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Enzyme kinetics is the study of how fast enzyme-catalysed reactions occur, and what affects that speed. When we talk about the rate of reaction, we mean how quickly a substrate is converted into a product.
The rate can be measured by:
The most useful measurement is the initial rate of reaction — the speed at the very beginning of the reaction, before the enzyme starts to slow down as substrate runs out.
Temperature has two opposing effects on enzyme activity:
At low temperatures: The rate of reaction is slow. Enzyme and substrate molecules move slowly and have little kinetic energy (energy of movement). This means they collide with each other less often, and even when they do collide, the collision may not have enough energy to form an enzyme-substrate complex. So fewer reactions happen per second.
As temperature rises: Molecules move faster. They collide more frequently and with more energy. This increases the rate of enzyme-substrate complex formation, so the rate of reaction increases.
At very high temperatures: The rate drops sharply. The bonds (especially hydrogen bonds and ionic bonds) that hold the enzyme's three-dimensional shape together start to break. This causes the tertiary structure (the precise folded shape) of the enzyme to change. The active site — the region where the substrate binds — loses its shape and the substrate can no longer fit. This permanent damage is called denaturation. A denatured enzyme cannot recover.
Every enzyme has an optimum temperature — the specific temperature at which it works fastest. For most human enzymes, this is around 37°C (body temperature). Temperatures above ~40°C start to denature human enzymes.
For every 10°C rise in temperature, the rate of reaction approximately doubles. This is expressed as a value called Q₁₀ = 2.
Example: If the rate at 5°C is x, the rate at 15°C is 2x.
A graph of temperature vs. rate of reaction produces a bell-shaped curve:
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