Enzymes and Metabolism
Explain enzyme specificity, activation energy, catalysts, and factors that affect enzyme activity. Interpret graphs of enzyme rate under changing temperature, pH, and substrate concentration.
A lesson of Biology, in Cells and Cellular Processes, about 10 minutes.
By the end of this lesson, you will be able to define metabolism and explain anabolism vs. catabolism, describe how enzymes work as catalysts, explain factors that affect enzyme activity, and apply the enzyme-substrate model to biochemical reactions.
The Big Idea
A cell performs thousands of chemical reactions simultaneously, most of which would proceed far too slowly at biological temperatures without a catalyst. Enzymes are those catalysts — proteins shaped precisely to accelerate specific reactions while remaining themselves unchanged. Without enzymes, metabolism would be impossibly slow, and life as we know it couldn't exist.
Metabolism Defined
Metabolism: the sum of all chemical reactions in a cell.
Anabolism (building up): small molecules → large molecules; requires energy (endergonic). Example: protein synthesis, DNA replication.
Catabolism (breaking down): large molecules → small molecules; releases energy (exergonic). Example: digestion, cellular respiration.
ATP: the universal energy currency. Exergonic reactions release energy captured as ATP; endergonic reactions consume ATP.
Enzymes as Catalysts
Catalyst: a substance that speeds up a reaction without being consumed or permanently altered.
Enzymes work by lowering the activation energy (the energy needed to start the reaction):
- Substrate binds to the enzyme's active site → forms the enzyme-substrate complex
- Enzyme stresses bonds in the substrate, making the reaction more likely
- Products are released; enzyme is free to catalyze another reaction
Lock-and-key model: the active site is a rigid, specific shape that fits only one substrate.
Induced fit model (more accurate): the active site changes shape slightly to accommodate the substrate, like a handshake.
Factors Affecting Enzyme Activity
Temperature:
- Increasing temperature → faster molecular motion → more collisions → higher activity
- Too high → enzyme denatures (shape changes irreversibly) → activity drops to zero
- Optimal temperature for most human enzymes: ~37°C
pH:
- Enzymes have an optimal pH; outside this range, H⁺/OH⁻ disrupt hydrogen bonds → shape changes → activity decreases
- Pepsin (stomach): optimal pH ≈ 2; Amylase (saliva): optimal pH ≈ 7