In medicine and industry, it is often important to understand precisely how fast a chemical reaction proceeds and which factors influence the reaction rate. This allows processes in nature and technology to be better understood and controlled. Reaction kinetics investigates how fast a chemical reaction proceeds – that is, how long it takes for a substrate (starting material) to be converted into a product (end product). It also examines which conditions and influences can change the speed of the reaction. When enzymes are used in reactions, Michaelis-Menten kinetics is often used for reaction kinetics. Therefore, the focus here is on this form of enzyme kinetics.
An Ice Cream Parlor as an Example of Enzyme Kinetics
The functioning of enzymes can be well compared to an ice cream parlor: The customers who want to buy an ice cream represent the substrates – i.e., the starting materials of the chemical reaction. The salespeople are the enzymes that help convert the substrates into products, i.e., served customers with ice cream. If only a few customers come to the ice cream parlor, they are served quickly. The salespeople have idle time until new customers arrive (Fig. 1a and 2a). The more customers there are, the more ice cream can be sold – the reaction rate increases (Fig. 1b and 2a). However, each salesperson can only serve a certain number of customers. If it gets too crowded, each service takes longer, and at some point, the salespeople are completely busy (Fig. 1c and 2a). From this point on, it is no longer beneficial to add more customers (substrates) – the number of served customers remains the same, as the salespeople have reached their maximum capacity (Vmax). Only if more salespeople (enzymes) are added (Fig. 1d and 2b) can the number of served customers be increased again.
A: Reaction with a constant number of salespeople (enzymes). B: Comparison of the reaction with a low number of salespeople (black curve) and an increased number of salespeople (red curve).
Michaelis-Menten Kinetics
In enzyme research, it is difficult to determine the exact point of maximum reaction rate (Vmax) because enzymes continuously bind substrates and release products. Instead, a measurable point in the course of the reaction is considered: the moment when the reaction rate reaches exactly half of Vmax (Fig. 3a). This value is particularly informative because, at this point, the enzyme is neither fully utilized nor barely active. This allows for a reliable estimation of how strongly a substrate binds to an enzyme. The corresponding substrate concentration is called the Michaelis-Menten constant (Km). It indicates at which substrate concentration the reaction proceeds at half the maximum possible speed. Km remains independent of the amount of enzyme or substrate and only changes if the binding properties between enzyme and substrate change (Fig. 3b).
You can calculate Km, but also see it in a diagram. In the diagram, the reaction rate (y-axis) is plotted against the substrate concentration (x-axis). The resulting curve is called a saturation curve, as the enzyme reaction reaches the saturation state at Vmax. In the saturation state, all enzymes are occupied by a substrate, so the reaction rate would not increase if more substrate were added.
A: vmax shows the theoretical maximum rate of the reaction. vmax/2 shows half of the maximum reaction rate. Km shows the substrate concentration at which half of the maximum reaction rate is reached. B: The black line shows a lower enzyme concentration, the red line an increased one. The Km value remains the same.
Tip: On Kniffelix, there is an experiment on the topic of Michaelis-Menten kinetics called “Influence of Sugar Concentration on Yeast Activity” with a knowledge box to print.
Ways to Influence Enzyme Activity
In addition to the amount of enzyme and substrate, environmental conditions also play a crucial role in the speed and efficiency of enzymatic reactions. Temperature and pH value, in particular, influence how well an enzyme can work. Both factors directly affect the structure of the enzyme and the probability of a successful enzyme-substrate interaction.
The activity of enzymes depends significantly on temperature. Depending on the origin of the enzyme, there is a specific temperature range in which it remains stable and active. Within this range, a higher temperature can accelerate the movement of molecules, thus increasing the probability that the enzyme and substrate will meet. When the optimal temperature is reached, the reaction can achieve its maximum speed. However, if the temperature rises above this point, there is a risk that the enzyme will denature and completely lose its catalytic activity.
The pH value of the medium also significantly influences enzyme activity. It affects the charge of the amino acids that form the active site of the enzyme. Only at an optimal pH value does the spatial structure of the active site remain intact, allowing the enzyme-substrate interaction to occur reliably. If the pH value is outside this range, the enzyme can become inactive or change in such a way that no reaction is possible.
In addition to a higher enzyme amount and optimal temperature and pH, the reaction rate can also be increased by optimally distributing the enzymes in the reaction space – much like in a well-organized ice cream parlor where all processes mesh smoothly. This is precisely the principle we demonstrate clearly in our experiment with the bubble column.