Lesson 4: Cellular Energetics — Enzymes and ATPAP

A cell is a chemical factory that must run thousands of reactions at exactly the right rates in a watery environment at roughly constant temperature. Unit 3 of the CED (Topics 3.1–3.4) covers how it is done: enzymes lower the activation-energy barrier so reactions proceed quickly and selectively, the environment tunes enzyme activity, and ATP shuttles energy from reactions that release it to reactions that need it. Nearly every part of this lesson appears in data-based exam questions — rate graphs, inhibitor tables, temperature curves — so learn it as a set of tools for reading experiments, not just as definitions.

How enzymes work

Most spontaneous reactions are too slow to sustain life because molecules must first reach an unstable, high-energy transition state. The extra energy needed is the activation energy (Eₐ). An enzyme lowers Eₐ by binding its substrate at a precisely shaped active site, forming an enzyme–substrate complex that strains bonds, orients reactants or provides a microenvironment (for example a pocket of different pH) favouring the transition state. The enzyme emerges unchanged and can be reused thousands of times. The modern model is induced fit: the active site moulds itself slightly around the substrate, improving catalysis. Specificity comes from shape and chemistry — which is why a single amino acid change in the active site can destroy function (Lesson 1's structure–function story, again).

Reaction rate depends on conditions. As substrate concentration rises, rate rises steeply at first, then levels off at Vmax when every active site is busy — the enzyme is saturated, and only adding more enzyme raises the rate further. Temperature increases rate up to an optimum, because collisions are faster, but beyond it the enzyme denatures (Lesson 1) and rate crashes. Each enzyme also has an optimal pH: pepsin works near pH 2 in the stomach; trypsin near pH 8 in the small intestine; moving away from the optimum distorts the charges on R groups that hold the fold and the active site together.

Cells regulate enzymes with inhibitors. Competitive inhibitors resemble the substrate and occupy the active site; their effect can be overcome by adding more substrate. Noncompetitive inhibitors bind a different site (an allosteric site) and deform the enzyme so the active site works poorly, whatever the substrate concentration. Allosteric regulation is also positive: activators stabilise the active form. Many metabolic pathways use feedback inhibition, in which the final product allosterically inhibits an early enzyme — the cell stops making what it already has.

ATP: the energy currency — and what that really means

Reactions that release free energy (exergonic, ΔG negative) are coupled to reactions that require it (endergonic, ΔG positive) through ATP. Hydrolysing ATP to ADP + inorganic phosphate has ΔG ≈ −30.5 kJ/mol under standard conditions (more inside cells). The energy does not sit "in a bond" waiting to burst out — breaking the terminal phosphate bond costs energy, but the products are far more stable (less charge repulsion, more resonance forms), so the overall reaction releases energy. In energy coupling, enzymes perform phosphorylation: they transfer ATP's terminal phosphate onto a reactant, making it reactive enough to proceed (as when glucose is phosphorylated in the first step of glycolysis), or onto a protein to change its shape, as in the sodium–potassium pump. ATP is regenerated continuously by cellular respiration and photosynthesis — an active human cell recycles its own mass in ATP many times per day.

Worked example 1: identifying an inhibitor from data

An enzyme converts substrate S to product. Without inhibitor, the reaction rate rises with substrate concentration and plateaus at 100 units. With inhibitor X added, the plateau is still 100 units, but much more substrate is needed to reach half-maximal rate. With inhibitor Y, the plateau falls to 45 units and cannot be restored by adding substrate.

Classify X and Y. X is a competitive inhibitor: it competes for the active site, slowing the approach to maximum rate, but enough substrate outcompetes it, so Vmax is unchanged. Y is a noncompetitive inhibitor: it binds elsewhere and disables a fraction of the enzyme molecules outright, so the maximum rate falls regardless of substrate concentration. When the exam gives you a graph, read it the same way: same plateau + shifted curve = competitive; lower plateau = noncompetitive.

Worked example 2: reading an enzyme-rate experiment

Students test the enzyme catalase (which breaks down hydrogen peroxide) using discs soaked in enzyme placed in peroxide solution; the time for each disc to float (lifted by oxygen bubbles) is recorded. At 10 °C the mean floating time is 84 s; at 25 °C it is 31 s; at 40 °C it is 19 s; at 60 °C no disc floats within 300 s.

Interpret the trend, identifying the optimum region and the anomaly. Rate is roughly the reciprocal of floating time, so activity rises from 10 °C to 40 °C as molecular collisions increase. The optimum under these conditions lies somewhere around 35–45 °C. At 60 °C the enzyme is denatured: the active site's shape is destroyed, so no oxygen is produced and the discs never float. Two exam-ready details: (i) the students measured time, so "faster floating = higher rate" must be stated, not assumed; (ii) a fair test repeats each temperature and controls peroxide concentration, disc size and enzyme concentration — exactly the control-of-variables points awarded in FRQ experimental-design questions.

Where this shows up in the real world

Many medicines are enzyme inhibitors. Statins competitively inhibit HMG-CoA reductase, an early enzyme in cholesterol synthesis, lowering the cholesterol the body makes. Methotrexate, a chemotherapy drug, competitively inhibits an enzyme needed to build nucleotides, starving fast-dividing cancer cells of DNA building blocks. Fever is the dangerous edge of temperature sensitivity: a few degrees above optimum begins to distort the human enzymes that run metabolism, which is one reason sustained fevers above about 41 °C are a medical emergency.

Common mistakes

• "Enzymes are used up in the reaction." They are catalysts — regenerated every cycle. If a question shows enzyme concentration staying constant, that is why.
• Claiming enzymes change ΔG or make endergonic reactions spontaneous. Enzymes change only the rate (via Eₐ), never the thermodynamics. An endergonic reaction still needs coupling to ATP or another energy source.
• Saying denaturation breaks peptide bonds. Heat and pH disrupt weak interactions (Lesson 1); the sequence survives.
• Reading a plateau as "the reaction stopped." A plateau in a rate-versus-substrate graph means saturation — the reaction is running at maximum speed.
• Stating that ATP hydrolysis "releases the energy stored in its bonds." Breaking any bond requires energy input; the net release comes from the greater stability of ADP + Pi. The College Board accepts "energy released by hydrolysis of ATP" but punishes the bond-storage misconception when it leads to wrong predictions.
• Confusing competitive and noncompetitive effects on Vmax. Competitive: Vmax unchanged, more substrate needed. Noncompetitive: Vmax falls.

AP Exam Experience

Question 1. An enzyme-catalysed reaction has a ΔG of −12 kJ/mol. When the enzyme is removed and the same reaction is measured under identical conditions, ΔG will be

• (A) more negative, because the enzyme was holding energy back
• (B) zero, because no reaction can occur without an enzyme
• (C) unchanged at −12 kJ/mol, but the reaction will proceed far more slowly
• (D) positive, because enzymes reverse the direction of reactions

Question 2. In a metabolic pathway A → B → C → D, compound D allosterically inhibits the enzyme that converts A to B. This is an example of

• (A) competitive inhibition by a substrate
• (B) feedback inhibition, which prevents overproduction of D
• (C) denaturation of the first enzyme
• (D) energy coupling through ATP

Question 3. A student claims that increasing substrate concentration indefinitely will keep increasing the rate of an enzyme-catalysed reaction. The claim fails because

• (A) enzymes denature when substrate is added
• (B) at high substrate concentrations all active sites are occupied, so the rate reaches a maximum set by the amount of enzyme
• (C) substrates inhibit their own enzymes at all concentrations
• (D) ATP is consumed by substrate molecules directly

Answer key

• 1. (C) — Catalysts lower activation energy only. ΔG is a property of reactants and products, so it is unchanged; the uncatalysed reaction is simply extremely slow.
• 2. (B) — The end product shutting down an early step is classic feedback (negative) inhibition: economical regulation that matches production to need.
• 3. (B) — Saturation: once every active site is working continuously, extra substrate must wait its turn, so the rate plateaus at Vmax.

Free-response question. Researchers study an enzyme from a bacterium living in a hot spring at 75 °C. They measure its activity at temperatures from 10 °C to 90 °C and compare it with the same enzyme from a soil bacterium. The hot-spring enzyme shows little activity below 40 °C, peak activity at 78 °C, and retains activity after boiling for 10 minutes; the soil enzyme peaks at 35 °C and is inactive after 10 minutes at 60 °C. (a) Describe the shape of the activity curve for the soil enzyme and explain the decline above 35 °C. (b) Explain how the hot-spring enzyme's structure could account for its heat stability. (c) Predict the effect of transferring the hot-spring bacterium to a pond at 20 °C on its growth, and justify the prediction using enzyme activity. (d) A student argues the hot-spring enzyme would work even better in human cells because "faster enzymes are always better." Evaluate the claim.

Scoring guideline

• (a) 1 point: activity rises to a peak at 35 °C then falls steeply to zero by 60 °C. 1 point: above the optimum, thermal energy disrupts hydrogen bonds/weak interactions, denaturing the enzyme and destroying the active site.
• (b) 1 point: more internal bonds/interactions (e.g., additional ionic bonds, disulfide bridges, tighter hydrophobic packing) make the fold harder to disrupt, so the active site survives high temperatures (accept any plausible stabilising structural feature).
• (c) 1 point: predicts poor or no growth. 1 point: at 20 °C the enzyme has little activity (molecules move slowly and the rigid fold is poorly suited to low temperatures), so metabolism cannot sustain growth.
• (d) 1 point: rejects or qualifies the claim — at human body temperature (37 °C) the enzyme shows little activity, and enzyme performance depends on the match between the enzyme's optimum and its environment, not on maximum possible speed (accept reference to specificity/substrate fit).