How temperature changes reaction rate
The dependence of the enzymatic reaction rate on ambient temperature is graphically represented as a classic bell-shaped curve. The entire process of activity changes can be divided into three key stages, each with its own strict biochemical patterns.
First, the ascending limb of the graph is formed. With a gradual increase in temperature from 0°C to 37°C, the rate of reaction product formation steadily rises. This process obeys the Van 't Hoff rule: for every 10°C temperature increase, the rate of a biochemical reaction increases by a factor of two to four. The fundamental reason for this acceleration is the rapid growth of the kinetic energy of reacting molecules.
Next, the graph reaches its peak. This is the optimal temperature zone, which for the overwhelming majority of human enzyme systems is a narrow range from 37°C to 40°C. It is under these conditions that protein catalysts work with maximum efficiency.
This is followed by the descending limb. If heating of the medium continues above 40–45°C, the reaction rate begins to drop rapidly until it stops completely. The reason lies in the protein nature of enzymes. Exposure to high temperatures causes irreversible thermal denaturation: the complex tertiary structure of the molecule is disrupted, and the spatial architecture of the active site is completely deformed, losing its ability to bind to the substrate.
Medical significance of hypothermia and hyperthermia
A deep understanding of temperature optima is critically important for modern medicine, especially in the fields of cardiac surgery, neurosurgery, and intensive care.
Artificial hypothermia is of enormous clinical significance. When performing complex and prolonged surgical interventions on brain tissues or on a "dry" heart, specialists need to temporarily stop blood flow. If such an operation is performed at normal body temperature (around 37°C), the resulting ischemia will very quickly lead to irreversible cell damage. This is because under optimal conditions, enzymes exhibit maximum activity, which means that cells continuously require a huge amount of ATP and oxygen.
General or local cooling of the patient to 25–30°C (and even lower) effectively protects tissues. Hypothermia sharply reduces enzymatic activity and globally inhibits all cellular metabolism. As a result, oxygen and glucose consumption drops severalfold, and surgeons are granted precious extra time during which cells can survive hypoxia without fatal consequences.
On the other hand, hyperthermia has also found application. Local exposure to extremely high temperatures causes immediate protein denaturation. This physical principle underlies electrosurgery — a method of surgical hemostasis. The destruction of enzyme proteins leads to local cell death and reliable "sealing" of damaged blood vessels.
Laboratory assessment: the lysozyme example
To accurately determine the properties of catalysts in a laboratory setting, it is essential to strictly maintain the temperature regime. If the incubation of the reaction mixture occurs at ordinary room temperature rather than in a specialized thermostat, the researcher will naturally obtain falsely low specific activity values. Due to a lack of kinetic energy, the molecules will interact too slowly.
Let us consider a standard practical experiment with the enzyme lysozyme, for which the optimal temperature is strictly 37°C. When the experiment is set up correctly under optimal conditions, 1 mg of purified enzyme is taken and incubated for 15 minutes. As a result of this reaction, 45 µmol of the final product is synthesized. It is precisely this reference data that makes it possible to calculate the true, rather than underestimated, specific activity of the substance.
Briefly: answers to topical questions
How exactly is the specific activity of an enzyme calculated in international units per milligram of protein?
The specific activity of an enzyme is calculated as the number of units of enzymatic activity per 1 mg of protein. Calculation formula: the amount of converted substrate (in µmol) is divided by the product of time (in minutes) and the amount of protein (in mg). The unit of activity is expressed in µmol/(min·mg).
What chemical factors, besides extreme temperatures, can cause irreversible denaturation of enzymes?
Chemical factors causing enzyme denaturation include concentrated alkalis, acids, oxidizing agents, and heavy metal ions. These substances interact with the functional groups of amino acids: COOH, NH, OH, and SH groups. Heavy metals — mercury, arsenic, antimony, thallium — act as thiol poisons by binding to SH groups.
The Van 't Hoff rule only works "uphill" (up to 37°C): heat by 10 degrees — the reaction accelerates. Cross 40°C — the enzyme is "cooked" (denaturation).
Class and exam questions
Why do enzymes stop working at temperatures above 45°C?
Thermal denaturation occurs: the spatial tertiary structure of the protein is disrupted. The active site is deformed, and the enzyme permanently loses its ability to bind the substrate.
Why is the body cooled during prolonged heart surgery?
Under controlled hypothermia (25–30°C), enzymes work much slower, which reduces overall metabolism. This radically decreases the oxygen demand of brain and heart tissues, allowing them to survive longer without blood flow.
Why does room temperature distort the results of laboratory tests?
At room temperature, the kinetic energy of molecules is significantly lower than at physiological 37°C. As a result, the reaction rate drops, and the researcher calculates an artificially underestimated specific activity of the enzyme.
What else to study on this topic
- Formulas and algorithms for calculating enzyme specific activity
- Physicochemical mechanisms of thermal protein denaturation
- Application of high-temperature coagulation in surgery
- Dependence of reaction rate on substrate concentration (Michaelis–Menten kinetics)
- Influence of other factors on enzymes: medium pH and activators
Master the entire topic in the "Biochemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Effect of temperature on enzyme activity" and 3,616 more pages on the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 115 video lessons on the subject
Other topics in the "Enzymology" section
Free video lessons: biochemistryShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.