Enzymes¶

Part of Module 2: Foundations in biology.

Enzymes give a first clear example of how protein structure controls biological function. They are biological catalysts, but the key point is deeper than that label: enzyme specificity, rate changes under different conditions, and careful interpretation of enzyme data all matter for understanding how living systems regulate their chemistry.

Learning Objectives¶

Enzyme Action¶

Enzymes are globular proteins with complex and unique tertiary structures. As biological catalysts, they increase the rate of chemical reactions without being consumed in the process — meaning a single enzyme molecule can be reused repeatedly to catalyse many successive reactions.

All chemical reactions require a certain amount of energy to get started — the activation energy — typically supplied as heat. Without sufficient activation energy, reactant molecules lack the energy to break their existing bonds and form new ones to produce products. Enzymes work by lowering the activation energy for a reaction, which means reactions can proceed at lower temperatures, such as the body temperature of an organism, that would otherwise be too cool for the chemistry to occur efficiently.

Intracellular and Extracellular Enzymes¶

Enzymes can be grouped into two functional categories:

Intracellular enzymes act within the cells that produce them. Catalase is a well-known example: it catalyses the breakdown of hydrogen peroxide into oxygen and water, preventing the accumulation of this toxic by-product inside cells.

Extracellular enzymes are secreted from the cells that produce them and act in the surrounding environment. Amylase is secreted by the salivary glands, pancreas, and small intestine to break down starch into maltose. Trypsin is secreted by the pancreas into the small intestine to break down proteins into smaller polypeptides.

The Active Site and Enzyme–Substrate Complexes¶

Enzymes have unique tertiary structures that determine the shape of their active site. This shape is complementary to the substrate — not identical, but a precise match in terms of shape and chemical properties. When the substrate binds to the active site, an enzyme–substrate complex forms. Temporary bonds — including hydrogen bonds and ionic interactions — form between the R groups within the active site and the substrate. These bonds help to lower the activation energy, either by straining the substrate's bonds or by holding reactants in the correct orientation. Once the reaction is complete, the products are released from the active site, leaving the enzyme free to catalyse the next reaction.

Lock-and-Key and Induced-Fit Models¶

Two models describe how enzymes bind their substrates:

The lock-and-key model was the original proposal. In this model, the substrate fits perfectly into the enzyme's active site in the same way that a key fits into a lock — both the enzyme and substrate are treated as rigid, complementary shapes.

The induced-fit model provides a more accurate account supported by more recent evidence. In this model, the substrate does not fit perfectly into the active site initially. As the substrate enters, the active site changes shape slightly to fit more closely around it. This conformational change puts strain on the substrate's bonds, which helps lower the activation energy. The induced-fit model is preferred because it better explains how the act of binding itself can contribute to catalysis.

Factors Affecting Rate¶

Four factors most commonly affect the rate of enzyme-catalysed reactions: temperature, pH, substrate concentration, and enzyme concentration. Each affects how frequently enzyme–substrate complexes form.

Temperature¶

All enzymes have an optimum temperature at which they work fastest, though this varies between enzymes. As temperature rises, the molecules have more kinetic energy, which increases the frequency of collisions between enzyme and substrate, forming more enzyme–substrate complexes and raising the rate of reaction. The maximum rate is reached at the optimum temperature.

Beyond the optimum, the excess kinetic energy causes bonds within the enzyme's tertiary structure to break, so the active site changes shape. The substrate can no longer fit, and the enzyme is said to be denatured. Once denatured, the change is effectively permanent — the enzyme loses its catalytic function.

Below the optimum temperature, reactions slow because molecules have less kinetic energy and collisions are less frequent, but the enzyme is not denatured and will recover if temperature rises.

Calculating Q10: The temperature coefficient Q10 expresses how much the rate of reaction changes for a 10 °C increase in temperature:

Q10 = R2 / R1

where R2 is the rate at the higher temperature and R1 is the rate at the lower temperature. For example, if the rate at 20 °C is 5 products per minute and at 30 °C is 10 products per minute, then Q10 = 10 / 5 = 2. A Q10 of 2 means the rate doubles for every 10 °C rise — a common value for biological reactions in a moderate temperature range.

pH¶

All enzymes have an optimum pH, which again varies by enzyme. In acidic conditions, excess H⁺ ions break ionic and hydrogen bonds within the enzyme's tertiary structure. In alkaline conditions, OH⁻ ions similarly disrupt these bonds. Either deviation from the optimum causes the active site to change shape, reducing the formation of enzyme–substrate complexes and decreasing the rate of reaction. Sufficiently extreme pH values will denature the enzyme entirely.

Substrate Concentration¶

As substrate concentration increases, there are more substrate molecules available to collide with enzyme active sites, so the rate of reaction increases. However, this relationship has a limit: once all active sites are occupied — the saturation point — adding more substrate has no further effect because enzyme concentration has become the limiting factor. The graph of rate against substrate concentration is therefore a curve that rises steeply and then plateaus.

Enzyme Concentration¶

Increasing enzyme concentration increases the rate of reaction when substrate is in excess, because there are more active sites available to form enzyme–substrate complexes. However, once there are more enzyme molecules than substrate molecules to fill them, substrate concentration becomes the limiting factor and the rate plateaus again.

Cofactors, Coenzymes, and Prosthetic Groups¶

Some enzymes require additional non-protein substances called cofactors to function. Cofactors bind to enzymes and increase their activity. They fall into distinct categories:

Inorganic cofactors are typically metal ions. Chloride ions (Cl⁻) are a cofactor for amylase; without them, the enzyme's activity is reduced.

Coenzymes are organic cofactors, often derived from vitamins, that associate temporarily with the enzyme during catalysis. They frequently act as carriers — picking up or donating atoms or groups between enzyme reactions. Because they are not permanently bound, they need to be continually recycled or replenished.

Prosthetic groups are cofactors that are permanently and tightly bound to the enzyme protein. Zinc ions (Zn²⁺) act as a prosthetic group in carbonic anhydrase, the enzyme that catalyses conversion of carbon dioxide and water to carbonic acid in red blood cells.

Enzyme Inhibitors¶

Inhibitors are molecules that bind to enzymes and reduce their activity. Inhibition can be reversible or irreversible depending on the type of bond formed: reversible inhibitors form weak bonds such as hydrogen or ionic bonds, while irreversible inhibitors form strong covalent bonds and permanently disable the enzyme.

Competitive Inhibitors¶

Competitive inhibitors have a molecular shape similar to the substrate. They bind at the active site of the enzyme, directly preventing the substrate from occupying that site. As a result, fewer enzyme–substrate complexes form and the rate of reaction decreases.

Because competitive inhibitors and substrate molecules both compete for the same binding site, increasing substrate concentration can overcome the inhibitor's effect: with a high enough substrate-to-inhibitor ratio, the probability that a substrate molecule occupies the active site increases and the rate recovers toward its uninhibited maximum (Vmax). Most competitive inhibitors are reversible, binding only temporarily.

Non-Competitive Inhibitors¶

Non-competitive inhibitors bind at a location on the enzyme that is distinct from the active site — the allosteric site. This binding changes the enzyme's tertiary structure so that the active site changes shape. The active site is no longer complementary to the substrate, so the substrate cannot bind and no enzyme–substrate complex forms.

Crucially, increasing substrate concentration has no effect on non-competitive inhibition. Because the inhibitor does not occupy the active site, adding more substrate cannot displace it. Vmax is therefore reduced, regardless of how much substrate is present.

Reversible vs Irreversible Inhibition¶

The distinction between reversible and irreversible inhibition matters clinically and biochemically. Many pharmaceutical drugs and poisons act as irreversible inhibitors. For example, organophosphate nerve agents irreversibly inhibit acetylcholinesterase by forming a permanent covalent bond, preventing breakdown of the neurotransmitter acetylcholine.

End-Product Inhibition¶

End-product inhibition is a specific, physiologically important form of non-competitive inhibition. In a metabolic pathway, the final product may act as an inhibitor of an early enzyme in that pathway. When sufficient product accumulates, it switches off its own synthesis — a classic negative feedback loop. When product levels fall, inhibition is released and the pathway resumes. This allows cells to regulate metabolic output efficiently, without wasteful overproduction of any particular molecule.

Interpreting Enzyme Data¶

Many practical investigations involve measuring enzyme activity indirectly: a colour change may be a proxy for substrate disappearance (as in iodine assays for starch) or product formation. When analysing results, it is important to identify what the measured variable actually represents.

Common graph shapes to recognise and explain:

• Rate vs temperature: rises to an optimum, then falls steeply as denaturation occurs.
• Rate vs pH: a bell-shaped curve peaked at the optimum, with steep drops either side.
• Rate vs substrate concentration: rises then plateaus at Vmax; if a non-competitive inhibitor is present, the plateau is lower. With a competitive inhibitor, the curve takes longer to reach Vmax but the same maximum is ultimately achievable.
• Rate vs enzyme concentration: rises linearly (when substrate is in excess), then plateaus when substrate becomes limiting.

Common Confusions¶

A plateau at high substrate concentration does not mean the enzyme has stopped working — it means enzyme concentration has become the limiting factor, with all active sites saturated.

Denaturation is not the same as inhibition. Denaturation permanently alters the enzyme's tertiary structure, destroying the active site. Inhibition (if reversible) temporarily reduces activity without fundamentally changing protein structure, and the enzyme can recover.

With a competitive inhibitor present, the maximum possible rate (Vmax) can still be reached if substrate concentration becomes high enough. This is not the case for non-competitive inhibitors acting on available enzyme molecules, where Vmax is irreversibly reduced relative to the uninhibited enzyme.

A rate increase at a higher temperature does not always mean denaturation has not begun; at temperatures just above the optimum, rising kinetic energy and beginning denaturation may overlap, producing a complex pattern.

Key Terms¶

• Enzyme: a globular protein that acts as a biological catalyst, speeding up chemical reactions by lowering their activation energy.
• Active site: the region of an enzyme, determined by its tertiary structure, where the substrate binds to form an enzyme–substrate complex.
• Substrate: the molecule upon which an enzyme acts.
• Activation energy: the minimum energy required for a chemical reaction to proceed.
• Enzyme–substrate complex: the temporary structure formed when a substrate binds to the active site of an enzyme.
• Lock-and-key model: a model of enzyme action in which the substrate fits rigidly into a pre-formed, complementary active site.
• Induced-fit model: a model of enzyme action in which the active site changes shape slightly as the substrate binds, placing strain on the substrate's bonds and contributing to catalysis.
• Intracellular enzyme: an enzyme that acts within the cell that produced it (e.g. catalase).
• Extracellular enzyme: an enzyme that is secreted and acts outside its producing cell (e.g. amylase, trypsin).
• Denaturation: an irreversible change to a protein's tertiary structure, altering the active site so the substrate can no longer bind.
• Optimum: the temperature or pH at which an enzyme's activity is greatest.
• Q10: the factor by which reaction rate changes for a 10 °C increase in temperature; calculated as R2/R1.
• Limiting factor: the factor in shortest supply that restricts the rate of a process.
• Saturation point: the substrate concentration at which all enzyme active sites are occupied and rate plateaus.
• Cofactor: a non-protein molecule or ion required for an enzyme to be active.
• Coenzyme: an organic cofactor, often vitamin-derived, that temporarily associates with an enzyme during catalysis.
• Prosthetic group: a cofactor permanently bound to an enzyme (e.g. Zn²⁺ in carbonic anhydrase).
• Competitive inhibitor: a molecule that binds at the active site and competes directly with the substrate; its effect can be overcome by increasing substrate concentration.
• Non-competitive inhibitor: a molecule that binds at the allosteric site, changing the active site shape; its effect cannot be overcome by increasing substrate concentration.
• Allosteric site: a region of an enzyme away from the active site where non-competitive inhibitors (and some regulatory molecules) bind.
• End-product inhibition: regulation in which the final product of a metabolic pathway inhibits an early enzyme in that pathway, acting as a negative feedback mechanism.
• Reversible inhibition: inhibition in which the inhibitor forms weak bonds and can dissociate from the enzyme.
• Irreversible inhibition: inhibition in which the inhibitor forms permanent covalent bonds, permanently disabling the enzyme.

Connected Pages¶

• 1.1.4 Evaluation
• 2.1.2 Biological molecules
• 2.1.3 Nucleotides and nucleic acids
• 2.1.5 Biological membranes