Enzyme active site (lock and key) sim

What the sim shows

An enzyme is a protein that speeds up a reaction without being used up. Amylase breaks starch down into maltose. The lock-and-key model: the enzyme's active site has a shape that fits its substrate exactly, as a key fits a lock. Starch fits; protein and fat do not, so amylase acts on starch only: enzymes are specific. Substrate and enzyme join as an enzyme–substrate complex, the starch is broken, and the products leave; the enzyme is free to work again. Temperature: warmer molecules move faster and collide more often and with more energy, so the rate rises (about doubling for each 10 °C near room temperature; the collision idea is IGCSE Supplement). But heat also breaks the weak bonds that hold the enzyme in its shape, so above the optimum the active site changes shape and the substrate no longer fits: the enzyme is denatured. Denaturing is permanent: cooling it does not bring the shape back. An enzyme is not alive, so it is denatured, not killed. Cold only slows an enzyme down. pH changes the charges in the active site and so its shape, so the enzyme works best at its optimum pH (about 7 for amylase). A small change of pH is reversible; a strong acid or alkali denatures it for good. The model: rate = 40 maltose a minute × e^(−Ea/RT) (Ea = 50 kJ/mol, scaled to 1 at 37 °C) × the share of enzyme molecules still folded (a two-state model, ΔH = 300 kJ/mol, Tm = 41.35 °C, chosen so the optimum is 37 °C) × the share with the right charges (pK 5.5 and 8.5, best at pH 7). At the optimum about 1 enzyme molecule in 6 has already lost its shape; above it, more are lost than the extra speed gains, so the rate falls. The model enzyme on the platform stands for a typical enzyme molecule: starch fits it while at least half of the active sites have the right shape. When it does not fit, the few active sites that still have the right shape explain why a minute can still make a little maltose. A minute's count scatters around the model rate, as real counts do. Time moves only when you press Run 1 minute. Model limits: the numbers are typical textbook values, not one enzyme's measured data; the pH curve is a simple two-group model; a pH of 3 or below, or 11 or above, is treated as denaturing at once; the model pieces are drawn flat, while real enzymes and substrates are folded 3D molecules millions of times smaller.

• The enzyme (amylase): its active site is the notch in the front edge
• Substrates: starch fits; protein and fat do not
• Products (maltose) collect in the right-hand tray

Use it in your lessons

• Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
• Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
• Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.

Free guide: 12 practicals as sims, and how to run one in a lesson →

Need a version for your exact set-up? Have one made: US$49 a figure in an order of 5.