Quick Overview
Enzymes are proteins that speed up reactions without being used up
They work by the "lock and key" model — substrate shape must fit enzyme's active site
Each enzyme only works with one specific substrate (enzyme specificity)
Temperature affects enzyme speed: increases activity up to optimum (37°C in humans), then denatures above it
Low temperatures slow enzymes down but don't denature them
pH affects enzyme shape: most optimal at pH 7, some at pH 2 (stomach) or pH 8–9 (duodenum)
Denaturation (breaking bonds) is irreversible and stops enzyme activity completely
What Are Enzymes?
Think of enzymes as the workforce of living cells. They're protein catalysts — special molecules that speed up chemical reactions without being changed or used up themselves. Without enzymes, your body would move at a snail's pace. For example, digesting one meal would take 2–3 weeks; with enzymes, it takes about 4 hours.
Why Enzymes Matter
Every living process — digestion, respiration, muscle contraction, even thinking — relies on enzymes to work fast enough to keep you alive. They maintain the speeds of your metabolic reactions (all the chemistry that keeps you functioning) at rates that your body can actually sustain.
Definition
An enzyme is a biological catalyst (a protein) that speeds up the rate of a chemical reaction without being changed or used up in the reaction.
Analogy: Think of an enzyme like a cooking assistant. It brings ingredients together, helps them react, then hands you the finished dish and walks away unchanged — ready to help with the next meal. It never gets eaten, never disappears. It just makes things happen faster.
Key Properties
- Proteins: Enzymes are made of chains of amino acids folded into a specific 3D shape
- Biological: They're made inside living cells, not in factories
- Catalysts: They speed reactions up without being changed themselves
- Essential: Life cannot happen at the right speed without them
💡 Exam Tip: Always remember — enzymes are NOT used up or changed by the reaction. If a question asks whether enzymes are consumed, the answer is no.
Practice Question
Why would a person die within a few weeks if their cells couldn't produce enzymes?
How Enzymes Work: The Lock & Key Model
Enzymes work through a simple but elegant system: each enzyme has a specific shape that perfectly fits only one (or a few) substrates — just like a lock fits only its matching key.
The Three-Step Process
Step 1: Random Collision
Enzymes and substrates move randomly through the cell in solution. Eventually, an enzyme and its matching substrate bump into each other.
Step 2: Formation of Enzyme-Substrate Complex
When the substrate's shape perfectly fits the enzyme's active site (the special pocket on the enzyme), they bind together. At this moment, they're called the "enzyme-substrate complex." The reaction happens here.
Step 3: Release of Products
Products form from the substrate. Because the products have a different shape than the substrate, they no longer fit the active site. They leave, and the enzyme is free to pick up another substrate. The enzyme is completely unchanged.
Key Terms
Substrate:
Active site:
Enzyme-substrate complex:
Product: