Preview
IB DP Biology — C: Interaction and Interdependence | SL | C1.1
📋 What You Will Learn
- Identify the structural features of enzymes that enable substrate binding and catalysis, including the active site and induced fit model
- Explain how enzymes lower activation energy and accelerate metabolic reactions without being consumed
- Analyse how enzyme activity is regulated through competitive inhibition, non-competitive inhibition, and allosteric control
- Interpret enzyme kinetics data (rate vs. substrate concentration graphs) and apply these to real biological contexts
- Evaluate how metabolic flux is controlled through feedback inhibition and enzyme regulation in metabolic pathways
🔍 Guiding Question How do enzymes control the rate and direction of metabolic reactions, and what mechanisms allow cells to regulate metabolic flux in response to changing conditions?
🌍 Why This Matters Every second, your liver cells perform over 500 distinct chemical reactions — digesting toxins, synthesising proteins, and managing blood glucose — all without boiling or exploding. This is only possible because enzymes act as precision-engineered molecular machines, each one tuned to catalyse exactly one reaction at exactly the right speed. When enzyme regulation fails, the consequences are immediate: uncontrolled cell division in cancer, metabolic collapse in phenylketonuria, or the toxic accumulation of substrates in lysosomal storage diseases.
The Problem the Cell Had to Solve
Life runs on chemistry. But most of the chemical reactions that sustain a cell — breaking down glucose, copying DNA, building proteins — would take years to occur spontaneously at body temperature. The cell needed a way to make reactions happen fast, selectively, and in the right order. The solution evolution arrived at was the enzyme: a protein catalyst that lowers the energy barrier for a specific reaction without being permanently changed by it.
Understanding enzymes is not just about memorising definitions. It is about understanding how cells control their own chemistry — how they speed up reactions when resources are plentiful, slow them down when products accumulate, and redirect metabolic traffic in response to signals. This is the story of metabolic control.
📖 Key Definition — Enzyme An enzyme is a biological catalyst — a globular protein (or, in some cases, an RNA molecule called a ribozyme) that increases the rate of a specific chemical reaction by lowering the activation energy required, without itself being permanently altered or consumed in the process.
📖 Key Definition — Activation Energy Activation energy is the minimum energy input required to initiate a chemical reaction — the energy needed to break existing bonds in reactant molecules so that new bonds can form. Enzymes lower this energy barrier, making reactions feasible at the temperatures found in living cells.
How Enzymes Work: The Active Site and Catalytic Mechanism
Picture an enzyme as a three-dimensional protein sculpture, folded into a precise shape by the sequence of its amino acids. Buried within that shape is a small, chemically specialised pocket called the active site. This is where catalysis happens. The active site is not just a passive hole — it is lined with specific amino acid residues whose R-groups create a unique chemical environment: charged, hydrophobic, or capable of forming hydrogen bonds in exactly the right configuration to bind one specific substrate.
📖 Key Definition — Active Site The active site is the specific region of an enzyme molecule, formed by a small number of amino acid residues, where the substrate binds and the catalytic reaction takes place. Its three-dimensional shape is complementary to the shape of the substrate molecule.
The Induced Fit Model
For decades, the "lock and key" model described the active site as a rigid, perfectly