Effect of temperature on reaction rate
The dependence of the enzymatic reaction rate on temperature is described by a characteristic bell-shaped curve. If we plot a graph with temperature in degrees Celsius on the abscissa (X-axis) and the reaction rate in µmol/min on the ordinate (Y-axis), several key phases can be distinguished.
- Growth phase (0–37 °C): As the temperature increases, catalytic activity gradually rises, which is similar to the behavior of ordinary chemical reactions.
- Temperature optimum: This is the point of maximum catalytic activity. For the vast majority of enzymes in the human body, this peak lies in the range of 37–38 °C.
- Decline phase (above 40 °C): When the temperature optimum is exceeded, the reaction rate drops sharply. The main cause of this phenomenon is thermal denaturation. The protein molecule of the enzyme disrupts its native structure, which inevitably leads to the complete loss of catalytic properties.
Dependence of activity on hydrogen ion exponent (pH)
The activity of any enzyme is strictly tied to the hydrogen ion exponent (pH) of the solution. The graph of this dependence also has a bell shape: for each catalyst, there is a narrow pH optimum at which it functions most efficiently.
Mechanism of pH influence: Fluctuations in the acidity of the medium alter the degree of ionization of the functional groups of amino acid residues in the protein molecule and in the substrate itself. The enzyme's ability to form an enzyme-substrate complex directly depends on proper ionization. If the pH is far from optimum, substrate binding to the enzyme's active site is disrupted.
Examples of temperature optima:
| Enzyme | Optimal environment | pH value |
|---|---|---|
| Pepsin | Strongly acidic | ~ 1.5–2.0 |
| Trypsin | Slightly alkaline / neutral | ~ 7.0–8.0 |
| Alkaline phosphatase | Alkaline | ~ 9.0–10.0 |
Effect of substrate concentration on reaction rate
Reaction kinetics is closely related to the concentration of reacting substances. If we fix the amount of enzyme as a constant value ($[E] = const$) and gradually add substrate ($[S]$), the graph of the initial reaction rate ($V$) versus the amount of substrate takes the form of a hyperbola.
The process dynamics include three stages:
- Rate increase: As substrate is added, the initial reaction rate rapidly increases.
- Saturation: A point is reached when substrate molecules become so numerous that they occupy all available active sites. The maximum possible formation of enzyme-substrate complexes for a given amount of catalyst occurs.
- Plateau: The rate of product formation reaches its peak and increases no further, no matter how much substrate we add.
The state of complete enzyme saturation is characterized by the parameter of maximum reaction rate ($V_{max}$). This constant value (for a given enzyme concentration) clearly reflects the catalytic activity of the protein under conditions of substrate excess.
Michaelis constant and substrate affinity
In addition to the maximum rate, the most important kinetic parameter is the Michaelis constant ($K_m$). It is numerically equal to the substrate concentration at which the reaction rate is exactly half of the maximum ($1/2 V_{max}$).
Analysis of hyperbolic curves (Michaelis-Menten kinetics) makes it possible to compare the properties of different enzymes. Suppose two enzymes have the same maximum rate, but different saturation curves:
- High affinity (steep rise): The curve rises sharply and is shifted to the left. The $K_m$ value is small. This means that the enzyme has a high affinity for the substrate—it requires very little reactant to reach $1/2 V_{max}$.
- Low affinity (gradual rise): The curve rises smoothly and is shifted to the right. The $K_m$ value is large. Such an enzyme (with low affinity) requires a much higher substrate concentration to achieve the same velocity.
Briefly: answers to topical questions
What is the mathematical Michaelis-Menten equation?
The Michaelis-Menten equation has the following form:
v = Vmax · [S] / (KM + [S])
In some sources, it is also written as:
$v = \frac{V_{max} \cdot [A]}{K_m + [A]}$
where:
- v — reaction velocity at a given substrate concentration;
- Vmax — maximum reaction velocity;
- [S] (or [A]) — substrate concentration;
- KM (or Km) — Michaelis constant.
How are Km and Vmax determined graphically using the Lineweaver-Burk method?
Using the Lineweaver-Burk method (double-reciprocal plot), a plot of 1/v versus 1/[S] is constructed, which is a straight line.
The values of the kinetic constants are determined from the intercepts of this line with the coordinate axes:
- The intercept on the ordinate axis corresponds to 1/Vmax (at 1/[S] = 0);
- The intercept on the abscissa axis corresponds to -1/Km (at 1/v = 0).
How do competitive and noncompetitive inhibitors differ in their effect on the Michaelis constant (Km) and Vmax?
The effect of competitive and noncompetitive inhibitors on kinetic parameters differs as follows:
| Parameter | Competitive inhibitor | Noncompetitive inhibitor |
|---|---|---|
| Vmax | Remains unchanged | Decreases (apparent Vmax decreases) |
| Km | Changes (apparent Km increases) | Unchanged |
A competitive inhibitor affects the affinity of the enzyme for the substrate, but not the catalytic efficiency. A noncompetitive inhibitor, conversely, decreases catalytic efficiency without affecting affinity.
What is the dependence of reaction velocity on enzyme concentration in the presence of excess substrate?
At a constant high concentration (excess) of substrate, the rate of substrate conversion is directly proportional to the enzyme concentration ([E]).
The graph of reaction velocity (v) versus enzyme concentration ([E]) is linear over a wide range of enzyme concentrations. At very high [E] values, a deviation from linearity may be observed.
The smaller the Michaelis constant (Km), the higher the affinity. Imagine that the enzyme needs "less fuel" (substrate) to quickly reach half of its maximum velocity.
Class and exam questions
Why does the reaction rate drop sharply when the temperature rises above 40 °C?
When optimal values are exceeded, thermal denaturation of the protein begins. The native structure of the enzyme is disrupted, leading to the irreversible loss of its catalytic properties.
What does the maximum reaction velocity (Vmax) indicate?
It reflects the catalytic activity of the enzyme. Vmax represents the limiting rate of product formation under conditions where all active sites of the enzyme molecules are fully saturated with substrate.
How to determine the enzyme with the highest affinity for the substrate using the Michaelis-Menten plot?
An enzyme with a high affinity has a velocity versus substrate concentration curve that rises more steeply and is shifted to the left, and its Michaelis constant (Km) value will be the lowest.
What else to study on this topic
- Mechanisms of thermal denaturation of the protein molecule
- The process of ionization of amino acid residues in the active site
- Stages of enzyme-substrate complex formation
- Mathematical derivation of the Michaelis-Menten equation
- The effect of allosteric enzyme sites on reaction kinetics
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 "Enzyme Kinetics" and 3,616 more pages of 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.