Enzymes

The One Thing

Enzymes are specific protein catalysts whose active sites bind particular substrates and lower activation energy without being consumed. Their activity depends on conditions that affect active-site shape and the frequency of successful enzyme–substrate collisions.

Reactions, Processes and Experiments

Key Terms

• Enzyme: A biological catalyst that increases the rate of a chemical reaction without being changed permanently.
• Catalyst: A substance that speeds up a chemical reaction by lowering the activation energy, without being used up.
• Substrate: The molecule on which an enzyme acts.
• Active site: The region of an enzyme where the substrate binds and the reaction takes place.
• Enzyme-substrate complex: The temporary combination formed when a substrate fits into an enzyme’s active site.
• Product: The substance or substances formed by an enzyme-controlled reaction.
• Specificity: The ability of an enzyme to bind only to a particular substrate or a small group of closely related substrates.
• Lock-and-key model: A model suggesting that the substrate fits the active site exactly because their shapes are complementary.
• Induced-fit model: A model suggesting that the active site changes shape slightly when the substrate binds, improving the fit and helping the reaction occur.
• Activation energy: The minimum amount of energy needed for a chemical reaction to begin.
• Optimum temperature: The temperature at which an enzyme works at its fastest rate.
• Optimum pH: The pH at which an enzyme has its highest activity.
• Denaturation: A permanent change in an enzyme’s three-dimensional shape, usually caused by high temperature or extreme pH, making the active site no longer complementary to the substrate.
• Limiting factor: A factor that prevents the reaction rate from increasing further when it is in short supply.
• Inhibitor: A substance that reduces enzyme activity by interfering with substrate binding or enzyme function.

Easily Confused

• Lock-and-key model vs induced-fit model: The lock-and-key model proposes an exactly complementary, fixed active site; the induced-fit model proposes that the active site changes shape slightly when the substrate binds.
• Optimum temperature vs denaturation: The optimum temperature gives the fastest activity while the enzyme retains its shape; temperatures above it may permanently alter the enzyme’s active site.
• Low temperature vs extreme pH: Low temperature slows activity by reducing kinetic energy and successful collisions, whereas extreme pH can alter the enzyme’s shape and cause loss of function.
• Substrate concentration vs enzyme concentration: Increasing substrate concentration raises the rate only until active sites are saturated, whereas increasing enzyme concentration can raise the rate when substrate is available in excess.
• Iodine solution vs Benedict’s solution: Iodine tests for starch and changes from yellow-brown to blue-black; Benedict’s solution tests for reducing sugars such as maltose and changes colour on heating.
• Amount-based rate vs reciprocal-time rate: Rate can be calculated as amount of product formed or substrate used divided by time, or, for a fixed amount of reaction, as rate = 1 ÷ time.

What Gets Asked

• Explain enzyme specificity using the complementary shapes of the active site and substrate, referring to the lock-and-key or induced-fit model. A common error is to omit the active site or imply that enzymes act on every substrate.
• Describe or interpret the effect of temperature. Marks are lost by stating only that activity decreases above the optimum without explaining that bonds maintaining the enzyme’s shape may break, causing denaturation.
• Describe or interpret the effect of pH. The essential point is that pH that is too high or too low can alter charges and bonds and change the active-site shape.
• Explain the effect of increasing substrate concentration. The rate initially rises, but levels off when all active sites are occupied; adding more substrate then has little or no further effect.
• Calculate reaction rate using rate = amount of product formed ÷ time, rate = amount of substrate used ÷ time, or rate = 1 ÷ time for a fixed amount of reaction. The specific quantities and units must match the equation used.
• Design or evaluate an enzyme investigation by identifying controlled variables: enzyme concentration, substrate concentration, pH, temperature, reaction time, and total volume. The relevant test may also involve iodine for starch or Benedict’s solution, on heating, for reducing sugars such as maltose.