The thousands of chemical reactions that keep you alive would, on their own, happen far too slowly to sustain life. Enzymes are the answer — molecular catalysts that speed reactions up by millions of times and let the cell control its chemistry with exquisite precision. They make metabolism possible.
Learning Objectives
- •Understand what enzymes do and how
- •Learn how enzyme activity is regulated
- •See how enzymes organize metabolism into pathways
Enzymes: biological catalysts
An ENZYME is a biological CATALYST — usually a protein — that speeds up a specific chemical reaction without being used up. It works by lowering the 'activation energy': the energy hurdle a reaction must clear to proceed. By making that hurdle smaller, enzymes let reactions that would otherwise take years happen in milliseconds. Nearly every reaction in your metabolism is run by a specific enzyme.
The active site: specificity
Each enzyme has an ACTIVE SITE — a precisely-shaped pocket that fits its specific target molecule(s), the 'substrate', like a lock and key (more accurately, the site flexes to grip the substrate — 'induced fit'). This shape-based specificity means each enzyme catalyzes one particular reaction. It's another instance of the deep theme that a protein's SHAPE determines its FUNCTION.
Regulation: turning enzymes up and down
Crucially, enzyme activity is REGULATED — the cell can speed enzymes up or slow them down to control its chemistry. Common methods: INHIBITORS that block the active site; ALLOSTERIC regulation where a molecule binds elsewhere and changes the enzyme's shape; and FEEDBACK INHIBITION, where a pathway's end product switches off an enzyme earlier in the pathway (so the cell stops making what it already has enough of). This regulation is how cells finely tune metabolism moment to moment.
reaction WITHOUT enzyme: big activation-energy hurdle → very slow reaction WITH enzyme: LOWERED hurdle → fast substrate → [ENZYME active site grips it] → product (enzyme reused) Regulated by: inhibitors · allosteric molecules · feedback inhibition
Metabolism: enzymes working in pathways
Cells don't run reactions in isolation — they chain enzymes into PATHWAYS, where the product of one enzyme's reaction becomes the substrate for the next, like a molecular assembly line. METABOLISM is the sum of all these pathways: catabolic ones that break molecules down for energy, and anabolic ones that build molecules up. By controlling individual enzymes (often the regulators you met in the Pathways course act here), the cell directs the flow through these networks.
Why many drugs and poisons work by blocking enzymes
A huge fraction of medicines work by INHIBITING a specific enzyme — statins block an enzyme in cholesterol synthesis; many painkillers block enzymes that make inflammatory molecules; some antibiotics block bacterial enzymes. Many poisons work the same way (cyanide blocks a key enzyme of energy production). Understanding enzymes is understanding how much of pharmacology actually works at the molecular level.
Enzymes & metabolism, by the numbers
- ▸Enzymes are catalysts (usually proteins) that lower reactions' activation energy
- ▸An enzyme's active site fits its specific substrate (shape = specificity)
- ▸Activity is regulated by inhibitors, allosteric molecules, and feedback inhibition
- ▸Metabolism is enzymes chained into catabolic and anabolic pathways
Enzymes are used up each time they catalyze a reaction, like fuel.
Enzymes are CATALYSTS — they speed reactions without being consumed, so a single enzyme can catalyze the same reaction over and over. They lower the activation-energy hurdle and are then released to work again.
Quick Check
How do enzymes speed up reactions?
Quick Check
What is feedback inhibition?
True or False
Many drugs work by inhibiting a specific enzyme.
Summary
- →Enzymes are catalysts that lower activation energy, speeding reactions without being consumed
- →An enzyme's active site fits its specific substrate (shape = specificity)
- →Activity is regulated by inhibitors, allosteric molecules, and feedback inhibition
- →Metabolism is enzymes chained into catabolic and anabolic pathways
Beyond running reactions, cells must talk to each other — at the molecular level. Next: cell signaling.