Concept 4: Enzymes
Success Criteria & Vocabulary
Click this drop-down menu to see the Success Criteria.
I can draw and explain the structural features of enzymes.
I can describe how enzymes speed up reactions.
I can describe the ‘Lock & Key’ and ‘Induced Fit’ models in terms of function.
I can discuss how certain factors affect enzyme activity.
I can discuss how denaturation happens and its consequences on enzyme activity.
Click this drop-down menu to see the list of Vocabulary.
Activation energy: Minimum amount of energy needed for a chemical reaction to occur.
Active site: Region on an enzyme that binds to a protein or other substance during a reaction.
Anabolic reaction: Type of enzyme reaction, where enzymes join two or more substrates to form a larger product.
Catabolic reaction: Type of enzyme reaction, where enzymes break apart a large substrate to form smaller products.
Catalyst: Substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.
Co-enzyme: Organic molecule that binds to an enzyme's active site, altering its shape to allow substrate to bind more tightly.
Co-factor: Inorganic molecule that binds to an enzyme's active site, altering its shape to allow substrate to bind more tightly.
Competitive inhibitor: Type of inhibitor that binds to the active site to block the substate from binding.
Denature: Loss of an enzyme's normal shape so that it no longer functions; caused by a less than optimal pH and temperature.
Disulfide bridge: Strong bond between two sulfur (thiol) groups from different amino acids, that contributes to the 3D shape of an enzyme/protein.
Enzyme: Biological catalyst that speeds up a chemical reaction.
Hydrogen bond: Weak bond between a hydrogen atom of one amino acid and an oxygen atom of another amino acid that contributes to the 3D shape of an enzyme/protein.
Induced fit model: Model of enzyme activity that explains how a substrate binding to the active site causes a temporary change in the enzyme's shape; perfectly moulds around the substrate.
Inhibitor/poison: Molecule that either binds with the active site OR binds outside of the active site in such a way that the shape of the active site changes so that substrate can no longer bind. A limiting factor of enzyme activity.
Lock & key model: Model of enzyme activity that explains how a particular enzyme will only fit with one particular type of substrate.
Non-competitive inhibitor: Type of inhibitor that binds to the enzyme outside of the active site, causing it to change shape.
pH: Limiting factor of enzyme activity where extreme changes (either high or low) cause denaturation (NOT temperature).
Product: Molecule made by an enzyme-controlled reaction.
Substrate: Molecule used in an enzyme-controlled reaction.
Temperature: Limiting factor of enzyme activity where very high levels cause denaturation, and very low levels decrease the rate of collisions/reaction.
Watch my teaching video on Enzymes
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Task called '2.4 Concept 4'.
Enzyme structure and function.
Factors affecting enzyme activity.
Page 71 - Protein Structure and Denaturation
Page 73 - Enzymes
Page 74 - Enzymes and Activation Energy
Page 77 - Factors Affecting Enzymes - Temperature
Page 79 - Factors Affecting Enzymes - pH
Page 82 - Factors Affecting Enzymes - Substrate Concentration
Page 83 - Factors Affecting Enzymes - Enzyme Concentration
Page 84 - Factors Affecting Enzymes - Cofactors and Inhibitors
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Concept 4: Support Notes
What are Enzymes?
ENZYMES are biological catalysts, which mean they speed up chemical reactions without being used up themselves.
Enzyme Structure is Important
Enzymes are made of a chain of amino acids, folded together in a 3D shape, held together by HYDROGEN BONDS and DISULFIDE BRIDGES.
The shape of an ENZYME is super important because it ultimately determines the function of that enzyme. The most important part of an enzyme’s 3D structure is the ACTIVE SITE. This is because the active site is the area where the SUBSTRATES or the chemicals involved in the reaction bind to the enzyme.
The chemicals that bind to the enzyme’s active site are called substrates. The shape of the enzyme’s active site is very specific and corresponds to the shape of the substrate that binds there. Due to the unique shape of each active site, enzymes are highly specific . This means an enzyme can only speed up or CATALYSE one kind of reaction.
Enzymes are proteins which means they are made of a long chain of amino acids. Amino acids are the building blocks of proteins. These individual amino acids are joined together with special chemical bonds called peptide bonds. A string of these amino acids joined together is called a polypeptide chain.
The sequence of amino acids in the polypeptide chain causes the enzyme to fold into a very specific shape. And this very specific shape is held in place by hydrogen bonds and disulfide bridges forming between different amino acids.
Two Types of Reactions
When a SUBSTRATE binds to an enzyme’s ACTIVE SITE, it undergoes various chemical reactions. The reactions result in the ENZYME forming new molecules called PRODUCTS.
There are two types of reactions catalysed by enzymes.
Building up reactions are called ANABOLIC REACTIONS - this is where enzymes can join together two or more smaller substrates to form a larger product.
An example of this is during PHOTOSYNTHESIS, when plants make glucose molecules from smaller substrate (water and carbon dioxide).
Breaking down reactions are called CATABOLIC REACTIONS - this is where enzymes break apart large substrates into two or more smaller products.
An example of this is when our body breaks down glucose to release energy during cellular RESPIRATION.
Enzymes speed up Reactions by Lowering the Activation Energy
All those chemical reactions (done by ENZYMES) require energy.
ACTIVATION ENERGY is the minimum amount of energy needed for a chemical reaction to occur. Most of the chemical reactions that occur within cells have a high activation energy.
Enzymes work to lower the activation energy of a reaction, which helps it to occur much faster! This is why enzymes are known as biological CATALYSTS.
Without enzymes acting as biological catalysts, chemical reactions in our cells, like respiration, DNA replication, and cell division, would take place too slowly. So without enzymes, we can’t live.
Two Proposed Models of Enzyme Activity
There are two proposed models that explain enzyme activity: ‘LOCK AND KEY’, and ‘INDUCED FIT’.
Lock and Key Model
The ‘LOCK AND KEY’ model proposes that the shape of an enzyme’s ACTIVE SITE and the SUBSTRATE it CATALYSES fit each other perfectly, just like how a key fits into a lock. The enzyme has a specific shape that corresponds to the shape of a particular substrate. The enzyme and substrate fit together perfectly at the enzyme’s active site.
When the enzyme and substrate fit together at the active site, they form an enzyme-substrate complex. Here, chemical bonds are formed or broken to create particular products, which are then released by the enzyme.
Induced Fit Model
This is the model that is widely accepted by scientists, and it is generally considered to be the more correct model of ENZYME activity. It is a recent modification of the older lock and key model.
In the ‘INDUCED FIT’ model, the enzymes are thought of as flexible structures that don’t have the exact same shape as their substrate before binding. Instead, the enzyme moulds to fit the substrate perfectly, forming an enzyme-substrate complex. When the final PRODUCT is released, the enzyme returns to its original shape.
Comparison of Models
The lock and key and induced fit models are similar in that the enzymes have a specific shape that corresponds to a particular substrate. But the important difference is that the lock and key model states the enzyme does not change shape when it binds with the substrate, whereas the induced fit model states that enzymes are flexible and change shape to fit the substrate.
Factors Affecting Enzyme Activity
There are several factors that can affect an ENZYME'S rate of activity. This is because enzymes can only function in specific conditions. Outside of these conditions, the enzymes will not function as well or not function at all.
Every type of ENZYME has a unique range of TEMPERATURES they can function in. Within this range, there will be a specific temperature where the rate of reaction is the fastest. This is called the OPTIMUM temperature. This is the temperature where the rate of enzyme reaction is highest.
If the temperature is too far below the enzyme’s optimum temperature, the molecules will move very slowly. This means the number or frequency of successful collisions between enzymes and their substrates decreases, causing the rate of reaction to slow down or stop completely.
If the temperature increases SLIGHTLY, it causes the enzymes and substrates to move faster. This causes them to collide more frequently, increasing the rate at which enzymes and substrates bind together at the active site. Overall, this will result in an increased rate of reaction up to a point.
If the temperature rises TOO FAR above the enzyme’s optimum temperature, the bonds holding the enzyme in its specific shape will become disrupted and begin to break.
Once these bonds are broken, there’s nothing holding the polypeptide chain together, and so the enzyme will unfold and lose its shape. As a result, the substrate can no longer fit into the enzyme’s active site and chemical reactions are unable to take place. At this point, the enzyme is said to be DENATURED.
Denaturing due to extreme temperatures is permanent and means that the rate of reaction will decrease or may stop completely. Always use the term ‘denature’ to describe an enzyme that has changed shape to the point where it can no longer bind to the SUBSTRATE and CATALYSE the reaction.
pH affects ENZYME activity, because each enzyme has a specific pH where it works at its optimum rate/efficiency. All enzymes have an optimum pH value where they are most active.
For example, catalase is an enzyme that breaks down hydrogen peroxide and it has an optimum pH of 7. Another enzyme called pepsin helps digest the proteins in our food. It is produced in the stomach, so its optimum pH is very acidic, around 1.5!
A slight change in pH can interfere with the bonds within an enzyme, temporarily changes the shape of its ACTIVE SITE. This causes the reaction rate to drop, as the corresponding substrate is no longer able to bind to the enzyme’s active site and the reaction is unable to be catalysed.
Small changes in pH do not actually denature the enzyme because the bonds that have been temporarily disrupted by the change in pH are able to reform if the pH returns to the optimum level.
But, if there is an extreme change in the pH away from the optimum, this can DENATURE the enzyme. So either too low or too high a pH will disrupt the hydrogen bonds that hold the enzyme in a 3D shape. Without these hydrogen bonds, the active site will change shape permanently, the enzyme is denatured. When this happens, the shape of the active site no longer fits the substrate, and the enzyme can’t catalyse the reaction (reaction slows or stops altogether).
INHIBITORS are molecules that interfere with ENZYMES, causing a permanent or temporary decrease in enzyme activity. Examples of inhibitors are often heavy metals such as lead, mercury, cadmium, or poisons.
Inhibitors stop the substrate from binding with the active site by either:
Binding to the active site themselves (these inhibitors are called COMPETITIVE INHIBITORS because they compete with the substrate to bind to the active site)
Binding to the enzyme outside the active site in such a way that causes the active site to change shape - these inhibitors are called NON-COMPETITIVE INHIBITORS.
Inhibitors down the rate of reaction and may even prevent it from taking place.
The concentration of ENZYMES also has an effect on enzyme activity. If there are many enzymes, the rate of reaction begins to increase. This is because there is a greater chance that a SUBSTRATE will successfully bind to an enzyme’s active site if there are more enzymes present.
But, if the number of enzymes continues to increase, the rate of enzyme activity will begin to level off because all of the available substrates will already be bound to an enzyme. This means any new enzymes would not have a substrate to bind with and would have no impact on the rate of reaction.
The concentration of SUBSTRATES will affect ENZYME reaction rates. An increase in the number of substrates available will also have an effect on enzyme activity.
The rate of enzyme reaction will increase with increasing substrate concentration, because more substrates will successfully combine with an enzyme’s active site.
But, if the substrate concentration continues to increase, eventually the rate of reaction will begin to level off. This is known as a saturation point, because the active sites of the enzymes are already saturated with substrate!
Some enzymes can’t catalyse reactions by themselves and need the help of other molecules called CO-FACTORS. Co-factors and CO-ENZYMES are molecules that bind to an enzyme’s ACTIVE SITE, altering its shape in a way that allows the corresponding SUBSTRATE to tightly bind with the enzyme. So the change in shape doesn’t cause DENATURATION, the change in shape is actually a good thing in this case.
Without co-factors and co-enzymes, the substrate and enzyme won’t fit together and the reaction won’t take place. The only difference between co-factors and co-enzymes is that co-factors are inorganic molecules (like ions like magnesium ions, copper ions), molecules that don’t contain carbon. Whereas co-enzymes are organic molecules (like vitamins and haem), molecules that contain carbon atoms.
The Rule of Limiting Factors
The last thing you need to know about enzymes is that:
"The rate of any enzyme reaction will always correspond to the factor which is in least supply/least optimal."
For example, if even if there is plenty of substrate, the other factors necessary for a reaction, such as enzyme concentration, temperature, pH, could become ‘limiting factors’. Those limiting factors also need to increase to bring about a further increase in the rate of the reaction.