Enzymes & Metabolism
Middle SchoolLife runs on reactions that, left alone, would take years. Enzymes are the biological catalysts that make cellular chemistry happen fast enough to sustain life — and the switches through which the cell controls its own metabolism.
Catalysis and activation energy
Every reaction must climb an energy barrier — the activation energy (Ea) — before reactants become products. An enzyme is a biological catalyst (almost always a protein; a few are RNA "ribozymes") that lowers Ea by providing an alternative reaction pathway. It does not change the reaction's equilibrium or its overall energy change (ΔG); it only makes the reaction reach that equilibrium faster. Enzymes are unchanged by the reaction and reused.
Each enzyme has an active site — a precisely shaped pocket that binds the substrate. The classic lock-and-key model treats the fit as rigid; the more accurate induced-fit model recognises that the active site changes shape as the substrate binds, straining the substrate's bonds and easing the reaction. Enzymes are highly specific because the active-site shape complements only certain substrates.
Factors affecting enzyme activity
- Temperature — rate rises with temperature (more kinetic energy, more collisions) up to an optimum, then falls sharply as heat denatures the enzyme (breaks the weak bonds holding its 3-D shape).
- pH — each enzyme has an optimum pH; extremes disrupt ionic and hydrogen bonds, altering active-site shape and denaturing it.
- Substrate concentration — rate rises then plateaus when all active sites are saturated (Vmax).
- Enzyme concentration — with excess substrate, rate is proportional to enzyme amount.
- Inhibitors — molecules that reduce activity (see below).
Inhibition and regulation
Competitive inhibitors resemble the substrate and block the active site; their effect is overcome by raising substrate concentration. Non-competitive inhibitors bind elsewhere (an allosteric site), changing the active site's shape; raising substrate does not relieve them. Cells regulate pathways by end-product (feedback) inhibition: the final product of a pathway inhibits an early enzyme, switching off its own production once enough is made — an elegant self-governing thermostat.
Why denaturation is not the same as "using up"
A common confusion: heating an enzyme past its optimum does not "use it up" or speed it into exhaustion — it destroys its shape. An enzyme's function is its 3-D structure, held together by many weak bonds (hydrogen, ionic) determined by the protein's amino-acid sequence. Heat and pH extremes shake those weak bonds apart, so the active site loses its precise geometry and the substrate no longer fits. This is usually irreversible — like an egg white cooking. That is why a fever of just a few degrees is dangerous: human enzymes are tuned to ~37°C, and even modest overheating begins to warp them.
Below the optimum, most enzyme-controlled reactions roughly double in rate for every 10°C rise (Q₁₀ ≈ 2). This is why cold-blooded animals are sluggish in cold weather and why refrigeration slows the enzymes of spoilage microbes — slowing chemistry, not stopping it.
Experiment: catalase and the effect of temperature
Catalase breaks down toxic hydrogen peroxide: 2 H₂O₂ → 2 H₂O + O₂. The oxygen released lets us measure its rate.
- Prepare substrate & enzyme. Use a fixed volume of hydrogen peroxide and equal-sized discs of potato or liver (rich in catalase).
- Control the variable. Equilibrate enzyme and substrate separately in water baths at set temperatures: 10, 20, 30, 40, 50, 60°C.
- Collect the product. Combine them and measure the volume of O₂ gas collected (over water in a measuring cylinder, or by displacement) in a fixed time, e.g. 60 s. Alternatively measure the height of oxygen foam.
- Calculate rate. rate = volume of O₂ ÷ time. Repeat each temperature three times and take a mean to reduce random error.
- Plot. Rate (y) against temperature (x). Expect a rise to a peak around 40°C, then a steep decline.
- Interpret. The rise reflects increasing kinetic energy and collision frequency; the fall past the optimum reflects denaturation — the curve is not symmetrical, because heating destroys the enzyme rather than simply reversing the speed-up. That asymmetry is the signature of a protein catalyst.
Test yourself
Q1 Enzymes lower activation energy. Does this mean they make more energy available or make the reaction more favourable? Explain.
Neither. An enzyme lowers the activation energy — the barrier that must be crossed — so a greater fraction of molecules have enough energy to react at a given temperature, and the reaction proceeds faster. It does not change the overall energy difference between reactants and products (ΔG) or the position of equilibrium; a reaction that is energetically unfavourable stays unfavourable, just reached faster in whichever direction it tends. Enzymes change kinetics, not thermodynamics.
Q2 You measure enzyme activity across pH 2–10 and find two peaks in a mixed sample. What does this suggest?
Two distinct pH optima suggest the sample contains two different enzymes with different optimum pH values — for example a stomach enzyme like pepsin (optimum ≈ pH 2) and an intestinal enzyme like trypsin (optimum ≈ pH 8). Each enzyme's shape and charge are tuned to a particular environment, so each shows peak activity where its active site is best maintained.
Q3 How would you distinguish competitive from non-competitive inhibition experimentally?
Vary the substrate concentration in the presence of the inhibitor. If increasing substrate restores the rate toward the normal Vmax, the inhibitor is competitive — more substrate out-competes it for the active site (same Vmax, higher apparent Km). If increasing substrate cannot restore full rate (Vmax stays lowered), the inhibitor is non-competitive — it binds an allosteric site and disables enzyme molecules regardless of how much substrate is present.
Q4 Explain end-product inhibition and why it is efficient.
In a metabolic pathway A→B→C→D, the final product D inhibits the first enzyme (usually by binding an allosteric site). When D accumulates, it shuts down the pathway that makes it; when D is used up, inhibition lifts and the pathway restarts. This negative-feedback loop prevents wasteful overproduction, conserves resources and precursors, and keeps product levels stable — the cell self-regulates without any external control, like a thermostat switching off the heater once the room is warm.
Q5 A student claims "boiling then cooling an enzyme restores its activity because cooling reverses the heating." Evaluate.
The claim is wrong for boiling. Gentle warming and cooling are reversible because they only change molecular motion. But boiling denatures the enzyme: the weak bonds (hydrogen, ionic) that hold its precise tertiary structure are broken and the polypeptide collapses into a tangled, often aggregated shape. Cooling does not spontaneously refold it to the exact active conformation — denaturation is generally irreversible (like a boiled egg not un-cooking). Activity is permanently lost.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1Catalase activity rises to 40C then falls sharply by 60C. Explain.
- Warmth speeds molecule collisions up to the optimum
- Above it the active site denatures
Example 2An enzyme works best at pH 2. Where in the body is it likely active?
- pH 2 is strongly acidic
- The stomach is acidic
Now you try
Work each one out first, then tap to reveal the worked answer.