Enzymes as proteins (overview) - Biology IGCSE Study Notes

Enzymes are biological catalysts that speed up chemical reactions in living organisms without being consumed in the process. As proteins, enzymes are composed of long chains of amino acids folded into specific three-dimensional shapes crucial for their function.

Key Definitions:

• Catalyst: A substance that increases the rate of a chemical reaction without being permanently altered
• Active Site: The specific region on an enzyme's surface where the substrate binds, possessing a complementary shape
• Substrate: The molecule upon which an enzyme acts
• Product: The substance(s) formed after the enzyme catalyses the reaction
• Enzyme-Substrate Complex: The temporary structure formed when substrate binds to the enzyme's active site
• Lock and Key Model: A theory explaining enzyme specificity where the substrate fits precisely into the active site like a key in a lock

Essential Characteristics:

Enzymes exhibit specificity—each enzyme catalyses only one type of reaction due to its uniquely shaped active site. They are reusable, functioning repeatedly without being consumed. As proteins, enzymes are denatured by extreme temperatures or pH levels, permanently losing their functional shape.

The Basic Enzyme Reaction:

Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product

Mnemonic for Enzyme Properties: "STREP"

• Specific to substrates
• Temperature sensitive
• Reusable catalysts
• Effected by pH
• Protein structure essential

Enzymes lower the activation energy required for reactions, enabling metabolic processes to occur rapidly at body temperature. This efficiency is vital for life processes including digestion, respiration, and photosynthesis.

Understanding Enzyme Action Through Real-World Analogies:

Think of an enzyme as a molecular scissors perfectly designed for one cutting job. Just as safety scissors (enzyme) can only cut paper (substrate) of certain thickness, producing cut pieces (products), enzymes only work with specific substrates.

Real-World Applications:

1. Digestive Enzymes: Amylase in saliva breaks down starch into maltose during chewing. Notice how bread tastes sweeter after prolonged chewing—this demonstrates enzyme action! Protease in the stomach digests proteins into amino acids, while lipase in the small intestine breaks fats into fatty acids and glycerol.

2. Industrial Applications: Biological washing powders contain proteases and lipases that break down protein stains (blood, egg) and grease at low temperatures, saving energy. This demonstrates enzyme efficiency compared to traditional high-temperature washing.

3. Food Production: Lactase is added to milk to produce lactose-free dairy products for lactose-intolerant individuals. Pectinase clarifies fruit juices by breaking down pectin, preventing cloudiness.

The Lock and Key Mechanism Visualised:

Imagine a busy reception desk (active site) designed for one specific key card (substrate). Only the correctly shaped card activates the door mechanism, allowing passage (product formation). No other card works—this illustrates enzyme specificity.

Why Protein Structure Matters:

Enzymes' three-dimensional shapes result from amino acid sequence and folding. Heat disrupts weak bonds maintaining this shape, like a twisted telephone cord losing its coil structure. Once denatured, the active site changes permanently, preventing substrate binding—the enzyme becomes non-functional, much like a melted key that no longer fits its lock.