Enzyme Catalysis
A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed. An enzyme is a biological catalyst — a protein molecule whose three-dimensional shape positions specific reactants precisely, enabling reactions that would otherwise be prohibitively slow at body temperature.
The mechanism rests on the concept of activation energy: the energy barrier that reactants must cross to reach the transition state before they can rearrange into products. Most chemical reactions are thermodynamically favorable — the products have lower energy than the reactants — but the reactants must first climb a hill before they can fall. Without catalysis, the hill may be too high for most molecules to surmount at room temperature, making the reaction effectively inert.
Enzymes lower the activation energy barrier. They do this by physically binding to the reactant molecules (substrates), straining their bonds, positioning them optimally, and sometimes providing an alternative reaction pathway altogether. The enzyme-substrate complex can then proceed to products with far less energy input.
Why Physics Explains Biology
Feynman uses enzyme catalysis to illustrate his broader point about the unity of science. The mechanism of an enzyme is not mysterious or "vital" — it is atomic physics. The enzyme is a specific arrangement of atoms; the substrate is another arrangement; their interaction is governed by the same electromagnetic forces that govern all chemistry. Once you have the right three-dimensional structure, the catalysis follows from physics.
This is why Feynman can say "biology is applied chemistry, which is applied physics." The reduction is real, not metaphorical. The biological phenomenon (life's chemistry happening fast enough to sustain living cells) traces back to atomic structures.
Scale
Enzymes are astonishingly specific and efficient. A single enzyme molecule can catalyze millions of reactions per second, and they typically catalyze only one specific substrate — the "lock and key" principle, where the enzyme's active site is shaped to accept only its partner molecule. The immune system exploits this specificity: antibodies recognize particular molecular shapes, and enzymes in metabolism handle only their designated substrates.
Connections
- atomic-hypothesis — enzymes work because chemistry is atomic
- dna-double-helix — enzymes are also responsible for replicating DNA (DNA polymerase) and transcribing genes
- activation-energy — the energy barrier that catalysts lower