Enzyme Structure and Classification — Revision Notes

⚡ 30-Second Revision

Enzymes: — Biological catalysts, mostly proteins.
Active Site: — 3D pocket for substrate binding.
Induced Fit: — Enzyme changes shape upon substrate binding.
Apoenzyme: — Inactive protein part.
Holoenzyme: — Active apoenzyme + cofactor.
Cofactors: — Non-protein helpers.

- Inorganic Ions: $Mg^{2+}$, $Zn^{2+}$. - Coenzymes: Organic, loosely bound (e.g., NAD+, FAD, Coenzyme A; from vitamins). - Prosthetic Groups: Organic, tightly/covalently bound (e.g., Heme).

6 Enzyme Classes (IUBMB):

1. Oxidoreductases: Redox reactions ($A_{red} + B_{ox} \rightleftharpoons A_{ox} + B_{red}$). 2. Transferases: Group transfer ($A-X + B \rightleftharpoons A + B-X$). 3. Hydrolases: Hydrolysis (add $H_2O$ to break bonds). 4. Lyases: Cleavage without $H_2O$ (often form double bonds). 5. Isomerases: Isomerization (rearrangement within molecule). 6. Ligases: Ligation/joining (form bonds with ATP hydrolysis).

Specificity: — High, due to active site shape.

2-Minute Revision

Enzymes are protein catalysts that speed up reactions by lowering activation energy. Their function relies on their specific 3D structure, particularly the active site, a unique pocket where the substrate binds.

The 'induced fit' model explains that the active site is flexible, molding around the substrate for optimal interaction. Many enzymes require non-protein cofactors for activity. An inactive enzyme without its cofactor is an apoenzyme, while the active form is a holoenzyme.

Cofactors can be inorganic ions (like $Mg^{2+}$), or organic molecules. Organic cofactors are either loosely bound coenzymes (often vitamin derivatives like NAD+) or tightly bound prosthetic groups (like heme).

Enzymes are systematically classified into six major classes by IUBMB based on the reaction type: Oxidoreductases (redox), Transferases (group transfer), Hydrolases (hydrolysis), Lyases (non-hydrolytic cleavage), Isomerases (isomerization), and Ligases (joining molecules with ATP).

This classification helps understand their diverse roles and high specificity in metabolism.

5-Minute Revision

Let's quickly review the essentials of enzyme structure and classification. Enzymes are biological catalysts, primarily proteins, that dramatically accelerate biochemical reactions by reducing the activation energy.

Their catalytic power stems from their precise three-dimensional structure. The active site is the crucial region, a specific pocket or groove formed by the enzyme's folded polypeptide chain, where the substrate binds.

This binding is often described by the induced fit model, where the enzyme's active site subtly changes shape to snugly accommodate the substrate, optimizing the catalytic process.

Many enzymes aren't fully functional on their own; they require cofactors, which are non-protein helper molecules. An enzyme without its cofactor is an inactive apoenzyme, while the complete, active unit is a holoenzyme.

Cofactors come in different forms: inorganic ions (e.g., $Mg^{2+}$, $Zn^{2+}$) or organic molecules. The organic cofactors are further divided: coenzymes are loosely bound (e.g., NAD+, FAD, Coenzyme A, often derived from vitamins), acting as carriers of chemical groups; prosthetic groups are tightly, often covalently, bound (e.

g., Heme in catalase), forming an integral part of the enzyme's active site.

For systematic understanding, enzymes are classified by the IUBMB into six major classes based on the type of reaction they catalyze:

1
Oxidoreductases (EC 1): — Catalyze oxidation-reduction reactions (electron/hydrogen transfer). *Example: Alcohol dehydrogenase.*
2
Transferases (EC 2): — Transfer a functional group from one molecule to another. *Example: Hexokinase (phosphate transfer).*
3
Hydrolases (EC 3): — Break bonds by adding water (hydrolysis). *Example: Lipase, Amylase.*
4
Lyases (EC 4): — Cleave bonds by elimination, often forming double bonds, without using water or oxidation. *Example: Aldolase.*
5
Isomerases (EC 5): — Rearrange atoms within a molecule to form an isomer. *Example: Phosphoglucose isomerase.*
6
Ligases (EC 6): — Join two molecules together, typically coupled with ATP hydrolysis. *Example: DNA ligase.*

This classification, along with understanding enzyme specificity (their ability to act on specific substrates and reactions), is vital for comprehending metabolic pathways and their regulation.

Prelims Revision Notes

Enzymes are biological catalysts, predominantly proteinaceous, that accelerate biochemical reactions by lowering activation energy. They are not consumed in the reaction. The unique three-dimensional structure of an enzyme is crucial for its function, particularly the active site.

The active site is a specific pocket or groove where the substrate binds. The induced fit model (Koshland) is the widely accepted explanation for enzyme-substrate interaction, where the active site is flexible and molds around the substrate upon binding, optimizing catalysis.

The older lock and key model (Fischer) proposed a rigid active site.

Many enzymes require cofactors for activity. An inactive enzyme without its cofactor is an apoenzyme, while the complete, active enzyme-cofactor complex is a holoenzyme. Cofactors can be:

Inorganic ions: — Metal ions like $Mg^{2+}$, $Zn^{2+}$, $Fe^{2+}$.
Organic molecules:

* Coenzymes: Loosely bound, transient carriers of chemical groups (e.g., NAD+ from Niacin, FAD from Riboflavin, Coenzyme A from Pantothenic acid). Often derived from vitamins. * Prosthetic groups: Tightly, often covalently, bound to the apoenzyme (e.g., Heme in catalase).

Enzymes exhibit high specificity (absolute, group, linkage, stereochemical) due to the precise shape and chemical properties of their active site.

IUBMB Classification (6 Classes):

1
EC 1: Oxidoreductases: — Catalyze oxidation-reduction reactions (transfer of electrons/hydrogen). *Example: Dehydrogenases, Oxidases.*
2
EC 2: Transferases: — Transfer a functional group (e.g., phosphate, amino, methyl) from one molecule to another. *Example: Kinases (Hexokinase), Transaminases.*
3
EC 3: Hydrolases: — Catalyze hydrolysis (cleavage of bonds by adding water). *Example: Lipases, Proteases, Amylases.*
4
EC 4: Lyases: — Catalyze cleavage of C-C, C-O, C-N bonds by elimination, often forming double bonds, without hydrolysis or oxidation. *Example: Aldolase, Decarboxylases.*
5
EC 5: Isomerases: — Catalyze the rearrangement of atoms within a molecule to form an isomer. *Example: Mutases, Racemases.*
6
EC 6: Ligases: — Catalyze the joining of two molecules, coupled with ATP hydrolysis. *Example: DNA ligase, Synthetases.*

Vyyuha Quick Recall

To remember the 6 enzyme classes: Over The Hill, Little Insects Lay.

Oxidoreductases
Transferases
Hydrolases
Lyases
Isomerases
Ligases