BIOL10008 · Foundations Of Biology: Life's Machinery
Enzymes
Almost every reaction in a cell would be far too slow without help. Enzymes are biological catalysts — usually proteins — that speed a specific reaction by lowering its activation energy (Ea) without being used up. This chapter covers the active site and induced fit (E + S → ES → E + P, enzyme reused), the crucial point that enzymes lower Ea but not ΔG (route, not destination), and the four factors that change rate — temperature and pH (which can denature past an optimum) and substrate and enzyme concentration. It then covers regulation: competitive inhibitors block the active site (beaten by more substrate) versus non-competitive / allosteric inhibitors that bind elsewhere and cap the maximum rate, and feedback inhibition, where a pathway's end-product allosterically switches off an early enzyme so the pathway regulates itself. Expect a mix of concept and graph-reading, plus the classic PFK feedback scenario.
What this chapter covers
- 01The active site & induced fit — the catalytic cycle E + S → ES → E + P
- 02Lowering activation energy: enzymes lower Ea, not ΔG
- 03Factors affecting rate: temperature, pH, [substrate], [enzyme]
- 04Reading activity-vs-pH and rate-vs-[substrate] curves
- 05Competitive vs non-competitive / allosteric inhibition
- 06Feedback inhibition — pathways that self-regulate (the PFK scenario)
Worked example: competitive or non-competitive? (the one decisive question)
- +1(a) Classify X: because more substrate restores the maximum rate, X is a competitive inhibitor — it can be out-competed.
- +1(a) Classify Y: because extra substrate cannot restore the maximum, Y is a non-competitive (allosteric) inhibitor.
- +1(b) Where each binds: X binds the active site itself (it resembles the substrate and blocks it); Y binds a different allosteric site and changes the enzyme's shape.
- +1(c) The decisive question: ask 'can more substrate restore the maximum rate?' Yes → competitive; No → non-competitive.
- +1(c) The mechanism: a competitive inhibitor only occupies active sites temporarily, so flooding with substrate wins the competition; a non-competitive inhibitor distorts the active site via the allosteric site, so adding substrate cannot help — the maximum rate is lowered.
Key terms
- Active site & induced fit
- The active site is a shape-specific pocket on an enzyme. When the substrate binds, the enzyme moulds slightly around it (induced fit), forming the enzyme–substrate (ES) complex, then converts substrate to product and releases it, leaving the enzyme unchanged and reusable.
- Activation energy (Ea)
- The energy barrier a reaction must cross. An enzyme provides an alternative route with a lower Ea, so more molecules can react and the reaction goes faster. The enzyme changes the route, not the destination.
- Enzymes lower Ea, not ΔG
- Enzymes lower only the activation-energy hump; they do not change ΔG (the energy gap between reactants and products), so they speed both directions equally and do not shift where equilibrium sits.
- Competitive vs non-competitive inhibition
- A competitive inhibitor resembles the substrate and blocks the active site — beaten by more substrate, maximum rate unchanged. A non-competitive (allosteric) inhibitor binds elsewhere and distorts the active site — not beaten by more substrate, maximum rate lowered.
- Feedback inhibition
- In a pathway A→B→C→D, the end-product D binds the allosteric site of an early enzyme and switches it off. When D is abundant it shuts down its own production; when D is used up the brake releases — the pathway regulates itself.
Enzymes FAQ
Do enzymes change how much energy a reaction releases?
No — this is the most-tested misconception. An enzyme lowers only the activation-energy hump, so the reaction reaches equilibrium faster. It does not change ΔG, the energy gap between reactants and products, which is fixed by the molecules themselves. So an enzyme does not make a reaction release more energy or shift where equilibrium sits; it just gets you there faster, like a tunnel through a hill rather than a path over it.
Why does enzyme activity rise then fall as temperature increases?
Activity rises with heat at first because molecules collide more often and with more energy, up to the enzyme's optimum. Past the optimum, the added heat gives the protein's R groups so much kinetic energy that the bonds holding its fold (H-bonds, ionic, LDF) break — the enzyme denatures, the active site loses its shape, and catalysis stops. Extreme pH disrupts the active site the same way. Heat denaturation is effectively irreversible, like a cooked egg.
How do I tell competitive from non-competitive inhibition on a graph?
Look at whether the rate-vs-[substrate] curve still reaches the same plateau. A competitive inhibitor reaches the same maximum rate if you add enough substrate (the curve is shifted right but still levels off at the top). A non-competitive inhibitor caps the rate lower — extra substrate cannot rescue it. The one-line test is: can more substrate restore the maximum? Yes = competitive, No = non-competitive.
What is the PFK feedback scenario the test uses?
Phosphofructokinase (PFK) is an early glycolysis enzyme normally inhibited at its allosteric site by abundant ATP — classic negative feedback that slows glucose breakdown when energy is plentiful. The scenario asks what happens if a drug blocks ATP binding to that allosteric site: the brake is gone, so glycolysis keeps running even when ATP is abundant, because the feedback signal can no longer register. Allosteric enzymes are exactly the ones regulated this way.
Exam move
Separate the chapter into concept and graph. For concept, be able to walk the catalytic cycle (E + S → ES → E + P) with induced fit, and nail the one line examiners reward most: enzymes lower Ea but NOT ΔG. For graphs, practise reading activity-vs-pH (each enzyme has an optimum, e.g. pepsin ~pH 2, trypsin ~pH 8) and rate-vs-[substrate] (it plateaus at saturation; competitive shifts right but reaches the same plateau, non-competitive caps it lower). For regulation, drill the one decisive question — can more substrate restore the maximum rate? — and have the PFK feedback scenario ready to explain in full.
