Botany · Ch 12 — Enzymes
Nature of Enzyme Action
Nature of Enzyme Action
Nature of Enzyme Action
Enzyme action begins when the substrate binds to a particular pocket on the enzyme called the active site, forming a temporary enzyme-substrate (ES) complex. Within this complex the reaction is carried out, after which the product is released and the enzyme is free to bind a fresh substrate molecule. In simple terms: E + S -> ES -> E + P.
Forming the ES complex is what allows the enzyme to lower the activation energy of the reaction. If the potential energy of the system is plotted against the progress of the reaction, an uncatalysed reaction has to climb over a tall energy barrier (the activation energy) before the substrate can be converted to product; when the enzyme is present, it stabilises an intermediate, lower-energy transition state, so the same conversion needs a much smaller energy input and therefore happens far more readily. …
What this figure shows. A line graph with the horizontal axis labelled 'Progress of the reaction' (running from reactants on the left to products on the right) and the vertical axis labelled 'Potential energy'. Two curves are drawn. The upper (dashed or lighter) curve represents the uncatalysed reaction: it rises from the energy level of the substrate (S) on the left, climbs to a single tall peak marked 'Transition state' with a double-headed arrow labelled 'Activation energy (without enzyme)' measuring the height from the substrate level up to this peak, then descends to the energy level of the product (P) on the right, which sits somewhat lower than the substrate's starting level (indicating the reaction releases energy overall). The lower (solid, highlighted) curve represents the same reaction in the presence of the enzyme: it follows a similar overall path from S to P, but its peak is noticeably lower and is labelled 'ES complex / lower transition state', with a shorter double-headed arrow labelled 'Activation energy (with enzyme)' showing that only a smaller energy input is needed to reach this peak. A small annotation or bracket beside the two activation-energy arrows makes clear that the enzyme's arrow is markedly s …