Enzymes — Revision Notes

⚡ 30-Second Revision

Definition — Biological catalysts, mostly proteins, lower activation energy.
Specificity — Highly specific (Lock & Key, Induced Fit).
Active Site — Region where substrate binds.
Factors Affecting Activity

- Temperature: Optimal $37^circ C$ (human), high temp causes denaturation. - pH: Optimal specific to enzyme (e.g., Pepsin pH 1.5-2.5, Trypsin pH 8). - Substrate Conc.: Rate increases then plateaus ($V_{max}$). $K_m$ is [S] at $V_{max}/2$. - Enzyme Conc.: Rate $propto$ [Enzyme].

Inhibition

- Competitive: Inhibitor (similar to substrate) binds active site; increases $K_m$, $V_{max}$ unchanged; overcome by high [S]. - Non-competitive: Inhibitor binds allosteric site; decreases $V_{max}$, $K_m$ unchanged; not overcome by high [S].

Classification (EC) — Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases.
Cofactors/Coenzymes — Non-protein helpers (e.g., metal ions, vitamins). Apoenzyme + Cofactor = Holoenzyme.
Key Principle — Enzymes do NOT change $Delta G$ or $K_{eq}$.

2-Minute Revision

Enzymes are essential biological catalysts, primarily protein in nature, that accelerate biochemical reactions by significantly lowering their activation energy. They are not consumed in the process and are highly specific, meaning each enzyme typically acts on a particular substrate or a limited range of substrates.

This specificity is explained by the 'induced fit' model, where the enzyme's active site dynamically molds around the substrate upon binding. Enzyme activity is critically dependent on environmental conditions.

Optimal temperature (around $37^circ C$ for human enzymes) and pH are crucial; deviations can lead to denaturation, an irreversible loss of the enzyme's three-dimensional structure and function. Reaction rate also depends on substrate and enzyme concentrations, reaching a maximum velocity ($V_{max}$) when the enzyme is saturated.

Inhibitors can reduce enzyme activity: competitive inhibitors bind to the active site and can be overcome by increasing substrate concentration, while non-competitive inhibitors bind elsewhere and reduce $V_{max}$ irrespective of substrate concentration.

Enzymes are systematically classified into six classes based on the type of reaction they catalyze, and many require non-protein cofactors or coenzymes for their activity.

5-Minute Revision

Enzymes are the highly specialized protein catalysts that drive nearly all biochemical reactions in living organisms. Their fundamental role is to dramatically increase reaction rates by lowering the activation energy, the energy barrier that reactants must overcome to form products.

Unlike inorganic catalysts, enzymes operate efficiently under mild physiological conditions (e.g., body temperature and neutral pH) and exhibit remarkable specificity. This specificity arises from their unique three-dimensional structure, particularly the 'active site' – a pocket or cleft perfectly shaped to bind specific substrates.

The 'induced fit' model explains this interaction, where the active site subtly changes shape upon substrate binding to achieve an optimal fit.

Several factors critically influence enzyme activity. Temperature has an optimal range; exceeding it causes denaturation, an irreversible unfolding of the protein structure, leading to loss of function.

Similarly, each enzyme has an optimal pH; extreme pH values disrupt ionic bonds and hydrogen bonds, causing denaturation. Reaction velocity increases with substrate concentration until all active sites are saturated, reaching $V_{max}$.

The Michaelis constant ($K_m$) represents the substrate concentration at half $V_{max}$ and indicates the enzyme's affinity for its substrate (lower $K_m$ means higher affinity). Enzyme concentration directly correlates with reaction rate, assuming ample substrate.

Enzyme activity can be regulated by inhibitors. Competitive inhibitors structurally resemble the substrate and bind reversibly to the active site, increasing the apparent $K_m$ but not affecting $V_{max}$; their effect can be overcome by high substrate concentrations.

Non-competitive inhibitors bind to an allosteric site, causing a conformational change that reduces catalytic efficiency, thus decreasing $V_{max}$ without altering $K_m$; their effect cannot be overcome by increasing substrate concentration.

Some enzymes also require non-protein cofactors (metal ions) or coenzymes (organic molecules, often vitamin derivatives) to be active; an apoenzyme (protein part) plus a cofactor forms a holoenzyme (active form).

Enzymes are classified into six major groups based on the reaction type: Oxidoreductases (redox), Transferases (group transfer), Hydrolases (hydrolysis), Lyases (non-hydrolytic cleavage), Isomerases (isomerization), and Ligases (joining molecules with ATP). Remember, enzymes do not change the overall free energy change ($Delta G$) or the equilibrium constant ($K_{eq}$) of a reaction; they only accelerate the rate at which equilibrium is reached.

Prelims Revision Notes

Enzymes: NEET UG Chemistry Revision Notes

1. Definition & Nature:

Enzymes — Biological catalysts, primarily proteins (globular proteins). Some RNA molecules (ribozymes) also have catalytic activity.
Function — Accelerate biochemical reactions by **lowering activation energy ($E_a$)**.
Consumption — Not consumed in the reaction; regenerated unchanged.
Specificity — Highly specific for their substrates.

2. Mechanism of Action:

Active Site — Specific region on enzyme where substrate binds.
Enzyme-Substrate (ES) Complex — Transient complex formed upon substrate binding.
Models

* Lock and Key Model: Rigid active site, perfect fit (simplified). * Induced Fit Model: Flexible active site, molds to substrate upon binding (more accurate).

3. Factors Affecting Enzyme Activity:

Temperature

* Activity increases with temperature up to an optimum (e.g., $37^circ C$ for human enzymes). * Beyond optimum, denaturation occurs (loss of 3D structure, irreversible), leading to loss of activity.

pH

* Each enzyme has an optimal pH (e.g., Pepsin pH 1.5-2.5, Trypsin pH 8). * Deviations from optimum pH cause denaturation.

Substrate Concentration ([S])

* At low [S], rate $propto$ [S]. * At high [S], enzyme becomes saturated, rate reaches **maximum velocity ($V_{max}$)**. * **Michaelis Constant ($K_m$)**: Substrate concentration at which reaction rate is $V_{max}/2$. * Low $K_m$ = high enzyme affinity for substrate. * High $K_m$ = low enzyme affinity for substrate.

Enzyme Concentration ([E])

* Rate $propto$ [E] (assuming ample substrate).

4. Enzyme Inhibition:

Competitive Inhibition

* Inhibitor (structurally similar to substrate) binds active site. * Reversible. * Effect: Increases apparent $K_m$, $V_{max}$ unchanged. * Overcome by: Increasing substrate concentration.

Non-competitive Inhibition

* Inhibitor binds to an allosteric site (not active site). * Reversible or irreversible. * Effect: Decreases $V_{max}$, $K_m$ unchanged. * Overcome by: Cannot be overcome by increasing substrate concentration.

Irreversible Inhibition — Inhibitor binds permanently (covalently) to enzyme, destroying activity.

5. Enzyme Classification (EC System - 6 Classes):

1
Oxidoreductases — Redox reactions (e.g., Dehydrogenases).
2
Transferases — Transfer functional groups (e.g., Kinases, Transaminases).
3
Hydrolases — Hydrolysis reactions (cleavage with water) (e.g., Lipases, Amylases, Proteases).
4
Lyases — Cleavage without hydrolysis/oxidation (e.g., Decarboxylases).
5
Isomerases — Rearrangement of atoms (isomerization) (e.g., Mutases).
6
Ligases — Joining molecules with ATP hydrolysis (e.g., DNA ligase, Synthetases).

6. Cofactors & Coenzymes:

Cofactor — Non-protein component required for enzyme activity.

* Metal Ions: Inorganic (e.g., $Mg^{2+}$, $Zn^{2+}$). * Coenzymes: Organic molecules, often derived from vitamins (e.g., NAD$^+$ from Niacin, FAD from Riboflavin, Coenzyme A from Pantothenic acid). * Prosthetic Group: Tightly/covalently bound coenzyme.

Apoenzyme — Protein part of enzyme (inactive).
Holoenzyme — Apoenzyme + Cofactor (active).

7. Important Points to Remember:

Enzymes do NOT change the **equilibrium constant ($K_{eq}$) or the Gibbs free energy change ($Delta G$)** of a reaction. They only affect the *rate* at which equilibrium is reached.
Enzymes are highly efficient; a single enzyme molecule can catalyze many reactions per second.

Vyyuha Quick Recall

Oh Try Hydrolyzing Large Interesting Lipids!

Oxidoreductases
Transferases
Hydrolases
Lyases
Isomerases
Ligases

(This mnemonic helps recall the six main classes of enzymes in order.)