Revise Enzymes and metabolism for IB Biology (C1.1)

For any reaction to occur, the reacting molecules must collide with enough energy to break existing bonds and start forming new ones. That minimum energy is the activation energy, $E_a$ — an energy barrier that reactants must climb before they can turn into products. At the mild temperatures inside a cell, very few molecules have enough energy to cross a high barrier, so uncatalysed reactions are extremely slow. An enzyme is a biological catalyst: it provides an alternative reaction route with a LOWER activation energy, so a far greater proportion of collisions are successful and the rate increases dramatically. Crucially, the enzyme does not change the overall energy of the reactants or products — it does not make the reaction release or require more energy overall, and it does not shift the position of equilibrium. It only lowers the barrier between them, so equilibrium is reached faster. And because the enzyme is regenerated unchanged at the end, it is not used up and can act repeatedly.

An enzyme is a globular protein: a long chain of amino acids folded into a precise three-dimensional shape. Within that folded shape is a small pocket or groove called the active site, whose shape and chemical properties are complementary to a particular substrate — the reactant the enzyme acts on. Only a substrate that matches the active site can bind, forming an enzyme-substrate complex, and this is why enzymes are specific: one enzyme catalyses only one reaction, or one type of reaction. The way the fit works is described by the induced-fit model. Rather than being a rigid lock waiting for its key, the active site is slightly flexible. When the correct substrate begins to bind, it induces a conformational change in the enzyme so the active site moulds more closely around the substrate. This tighter grip puts strain on the substrate's bonds, which helps lower the activation energy and makes the reaction proceed. Once the product forms and leaves, the active site returns to its original shape, ready to bind again.

The rate of an enzyme-catalysed reaction depends on four measurable factors: temperature, pH, substrate concentration and enzyme concentration. Each has a characteristic graph, and in the exam you are expected to draw or interpret the shape AND give the reason for it. The unifying idea is collision theory: a reaction happens when substrate molecules collide with the active sites of enzyme molecules with enough energy. Anything that changes how often, or how successfully, those collisions happen changes the rate — until some other factor becomes limiting or the enzyme is damaged.

As temperature rises from low values, the rate increases: molecules gain kinetic energy, so enzyme and substrate move faster, collide more frequently, and more collisions have enough energy to react. The rate reaches a maximum at the optimum temperature (about 37°C for many human enzymes). Above the optimum the rate falls sharply, because the increasing thermal energy begins to break the weak bonds (such as hydrogen bonds) that hold the enzyme's tertiary structure together. The active site changes shape, the substrate can no longer bind, and the enzyme denatures. The graph is therefore a curve that rises to a peak at the optimum, then drops steeply — not a symmetrical bell, because denaturation causes a faster fall than the rise. The key contrast to make is: below the optimum, more kinetic energy → faster; above the optimum, denaturation → slower.

Each enzyme has an optimum pH at which its rate is fastest, and moving away from that pH in either direction lowers the rate. The graph is a curve that peaks at the optimum pH and falls away on both sides. The reason is that pH affects the charges on the amino-acid side chains (R groups) that form the active site and hold the enzyme's shape. Away from the optimum, these interactions are disrupted, the active site changes shape, and the substrate binds less well; at extreme pH the enzyme denatures. Different enzymes have different optima suited to where they work — for example pepsin, in the acidic stomach, has an optimum near pH 2, whereas most enzymes inside cells work best near neutral pH.

With a fixed amount of enzyme, increasing the substrate concentration first increases the rate: more substrate molecules mean more frequent collisions with active sites, so more enzyme-substrate complexes form per second. But the curve then levels off and plateaus. At high substrate concentration the active sites become saturated — essentially all of them are occupied at any instant, so adding more substrate cannot speed things up. The rate is now limited by how fast enzymes can process substrate, i.e. by the enzyme concentration, not the substrate. The characteristic shape is therefore a curve that rises steeply, then bends over to a plateau at the maximum rate.

With substrate in excess, increasing the enzyme concentration increases the rate in direct proportion: more enzyme molecules mean more active sites available, so more substrate can be processed at once and more enzyme-substrate complexes form per second. The graph is a straight line through the origin — as long as there is always plenty of substrate. If substrate ever became limiting, the line would level off, because extra active sites would have no substrate to act on.

Denaturation is a permanent change to the three-dimensional shape of an enzyme. High temperature or extreme pH breaks the relatively weak bonds — hydrogen bonds and other interactions between R groups — that hold the folded tertiary structure in place. As the folding unravels, the active site loses its precise shape, so it is no longer complementary to the substrate; the substrate cannot bind, no enzyme-substrate complex forms, and catalytic activity is lost. The single most important exam point is what denaturation does NOT change: it does not alter the sequence of amino acids (the primary structure). Only the shape is affected. Because the bonds do not spontaneously reform correctly, denaturation is generally irreversible.

Inhibitors are molecules that reduce the rate of an enzyme-catalysed reaction, and there are two main types to contrast. A competitive inhibitor has a shape similar to the substrate, so it binds to the active site itself and blocks the substrate from entering. Because inhibitor and substrate compete for the same site, the effect depends on their relative amounts: adding more substrate makes it more likely a substrate molecule reaches the active site first, so competitive inhibition can be overcome, and the maximum rate is still reached — it just takes a higher substrate concentration to get there. A non-competitive inhibitor binds instead to a different site (an allosteric site) away from the active site. This binding changes the enzyme's overall shape, which in turn changes the shape of the active site so the substrate no longer fits well. Because the inhibitor is not competing for the active site, adding more substrate does NOT overcome it, and the maximum rate is lowered. Distinguishing the two on a graph of rate against substrate concentration: the competitive curve reaches the same plateau as the uninhibited enzyme but only at higher substrate concentration, whereas the non-competitive curve plateaus at a lower maximum rate.

Metabolism is the sum of all the enzyme-catalysed chemical reactions occurring in a cell or organism. These reactions fall into two categories. Anabolism is the building up of larger, more complex molecules from smaller ones — for example joining amino acids into proteins, or fixing carbon dioxide into glucose in photosynthesis — and it requires an input of energy (it is endergonic). Catabolism is the breaking down of larger molecules into smaller ones — for example the digestion of starch to glucose, or the oxidation of glucose in cell respiration — and it releases energy (it is exergonic). The two are linked: the energy released by catabolic reactions is used to drive anabolic ones, so metabolism as a whole keeps the cell supplied with both the molecules and the energy it needs.

The reactions of metabolism are not random — they are organised into metabolic pathways, sequences in which the product of one reaction becomes the substrate for the next. Every individual step is catalysed by its own specific enzyme. Because each step needs its own enzyme, the cell can control the whole pathway with great precision: by making more or less of a particular enzyme, or by switching enzymes on and off, it regulates which reactions run and how fast. Pathways can be linear, like glycolysis, with a clear start and end, or cyclical, like the Krebs cycle, where the starting molecule is regenerated each turn. Breaking a big chemical change into many small enzyme-controlled steps also lets energy be released or captured in manageable amounts, and provides many points at which the cell can regulate the flow. Enzymes are therefore not just catalysts but the control points of metabolism.

Explanation questions in C1.1 are marked analytically: each distinct valid biological point earns one mark. There is no bonus for elegant writing and no penalty for a clumsy sentence — the engine looks for named, creditable ideas. Study how the four marks below attach to four separate points, how error-carried-forward (ECF) protects a chain of reasoning, and exactly which loose phrasing the engine refuses to credit.

Enzymes underpin almost everything else in biology. The same active-site and induced-fit ideas explain how the enzymes of cell respiration release energy from glucose, how the enzymes of photosynthesis fix carbon, how DNA polymerase and other enzymes copy and repair the genome, and how digestive enzymes break down food. The rate-controlling factors you have met here — temperature, pH, substrate concentration and inhibition — return whenever biologists design experiments or model metabolism, and inhibition in particular is the basis of how many drugs and poisons work. Master the reasoning template — bind at the active site, lower the activation energy, and watch what changes the collisions or the shape — and you can explain enzyme behaviour in any context the exam sets.