What does each enzyme name do?

• all enzymes end with ase

How do enzymes recognise substrates ?

used to have a lock key - enzyme has a specific shape in which only that substrate can fit - explains why each enzyme only catalyses one type of reaction

Now, induced fit:

• enzyme is slightly flexible

• changes shape when substrate binds

• like a glove moulding to your hand

• explains how catalysis works

Why does specificity matter for drugs?

• active site has a specific shape

• design a molecule that fits but cant be processed

• it sits in active site and jamms it + enzyme inhibitor

specificity=selectivity = fewer side effects

How do you measure enzyme activity - enzyme kinetics?

• the Michaelis-Menten curve

The experiment :

• fixed amount of enzyme

• vary substrate conc

• measure how fast the product is made - velocity

• plot velocity vs ( s)

observations :

Low [S]

• Rate increases as you add more substrate

• Enzyme has spare capacity

High [S]:

• Rate levels off

• Every enzyme molecule is busy

Maximum rate = Vmax= all full

• All enzyme molecules occupied

Shape = hyperbolic curve

What are the two parameters of the graph?

Vmax — Maximum velocity:

• Rate when ALL enzyme molecules have substrate bound

• Depends on: how much enzyme, how fast it works

• Maximum speed of the production line

The equation:

v = (Vmax × [S]) / (Km + [S])

When [S] = Km, then v = 1⁄2Vmax

Km — The Michaelis constant:

• it is [S] that gives 1⁄2 Vmax

• What it tells You - Enzyme-substrate affinity

Low Km High affinity: Enzyme grabs substrate tightly, works at low [S]

High Km Low affinity: Needs lots of substrate to work

Units Concentration (mM, μM)

Why do these parameters matter?

Why this matters for drugs:

Competitive inhibitors

• Km ↑ (enzyme seems less keen)

• vmax unchanged

Non-competitive inhibitors

• Vmax ↓ (capacity reduced)

This is how you tell them apart on the graph.

What is an inhibitor?

• block enzymes, blocking chemical reactions

• prevent substrate to product conversion

comparison of antagonist and inhibitor

What is competitive inhibition?

Mechanism :

• inhibitor resembles substrate

• competes for active iste

• prevent substrate binding

• overcome by adding more substrate

Effects on parameters:
- km increases, decrease affinity for substrate, harder to bind as it is competing

-vmax unchanged, as at a high enough substrate concentration, it can outcompete the inhibitor

What is non-competetitive inhibition ?

Mechanism :

• binds to allosteric site ( not active site )

• changes enzyme shape-function

• Substrate can still bind, but the enzyme does not work properly

• is not overcome by adding more s - maximum capacity permanently reduced

Effect on kinetics/parameters:

• km unchanged as substrates still bind to the active site normally

• vmax decreases as the enzyme can’t work at full speed

What is irreversible blocking ?

Mechanism :

• forms a covalent bond with the enzyme

• enzyme permanently inactive

• body must synthesise a new enzyme protein

• effects last days to weeks

Why irreversible?

• can’t outcompete with more substrate

• the only recovery would be getting a new protein enzyme

summary of three types of inhibition

some example of enzymes we know

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