IB Biology HL C1.1 Enzymes and metabolism Notes

Enzymes are mostly protein biological catalysts, with some RNA examples. They speed up reactions in small amounts and remain unchanged after catalysis, so cells can run reactions quickly without using up the enzyme.

Metabolism is all enzyme-controlled chemical reactions in cells. Specific enzymes control which metabolic reactions occur and when, so metabolism is not random chemistry; it is coordinated pathway control.

Metabolism includes anabolism and catabolism. Anabolism builds complex molecules from smaller ones, often by condensation, and requires energy. Catabolism breaks molecules down by hydrolysis or oxidation and releases energy.

Enzymes are usually globular proteins with a small active-site pocket. The active site binds the substrate, forms an enzyme-substrate complex, and catalyses the reaction. Active-site shape and chemistry are why enzymes are specific.

The folded protein creates the pocket that does the chemistry.

In induced fit, substrate binding causes shape changes in both enzyme and substrate. This aligns catalytic groups and raises the substrate toward the transition state, making reaction more likely.

Binding helps create the best catalytic fit rather than matching a rigid template from the start.

Molecular motion brings substrates and active sites into collision. Higher kinetic energy can increase successful active-site collisions, but only up to the point where enzyme structure remains intact.

Active-site amino acid arrangement determines substrate specificity. Denaturation changes tertiary structure and active-site shape, reducing activity; it does not usually break peptide bonds in the primary structure.

Temperature, pH, and substrate concentration change enzyme activity in different ways. Temperature increases rate to an optimum, then denaturation lowers activity. pH changes bonding in the active site. Substrate concentration increases rate until active sites are saturated and the curve plateaus.

The graph shape tells you the mechanism.

Enzyme reaction rate can be measured by substrate loss or product formation per unit time. Good assays use initial rate, replicates, controls, and graphs. Examples include amylase assays tracking starch loss or catalase assays tracking oxygen production.

Enzymes lower activation energy by providing an alternative pathway and stabilizing the transition state. They do not remove the energy barrier completely, and they do not change the overall energy difference between reactants and products.

The enzyme changes the route, not the starting and ending energy levels.

HL distinguishes where enzymes act. Intracellular enzymes act inside cells in cytoplasm, organelles, or membranes. Extracellular enzymes are secreted and act outside cells, such as digestive enzymes in the gut lumen.

Exergonic metabolic reactions inevitably release some energy as heat. Endotherms use metabolic heat to maintain body temperature, so metabolism is linked not only to making ATP but also to thermal regulation.

Metabolic pathways can be linear or cyclical. Linear pathways run from substrate to final product, such as glycolysis. Cyclical pathways regenerate an acceptor or starting molecule, such as the Krebs cycle and Calvin cycle.

Pathway shape changes how you reason about intermediates and repeated turns.

Non-competitive inhibitors bind away from the active site at allosteric sites. Binding changes enzyme shape and reduces activity despite substrate presence, so increasing substrate concentration does not remove the inhibition in the same way as competitive inhibition.

The inhibitor works through shape change, not active-site competition.

Competitive inhibitors resemble the substrate and bind reversibly to the active site. They prevent substrate binding, but increasing substrate concentration can reduce their effect because substrate outcompetes the inhibitor. Examples include malonate competing with succinate for succinic dehydrogenase.

Competitive inhibition is a contest for the active site.

Feedback inhibition controls metabolic pathways. When an end product accumulates, it inhibits an earlier pathway enzyme, often the first committed enzyme, by binding to an allosteric site. This prevents overproduction; when end product concentration falls, inhibition can be relieved. Isoleucine synthesis is a common example.

The product controls its own pathway by feeding back to an earlier step.

Mechanism-based inhibitors become irreversible after an active-site reaction starts. The enzyme begins to act on the inhibitor as if it were a substrate, but the reaction traps or permanently inactivates the enzyme. Penicillin inhibits transpeptidase and prevents bacterial cell wall cross-linking.

Enzymes are biological catalysts that lower activation energy and remain unchanged. Their globular protein shape creates active-site specificity; induced fit aligns substrates; molecular motion and collisions affect rate; temperature, pH, and substrate concentration change activity; assays measure substrate loss or product formation over time.

Use this for core questions on enzyme definition, metabolism, active sites, induced fit, rate curves, enzyme assays, and activation energy.

Enzymes can act inside or outside cells, metabolic reactions release heat, pathways may be linear or cyclical, and pathway output is regulated by inhibition. Competitive inhibitors bind active sites and can be overcome by more substrate; non-competitive inhibitors bind allosteric sites; feedback inhibition uses end products to inhibit earlier enzymes; mechanism-based inhibitors trap enzymes after reaction begins.

Use this for HL questions on enzyme location, metabolic heat, linear/cyclical pathways, and inhibition types.