Calculate the rate of enzyme-controlled reaction from the change in product (or substrate) over time.
Units: mg, cm³, mmol, etc.
Units: seconds, minutes, etc.
Calculate how reaction rate changes with a 10°C temperature rise, or find the rate at a new temperature.
Leave blank to calculate Q10 from two rates
Leave blank to predict rate from Q10
Calculate reaction velocity using the Michaelis-Menten equation: V = Vmax[S] / (Km + [S])
e.g. µmol/min
mM or same units as [S]
mM (same units as Km)
Enzyme activity describes how quickly an enzyme catalyses a reaction. It is affected by several key variables, each of which is a standard topic in GCSE and A-Level biology.
Increases activity up to optimum (~37°C for human enzymes), then denaturation causes rapid decline.
Each enzyme has an optimum pH. Deviation alters ionisation of amino acids, distorting the active site.
Increases rate until all active sites are occupied (Vmax). Beyond this, adding more substrate has no effect.
More enzyme = more active sites = faster rate, provided substrate is in excess.
Competitive inhibitors block the active site; non-competitive inhibitors bind allosteric sites, reducing Vmax.
Some enzymes require inorganic ions (e.g. Cl− for amylase) or organic coenzymes (e.g. NAD) for activity.
The Q10 temperature coefficient quantifies how much a reaction rate increases for every 10°C rise in temperature. For most biological reactions, Q10 ≈ 2, meaning the rate roughly doubles with each 10°C increase. This continues until the enzyme approaches its optimum temperature.
Above the optimum temperature, the kinetic energy of molecules exceeds the activation energy of the non-covalent bonds holding the enzyme in its tertiary structure. These bonds break, the active site changes shape, and the enzyme is denatured. For most human enzymes, significant denaturation begins around 40–45°C. Denaturation is irreversible under most conditions.
pH affects the ionisation state of amino acid residues in the enzyme, particularly those at the active site. Changes in pH alter the charge on these groups, disrupting the hydrogen bonds and ionic interactions that maintain the active site's precise shape. The result is a characteristic bell-shaped curve of activity vs pH, peaking at the enzyme's optimum pH.
Examples of enzyme optimum pH:
At A-Level, you are expected to understand quantitative enzyme kinetics. The Michaelis-Menten equation describes the relationship between substrate concentration and reaction velocity:
Where:
Km is an important measure of enzyme-substrate affinity. A low Km indicates that the enzyme achieves half its maximum velocity at a low substrate concentration — the enzyme has a high affinity for its substrate (they bind tightly). A high Km indicates low affinity (requires high substrate concentration to half-saturate the enzyme).
Typical Km values range from 10−³ to 10−&sup6 mol/L (mM to µM range). Hexokinase (glucose phosphorylation in glycolysis) has a Km of approximately 0.1 mM for glucose, while haemoglobin's O2 binding is described by analogous half-saturation constants.
Competitive inhibition: The inhibitor has a similar molecular shape to the substrate and competes for the active site. If substrate concentration is increased sufficiently, the inhibitor is outcompeted and Vmax is unchanged. The apparent Km increases. Examples: statins inhibit HMG-CoA reductase (cholesterol synthesis); many pesticides are competitive inhibitors of acetylcholinesterase.
Non-competitive inhibition: The inhibitor binds to an allosteric site (different from the active site), causing a conformational change that reduces the enzyme's catalytic efficiency. Increasing substrate cannot overcome this; Vmax decreases. Km is unchanged. Examples: heavy metal ions (Pb²+, Hg²+) act as non-competitive inhibitors of many enzymes.
End-product (feedback) inhibition: The final product of a metabolic pathway inhibits an enzyme earlier in the pathway. This is a vital regulatory mechanism in cells. For example, in amino acid synthesis pathways, the final amino acid inhibits the first enzyme in the pathway when sufficient quantities have been made.
Taking the reciprocal of both sides of the Michaelis-Menten equation gives the Lineweaver-Burk equation, which produces a straight line when 1/V is plotted against 1/[S]. This allows graphical determination of Vmax and Km from the y-intercept (1/Vmax) and x-intercept (-1/Km) respectively. The Lineweaver-Burk plot is also used to distinguish between types of inhibition by how the lines change in the presence of an inhibitor.
Most human body enzymes have an optimum temperature of approximately 37°C, which matches normal body temperature. Below this temperature, enzyme activity decreases because molecules have less kinetic energy and collide less frequently with the active site. Above 37°C, the enzyme begins to denature as heat energy disrupts the weak bonds maintaining the active site's shape. Some industrial enzymes (e.g. from thermophilic bacteria) have optima above 70°C.
Above the optimum temperature, enzymes denature. Excess heat energy breaks the hydrogen bonds, ionic bonds, and hydrophobic interactions that maintain the enzyme's tertiary structure. The active site changes shape and the substrate can no longer bind effectively (lock-and-key or induced-fit model fails). Enzyme activity falls rapidly to zero. Importantly, denaturation is irreversible — the enzyme cannot regain its original shape on cooling, unlike a reversible temperature decrease.
The Michaelis constant (Km) is defined as the substrate concentration at which the reaction velocity equals half the maximum velocity (Vmax/2). A low Km indicates high affinity between enzyme and substrate — the enzyme achieves half its maximum rate at low substrate concentration. A high Km indicates low affinity. Km is characteristic for a specific enzyme-substrate pair at a given temperature and pH, and it is measured in units of concentration (typically mM or µM).
Competitive inhibitors bind reversibly to the enzyme's active site, directly competing with the substrate. They increase the apparent Km (the enzyme appears to have lower affinity for the substrate), but Vmax remains the same because adding excess substrate can displace the inhibitor. Non-competitive inhibitors bind to an allosteric site away from the active site, causing a conformational change that decreases Vmax, but Km is unaffected because the inhibitor does not affect substrate binding affinity.
The Q10 temperature coefficient describes how much a reaction rate increases for every 10°C rise in temperature. For most biological enzyme-catalysed reactions, Q10 ≈ 2, meaning the rate approximately doubles with every 10°C increase (within the range where denaturation has not yet occurred). This occurs because higher temperatures increase the kinetic energy of molecules, leading to more frequent and more energetic collisions between enzyme and substrate. Q10 can be calculated as: (Rate at T2 / Rate at T1)^(10/(T2-T1)).
Each enzyme has an optimum pH determined by the specific amino acid residues in and around its active site. Pepsin is a protease secreted by chief cells in the stomach lining, where hydrochloric acid maintains pH 1–3. Pepsin's active site contains aspartate residues that function correctly only when protonated at low pH. Most enzymes in blood, cytoplasm, and the small intestine operate near neutral pH (7–8), where their amino acid R-groups carry the correct charges for substrate binding and catalysis.
The rate of reaction is calculated as the change in product formed (or substrate consumed) divided by the time taken. For example, if a reaction produces 3.6 mg of product in 60 seconds, the rate = 3.6 ÷ 60 = 0.06 mg/s. Initial rate is measured from the steepest part of the progress curve (early in the reaction, before substrate depletion or product inhibition occurs). In GCSE experiments, this is often measured by timing how long it takes to produce a fixed amount of product (e.g. making milk go clear in a lipase experiment).