CH 19+20 STUDY NOTES

Biological Macromolecules: Proteins and Enzymes

Overview

This section covers the structure and function of proteins, with a focus on enzymes as biological catalysts. Understanding protein structure is essential for grasping how enzymes work, how they are regulated, and how their activity is affected by environmental factors.

Protein Structure

Levels of Protein Structure

• Primary Structure: The linear sequence of amino acids in a polypeptide chain, held together by peptide bonds.

• Secondary Structure: Local folding or coiling of the polypeptide, commonly forming alpha helices and beta sheets, stabilized by hydrogen bonds between backbone atoms.

• Tertiary Structure: The overall 3D shape of a single polypeptide chain, resulting from interactions among side chains (R groups), including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.

• Quaternary Structure: The arrangement of two or more polypeptide chains (subunits) into a functional protein complex.

Example: Hemoglobin is a protein with quaternary structure, composed of four polypeptide subunits.

Tertiary Structure Details

The tertiary structure is determined by all interactions involving the side chains (R groups) of amino acids. These interactions include:

• Hydrophobic interactions (nonpolar side chains cluster away from water)

• Hydrogen bonds (between polar side chains)

• Ionic bonds (between charged side chains)

• Disulfide bridges (covalent bonds between cysteine residues)

Example: The folding of an enzyme's active site is a result of its tertiary structure.

Amino Acids and Ion Pairing

Amino acids can interact via their side chains, especially at physiological pH (around 7). Acidic and basic side chains can form ion pairs (salt bridges).

• Practice Question: Which of the following amino acids can form an ion pair with glutamic acid at pH 7?

  • Glutamine

  • Glycine

  • Arginine

• Answer: Arginine (a basic amino acid) can form an ion pair with glutamic acid (an acidic amino acid) at pH 7.

Additional info: Glutamic acid has a negatively charged side chain at pH 7, while arginine has a positively charged side chain.

Classification of Amino Acids

Amino acids are classified based on the properties of their side chains:

• Nonpolar (hydrophobic)

• Polar uncharged (hydrophilic)

• Polar acidic (negatively charged at pH 7)

• Polar basic (positively charged at pH 7)

Example: Lysine and arginine are basic; aspartic acid and glutamic acid are acidic.

Enzymes: Biological Catalysts

Enzyme Function and Mechanism

Enzymes are proteins that act as catalysts, speeding up chemical reactions by lowering the activation energy required for the reaction to proceed.

• Activation Energy ($E_a$): The minimum energy required to initiate a chemical reaction.

• Effect of Enzymes: Enzymes provide an alternative reaction pathway with a lower activation energy.

Equation:

$\text{Enzyme} + \text{Substrate} \rightarrow \text{Enzyme-Substrate Complex} \rightarrow \text{Enzyme} + \text{Product}$

Example: The enzyme sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Functions of Enzymes

• Allow reactions to occur rapidly that would otherwise take years under normal conditions.

• Enable biological processes to proceed at rates necessary for life.

Example: Sucrose hydrolysis:

$\ce{C12H22O11 + H2O \xrightarrow{sucrase} C6H12O6 + C6H12O6}$

(Sucrose + Water → Glucose + Fructose)

Enzyme Structure and Specificity

• Enzymes are proteins with unique 3D shapes, determined by their amino acid sequence and folding.

• The active site is a specific region where the substrate binds and the reaction occurs.

• Enzyme specificity is due to the precise arrangement of amino acids at the active site, allowing only certain substrates to bind.

Example: Lactase has an active site that binds specifically to lactose.

Active Site and Substrate Interaction

• The substrate enters the active site, forming an enzyme-substrate complex.

• The enzyme may change shape slightly to accommodate the substrate (induced fit model).

• After the reaction, products are released and the enzyme is free to catalyze another reaction.

Enzyme Reactions

Enzyme-catalyzed reactions typically follow these steps:

1. Substrate binds to the enzyme's active site.

2. Enzyme-substrate complex forms.

3. Reaction occurs, converting substrate to product.

4. Product is released; enzyme is unchanged.

Classification of Enzymes

Major Classes of Enzymes

Enzymes are classified based on the type of reaction they catalyze:

Class	Description	Example
Isomerases	Catalyze rearrangement of atoms within a molecule	Phosphoglucose isomerase
Transferases	Transfer functional groups between molecules	Kinases, transaminases
Ligases	Join two molecules together (often using ATP)	DNA ligase
Hydrolases	Catalyze hydrolysis reactions (breaking bonds with water)	Proteases, lipases
Lyases	Remove groups from or add groups to double bonds	Aldolase
Oxidoreductases	Catalyze oxidation-reduction reactions	Dehydrogenases, oxidases

Additional info: Knowing the class helps predict the type of reaction an enzyme will catalyze.

Protein Denaturation

What is Denaturation?

• Denaturation is the loss of a protein's native structure due to external stress (heat, acid, base, heavy metals, agitation).

• Denatured proteins lose their biological activity because their shape (especially the active site) is altered.

Example: Cooking an egg denatures egg proteins, causing them to solidify.

Enzyme Denaturation

• Enzymes, being proteins, can also be denatured.

• Denaturation can be caused by:

  • High temperature

  • Extreme pH

  • Heavy metal ions

• Each enzyme has an optimum temperature and optimum pH for activity.

Graphical Example: Enzyme activity increases with temperature up to an optimum, then drops sharply as denaturation occurs. Similarly, activity is highest at an optimum pH and decreases outside this range.

Enzymatic Regulation

Allosteric Regulation

• Enzyme activity can be regulated by molecules that bind to sites other than the active site (allosteric sites).

• Positive allosteric regulation: Activator binds, increasing enzyme activity.

• Negative allosteric regulation: Inhibitor binds, decreasing enzyme activity.

Example: Feedback inhibition, where the end product of a pathway inhibits an earlier enzyme to prevent overproduction.

Feedback Control

• In metabolic pathways, the final product can inhibit the first enzyme in the pathway (feedback inhibition).

• This helps maintain homeostasis by regulating the amount of product formed.

Example: Isoleucine inhibits threonine deaminase when isoleucine levels are high.

Summary: Connections Across Chemistry

• Protein structure and enzyme function involve bonding, thermodynamics, kinetics, and organic chemistry.

• Enzyme activity is influenced by solution conditions (pH, temperature, electrolytes).

Additional info: Understanding enzymes integrates concepts from multiple areas of GOB Chemistry, including molecular structure, reaction energetics, and biological function.

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