Properties of Enzymes

Characteristic Features of Enzymes

Enzymes are biological catalysts that accelerate chemical reactions in living organisms. They possess several defining features that distinguish them from non-biological catalysts.

• High catalytic efficiency: Enzymes can increase reaction rates by factors of 106 to 1017 compared to uncatalyzed reactions.

• Specificity: Enzymes are highly specific for their substrates and the reactions they catalyze, often showing stereospecificity (acting only on specific isomers).

• Regulation: Enzyme activity can be regulated by various mechanisms, allowing cells to control metabolic pathways.

Example: Urease catalyzes the hydrolysis of urea to carbon dioxide and ammonia, but does not act on other amides.

Enzyme Catalytic Proficiency

Enzymes exhibit remarkable catalytic proficiencies, as shown by comparing their rate constants to those of nonenzymatic reactions.

• Rate enhancement: The ratio of the enzymatic rate constant () to the nonenzymatic rate constant () is called catalytic proficiency.

Additional info: Catalytic proficiency is a measure of how much faster an enzyme-catalyzed reaction proceeds compared to the uncatalyzed reaction.

Enzyme Specificity and Active Sites

Enzymes are highly specific for their substrates due to the precise arrangement of amino acids in their active sites.

• Substrate specificity: Only certain molecules (substrates) fit into the enzyme's active site.

• Active site: The region of the enzyme where substrate binding and catalysis occur.

• Stereospecificity: Enzymes often distinguish between different stereoisomers of a substrate.

Example: Proteases cleave peptide bonds at specific amino acid residues, producing a carboxyl component and an amino component.

Regulation of Enzyme Activity

Enzyme activity can be regulated to control the flow of metabolites through metabolic pathways.

• Allosteric regulation: Enzymes can be activated or inhibited by molecules that bind at sites other than the active site.

• Feedback inhibition: The end product of a pathway inhibits an early enzyme, preventing overproduction.

Example: In glycolysis, phosphofructokinase is regulated by ATP (inhibitor) and AMP (activator).

Major Enzyme Classes

Classification of Enzymes

Enzymes are classified into six major classes based on the type of reaction they catalyze:

• Oxidoreductases: Catalyze oxidation-reduction reactions (e.g., lactate dehydrogenase).

• Transferases: Transfer functional groups between molecules.

• Hydrolases: Catalyze hydrolysis reactions (cleavage with water).

• Lyases: Remove atoms to form double bonds or add atoms to double bonds.

• Isomerases: Rearrange functional groups within a molecule.

• Ligases/Synthetases: Join two molecules together, usually coupled to ATP hydrolysis.

Example: Lactate dehydrogenase (an oxidoreductase) catalyzes the conversion of L-lactate to pyruvate.

Enzyme Cofactors

Role and Types of Cofactors

Many enzymes require non-protein molecules called cofactors to be active. Cofactors can be:

• Metal ions: Such as Mg2+, Zn2+, Fe2+.

• Coenzymes: Organic molecules, often derived from vitamins (e.g., NAD+, FAD, coenzyme A).

• Prosthetic groups: Tightly bound coenzymes or metal ions.

• Holoenzyme: The active enzyme with its cofactor.

• Apoenzyme: The protein part of the enzyme, without its cofactor (inactive).

Enzyme-Substrate Complex & Active Site

Formation and Function

Enzymes bind substrates to form an enzyme-substrate complex at the active site, facilitating the chemical reaction.

• Active site: The specific region where substrate binding and catalysis occur.

• Transition state: The enzyme stabilizes the transition state, lowering the activation energy required for the reaction.

Example: Structural models show substrates bound in the active site, highlighting key amino acids involved in catalysis.

Kinetics of Enzyme-Catalyzed Reactions

Reaction Coordinate and Rate

Enzyme-catalyzed reactions proceed via a transition state, with the enzyme lowering the activation energy () compared to the uncatalyzed reaction.

• Reaction rate equation: , where is the activation energy.

• Enzymes increase the rate by stabilizing the transition state and reducing .

Example: The reaction progress diagram shows a lower energy barrier for the catalyzed reaction.