C1.1 Enzymes and metabolismIB Biology SL: Revision notes

Enzymes are biological catalysts: they increase the rate of reactions without being used up. Faster reactions let cells carry out the processes of life quickly enough at body temperature.

Metabolism is the complex network of interdependent and interacting chemical reactions in an organism. Because enzymes are specific, a cell needs many different enzymes, and it can control its metabolism by controlling them.

• Anabolism builds macromolecules from monomers by condensation: protein synthesis, glycogen formation, photosynthesis.
• Catabolism breaks molecules down: hydrolysis of macromolecules into monomers in digestion, and oxidation of substrates in respiration.

Enzymes are globular proteins with an active site. The active site is made of only a few amino acids, but interactions between amino acids across the whole 3D structure give it the shape and chemical properties needed for catalysis.

Induced fit: when the substrate binds, both the substrate and the enzyme change shape, straining bonds in the substrate.

Specificity: only substrates with a complementary shape and chemistry bind. Denaturation is a change in the 3D structure (caused by high temperature or extreme pH) that alters the active site so the substrate no longer binds; it is usually permanent.

For catalysis, a substrate molecule and an active site must collide in the correct orientation. Movement is needed for this; in solution, both usually move randomly.

Sometimes one partner is immobilised: large substrate molecules may be fixed, while some enzymes are embedded in membranes. The other partner must then move to it.

• Temperature: rate rises as molecules gain kinetic energy and collide more often; above the optimum bonds in the enzyme break and it denatures, so rate falls steeply.
• pH: each enzyme has an optimum pH; away from it, charges on R groups change, reducing binding, and extreme pH denatures the enzyme.
• Substrate concentration: rate rises as collisions become more frequent, then levels off when all active sites are occupied (saturation).

NOS: sketch graphs are models of these relationships. Describe the trend in words, and use experimental results to evaluate whether a model fits a real enzyme.

Rate = amount of product formed (or substrate used) ÷ time. You can measure it directly (e.g. volume of oxygen per second from catalase) or, when an endpoint is timed, calculate rate = 1/time. Control other variables (temperature, pH, enzyme and substrate concentration), repeat measurements and calculate means.

Energy is needed to break bonds in the substrate, and energy is released when bonds form in the products. The activation energy is the energy needed to start the reaction. Enzymes lower the activation energy, so reactions occur quickly at body temperature. They do not change the overall energy released by the reaction.