The enzyme alcohol dehydrogenase prevents you from getting what is called the 'Asian flush', by PDB via Wikimedia Commons, licensed under CC0
Enzymes are special proteins that are critical in many chemical reactions that occur in living organisms. Knowing how enzymes work is necessary for understanding how metabolic processes are controlled and maintained within cells. This has broader applications in agriculture, medicine and even in textiles. Use this resource to learn about the structure and function of enzymes.
Biological catalysts
Enzymes are proteins that act as catalysts; they speed up chemical reactions by providing another pathway for the reaction to occur that requires less energy. This makes processes occur more efficiently. [transcript]
Line graph showing energy changes during a reaction
A line graph showing how energy changes as a reaction occurs. There are two lines on the graph: a solid line labelled "Without enzyme" and a dashed line labelled "With enzyme".
The x-axis is labelled "Reaction progress" and the y-axis is labelled "Energy". Neither have values, as the graph shows a rough profile of how energy changes.
The without enzyme line begins at about two-fifths of the way up the y-axis and continues at this energy level as the reaction progresses, until it increases and reaches a peak close to the maximum value of the y-axis. The energy then decreases gradually until it plateaus at a lower energy than when the reaction started.
The with enzyme line looks very similar to the without enzyme one. The only difference is that the peak occurs at the lower energy value.
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Enzymes play critical roles in a huge range of cellular functions, like digestion, energy production, protein synthesis and DNA replication. Examples of enzymes include RNA polymerase which catalyses the transcription of DNA into RNA, amylase which breaks down complex sugars from your diet into simple sugars that your body can use, and lactase which catalyses the digestion of lactose found in dairy foods.
Usually, enzymes work by bringing together two or more molecules, called substrates and joining them, or breaking one substrate into multiple products. Their ability to do this depends on their structure. The site where the enzyme binds the substrates, called the active site needs to the correct shape to bind the substrates.
Once the substrates bind to the enzyme, they form an enzyme–substrate complex. The reaction occurs to form an enzyme–products complex, then the products are released and the enzyme is free to catalyse the next reaction. Enzymes are not consumed in catalysis.
An enzyme is shown with a single substrate (green) matching the shape of its active site nearby.
The substrate binds to the active site of the enzyme to form an enzyme/substrate complex.
A reaction occurs to form an enzyme/product complex. This is clear because the original green substrate is now two products coloured in shades of pink.
The two products are released from the active site of the enzyme.
Models of enzyme function
The function of enzymes is described by two models: the lock-and-key model and the induced fit model.
Lock-and-key model
The lock-and-key model of enzyme action was proposed by German chemist Emil Fisher in 1984. He theorised that the shape of an enzyme’s active site was rigid and specifically matched the shape of its substrates, just like a lock has a unique key. [transcript]
Lock-and-key model of enzyme action
An enzyme is shown with two substrates (yellow and blue) matching the shape of its active site nearby.
The substrates bind to the active site of the enzyme to form an enzyme/substrate complex.
A reaction occurs to form an enzyme/product complex. This is clear because the two original substrates are now a single product coloured in green.
The product is released from the active site of the enzyme.
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Induced fit model
The induced fit model of enzyme action was proposed by American biochemist Daniel E. Koshland in 1958. It is an extension of Fisher’s lock-and-key model, where the enzyme changes shape slightly (or undergoes a conformation change) to bind substrates. This model helped to explain how enzymes can bind more than one type of substrate. [transcript]
Induced fit model of enzyme action
An enzyme is shown with two substrates (yellow and blue) nearby.
The substrates bind to the active site of the enzyme to form an enzyme/substrate complex. A conformational change in the enzyme active site is required to perfectly fit the substrates.
A reaction occurs to form an enzyme/product complex. This is clear because the two original substrates are now a single product coloured in green.
The product is released from the active site of the enzyme. The active site returns to its original shape.
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Factors affecting enzyme action
Several factors can affect enzyme action or activity, which then affects the efficiency of chemical reactions. These include:
temperature
pH
substrate concentration
enzyme concentration
the presence or absence of inhibitors and activators.
Enzymes have an optimal temperature range in which they operate best. Below this temperature range, enzymes activity is reduced. At very low temperatures, enzymes can even become inactive. Above this temperature range, enzymes can become denatured. Denaturation involves a permanent change to the enzyme’s structure, causing it to lose its ability to catalyse reactions. Even if the temperature is reduced following denaturation, enzyme activity cannot be recovered, as the shape of the enzyme has changed irreversibly. [transcript]
Line graph showing effect of temperature on enzyme action
A line graph showing how enzyme action is affected by temperature.
The x-axis is labelled "Temperature" and the y-axis is labelled "Enzyme activity". Neither have values, as the graph shows a rough profile of how enzyme activity is affected.
The line starts at 0, gradually increases to a maximum, then more quickly decreases back down to 0. At the far left end is the label "Enzyme inactivated (reversible)" and at the far right end is the label "Enzyme denatured (permanent)".
The peak of the curve is labelled "Optimum temperature", with a dashed line extending from the line down to the temperature axis.
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Just like temperature, the pH level of the environment in which the enzyme is present affects its activity. This is because enzymes commonly contain acidic and basic groups, which change the electric charge on an enzyme depending on pH.
When the pH of the surroundings is low, there are more \(\ce{H}^{+}\) ions available. The basic groups on the enzyme are more likely to accept protons and the acidic groups on the enzyme are less likely to donate protons, making the molecule less negative.
When the pH of the surroundings is high, there are less \(\ce{H}^{+}\) ions available. The basic groups on the enzyme are less likely to accept protons and the acidic groups on the enzyme are more likely to donate protons, making the molecule more negative.
In some cases, enzymes may rely on their charges to bind with substrates, thus the absence of these charges may affect their ability to perform their function. [transcript]
Line graph showing effect of pH on enzyme action
A line graph showing how enzyme action is affected by pH.
The x-axis is labelled "pH" and the y-axis is labelled "Enzyme activity". Neither have values, as the graph shows a rough profile of how enzyme activity is affected.
There are two curves. The red curve extends over the lower pH range and is labelled with "High H+". It increases from no enzyme activity to a maximum, before decreasing back down to 0. The peak of the curve is labelled "Optimum pH for enzyme 1", with a dashed line extending from the line down to the pH axis.
The blue curve extends over the higher pH range, and is labelled "Low H+". It has a similar bell shape as the red curve, but is a bit wider. The peak of the curve is labelled "Optimum pH for enzyme 2", with a dashed line extending from the line down to the pH axis.
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As the concentration of enzyme or substrate increases, the enzyme activity increases… but only until a certain point. If there is a finite amount of substrate and they are all bound to enzymes already, increasing the concentration of enzymes will not further increase enzyme activity. In the same way, if there is a finite amount of enzymes, increasing the concentration of substrates will increase enzyme activity. Activity will reach a plateau. [transcript]
Line graph showing effect of enzyme or substrate concentration on enzyme action
A line graph showing how enzyme action is affected by concentrations of enzyme or substrate.
The x-axis is labelled "Enzyme or substrate concentration" and the y-axis is labelled "Enzyme activity". Neither have values, as the graph shows a rough profile of how enzyme activity is affected.
There is one curve, beginning at 0 and increasing at a steady rate before it plateaus.
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The presence of inhibitors and activators can decrease or increase enzyme activity, respectively.
There are three types of inhibitors:
Competitive inhibitors bind to the active site of the enzyme, blocking the substrate from binding. This is direct competition.
Non-competitive inhibitors bind to a different part of the enzyme and cause a conformational change that reduces its activity.
Uncompetitive inhibitors bind to the enzyme–substrate complex and prevents the release of the product.
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Enzyme inhibitors
Competitive inhibitor (left): A competitive inhibitor (green) is bound to the active site of the enzyme, stopping the blue and yellow substrates from binding.
Non-competitive inhibitor (middle): A non-competitive inhibitor (green) is bound to another site on the enzyme, causing a change in the shape of the active site. This stops the substrates from binding.
Uncompetitive inhibitor (right): An uncompetitive inhibitor (green) is bound to the enzyme/substrate complex, stopping the products from being released.
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Activators can increase enzyme activity by helping the substrates bind. They might do this by binding to specific sites on the enzyme that help stabilise its structure.
Exercise
See how well you understand the structure and function of enzymes with a quick quiz.
Data drill
Read the scenario and use the information provided to answer the questions in the quiz.
Dr Catherine de List is studying the effect of pH on the activity of three enzymes: enzyme A, B and C. She measures their activity in enzyme units per millilitre over the whole pH spectrum. This is presented in the following graph. [transcript]
Line graph showing effect of pH on enzyme action
A line graph showing pH affects the action of three enzymes: Enzyme A, Enzyme B and Enzyme C.
The x-axis is labelled "pH", starting at 0 and ending at 14, with increments of 14. The y-axis is labelled "Enzyme activity (U/mL)", starting at 0 and ending at 60, with increments of 10.
There are three curves, each representing one enzyme.
The curve for Enzyme A starts with no enzyme activity at a pH of 1. Activity gradually increases until it reaches a peak of around 55 U/mL at a pH of 2.5. After the peak, the enzyme activity gradually decreases until it returns to no enzyme activity at a pH of 4.
The curve for Enzyme B starts with no enzyme activity at around pH 3.7. Activity gradually increases until it reaches a peak of around 15 U/mL at a pH of 7. After the peak, the enzyme activity gradually decreases until it returns to no enzyme activity at a pH of 10.5.
The curve for Enzyme C starts with no enzyme activity at a pH of 7. Activity gradually increases until it reaches a peak at around 40 U/mL at a pH of 9. After the peak, the enzyme activity gradually decreases until it returns to no enzyme actvity at a pH of 11.
