Enzymes and Biological Reactions
1. What Are Enzymes?
Learning outcomes
- I can define enzymes as biological catalysts.
- I can explain how enzymes speed up chemical reactions in living organisms.
- I can describe the role of enzymes in metabolism.
- I can explain why enzymes are essential for life.
- I can identify examples of enzymes involved in biological processes.
What Is an Enzyme?
An enzyme is a biological catalyst.
A catalyst is a substance that increases the rate of a chemical reaction without being used up permanently by the reaction.
Enzymes perform this role inside living organisms.
In simple terms:
Enzymes help chemical reactions happen faster.
This is essential because cells depend on thousands of chemical reactions every second.
Without enzymes, many of these reactions would occur far too slowly to support life.
Biological Catalysts
The word biological means associated with living organisms.
The word catalyst means something that speeds up a chemical reaction without being consumed overall.
Therefore:
Biological catalyst = a catalyst produced by living organisms
Enzymes can participate repeatedly in reactions because they are not consumed as reactants.
A simplified reaction can be represented as:
Substrate → Product
with the enzyme helping the reaction occur more quickly.
What Are Enzymes Made Of?
Most enzymes are proteins.
Proteins are large biological molecules made from chains of smaller molecules called:
amino acids
The amino-acid chain folds into a specific three-dimensional shape.
The shape of an enzyme is extremely important because it determines which molecules can interact with it.
Some RNA molecules can also act as biological catalysts. These catalytic RNA molecules are called ribozymes, but most enzymes encountered in introductory biology are proteins.
Enzymes and Chemical Reactions
Living organisms depend on chemical reactions.
Examples include reactions involved in:
- digestion
- respiration
- photosynthesis
- DNA replication
- protein synthesis
- breaking down toxins
- building new molecules
- releasing energy from nutrients
These reactions must occur at suitable rates.
Enzymes make this possible.
Activation Energy
For a chemical reaction to begin, reacting particles must overcome an energy barrier called the activation energy.
Enzymes increase reaction rates by providing a reaction pathway with a lower activation energy.
Think of activation energy as a hill that reactants must get over.
Without an enzyme:
larger energy barrier
With an enzyme:
smaller energy barrier
The enzyme does not simply "give" the molecules the required energy. Instead, it provides a pathway that requires less activation energy.
Enzymes Do Not Supply Energy
This distinction is important.
An enzyme does not provide energy to the reaction.
Instead, it:
lowers the activation energy required for the reaction to proceed
This allows a greater proportion of reactant molecules to react successfully under cellular conditions.
Enzymes and Reaction Rate
Because enzymes lower activation energy, reactions can occur much more rapidly.
Consider a chemical reaction in a cell that would naturally occur very slowly.
Without an enzyme:
Substrate → Product
may take too long to support normal cellular activity.
With the appropriate enzyme:
Substrate → Product
can occur much faster.
The enzyme remains available to catalyze additional reactions.
Substrates
The molecule or molecules upon which an enzyme acts are called:
substrates
For example, the enzyme amylase acts on:
starch
Therefore:
Enzyme = amylase
Substrate = starch
Products = smaller sugars
The Active Site
An enzyme contains a region called the active site.
The active site is the part of the enzyme where the substrate binds and the catalyzed reaction takes place.
Its shape and chemical properties allow particular substrates to interact with it.
This helps explain why enzymes are usually highly specific.
Enzyme Specificity
Different enzymes catalyze different reactions.
For example:
Amylase
acts on starch.
Lipase
acts on lipids.
Proteases
act on proteins.
An enzyme that digests starch cannot simply replace an enzyme needed to digest proteins.
The structure and chemical properties of the active site determine which substrates can bind effectively.
The Lock-and-Key Model
A simple model for enzyme action is the lock-and-key model.
Imagine:
Enzyme = lock
Substrate = key
Only a key with an appropriate shape fits the lock.
Similarly, a substrate with an appropriate shape and chemistry can bind to an enzyme's active site.
This model is useful for introducing enzyme specificity.
However, real enzymes are flexible rather than completely rigid.
The Induced-Fit Model
A more detailed explanation is the induced-fit model.
When a substrate approaches the active site, interactions between the substrate and enzyme can cause a slight change in the enzyme's shape.
This allows the active site to fit the substrate more effectively and helps the reaction occur.
The enzyme is therefore not simply a rigid lock.
Its structure can adjust during substrate binding.
The Enzyme-Substrate Complex
When a substrate binds to an enzyme, they temporarily form an:
enzyme-substrate complex
The general process is:
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
The enzyme helps convert the substrate into product.
The product is then released.
The enzyme can be used again.
Step 1: Substrate Approaches
A substrate moves toward the enzyme.
The substrate must encounter the appropriate active site.
Step 2: Substrate Binds
The substrate binds to the active site.
An:
enzyme-substrate complex
forms.
Step 3: Reaction Occurs
The enzyme helps the chemical reaction occur by lowering the activation-energy barrier.
Depending on the reaction, this may involve:
- breaking bonds
- forming bonds
- rearranging atoms
- transferring chemical groups
The substrate is converted into one or more products.
Step 4: Products Leave
The products no longer interact with the active site in the same way and are released.
The enzyme itself remains available.
It can interact with another substrate molecule and repeat the process.
Enzymes Can Break Molecules Apart
Some enzymes help break large molecules into smaller molecules.
For example, digestive enzymes break food molecules into smaller molecules that can be absorbed.
A simplified example is:
Large molecule + water → smaller molecules
Digestive enzymes frequently catalyze reactions called hydrolysis reactions.
Enzymes Can Build Molecules
Enzymes are not only used to break substances apart.
Other enzymes help cells construct larger molecules.
For example, enzymes participate in:
- DNA synthesis
- protein synthesis
- glycogen formation
- lipid synthesis
Cells therefore use enzymes for both:
breaking down molecules
and:
building molecules
Metabolism
All of the chemical reactions occurring within a living organism are collectively called:
metabolism
Metabolism includes thousands of interconnected reactions.
Almost every metabolic pathway depends on enzymes.
Catabolic Reactions
Catabolic reactions break larger molecules into smaller molecules.
These reactions can release energy or make smaller molecules available for other processes.
Examples include:
- digestion of food
- breakdown of glucose during cellular respiration
- breakdown of stored molecules
Enzymes control the individual reaction steps.
Anabolic Reactions
Anabolic reactions build larger molecules from smaller molecules.
Examples include:
- protein synthesis
- DNA synthesis
- glycogen synthesis
- production of some lipids
These reactions also depend on enzymes.
Therefore:
Metabolism = catabolic reactions + anabolic reactions
Enzymes and Metabolic Pathways
Many cellular processes require a sequence of reactions rather than a single reaction.
For example:
Substance A → Substance B → Substance C → Substance D
Each step may require a different enzyme:
A → B: Enzyme 1
B → C: Enzyme 2
C → D: Enzyme 3
This organization allows cells to control complex chemical processes.
Why Are Enzymes Essential for Life?
Living organisms must maintain conditions that are compatible with cells.
Human body temperature, for example, is normally around:
37°C
Many important biochemical reactions would occur too slowly at normal biological temperatures without enzymes.
Increasing temperature dramatically could speed some chemical reactions, but high temperatures would damage cells and biological molecules.
Enzymes allow reactions to occur rapidly under conditions suitable for life.
Enzymes and Digestion
Digestion is one of the easiest places to see enzyme function.
Large food molecules must be broken into smaller molecules before they can be absorbed and used effectively.
Major digestive enzymes include:
- amylase
- proteases
- lipases
Amylase
Amylase is an enzyme involved in carbohydrate digestion.
It breaks down:
starch
into smaller sugars.
Amylase is produced in locations including:
- salivary glands
- pancreas
Salivary amylase begins starch digestion in the mouth.
Proteases
Proteases are enzymes that break down proteins.
Proteins are ultimately broken into smaller peptides and amino acids through digestive processes.
Examples of proteases include:
- pepsin
- trypsin
Proteases are important because amino acids can be used to build new proteins.
Lipases
Lipases catalyze the digestion of lipids.
Lipids are broken down into products including:
fatty acids and glycerol
These products can then be absorbed and used by the body.
Lactase
Lactase catalyzes the breakdown of:
lactose
Lactose is a sugar found in milk.
The reaction produces the simpler sugars:
glucose and galactose
Lactase is another example of enzyme specificity.
Catalase
Cells can produce hydrogen peroxide during normal metabolic processes.
Hydrogen peroxide can damage cells if it accumulates.
The enzyme catalase rapidly breaks hydrogen peroxide down.
The reaction is:
hydrogen peroxide → water + oxygen
Balanced chemical equation:
2H₂O₂ → 2H₂O + O₂
The oxygen produced can often be observed as bubbles during catalase experiments.
Enzymes in Cellular Respiration
Cells release usable energy from nutrients through cellular respiration.
This is not a single reaction.
It is a series of enzyme-controlled reactions.
During aerobic respiration, glucose and oxygen are ultimately used to produce carbon dioxide and water while energy is transferred into forms the cell can use.
A simplified word equation is:
glucose + oxygen → carbon dioxide + water
Enzymes control the many individual stages involved.
Enzymes and DNA
Enzymes are also essential when cells copy and use genetic information.
For example:
DNA polymerase helps build new DNA strands during DNA replication.
Other enzymes:
- separate DNA strands
- join DNA fragments
- repair damaged DNA
- help produce RNA
Without these enzymes, cells could not accurately reproduce their genetic information.
Enzymes and Photosynthesis
Plants also depend on enzymes.
Photosynthesis involves a series of reactions rather than one simple chemical step.
Enzymes help control reactions involved in:
- carbon fixation
- production of carbohydrates
- processing energy-rich molecules
One important enzyme is:
RuBisCO
RuBisCO participates in carbon fixation during the Calvin cycle.
Enzymes and Defense
Some enzymes help protect organisms.
For example, lysozyme can damage bacterial cell walls.
Lysozyme can be found in substances such as:
- tears
- saliva
- mucus
Enzymes therefore participate not only in metabolism and digestion but also in biological defense.
Enzymes and Blood Clotting
Blood clotting involves a sequence of enzyme-controlled reactions.
When a blood vessel is damaged, proteins involved in the clotting process become activated in a carefully controlled sequence.
This eventually contributes to the formation of a stable blood clot.
This illustrates how enzyme-controlled pathways can produce a rapid biological response.
Enzymes Work in Cells and Outside Cells
Many enzymes function inside cells.
Examples include enzymes involved in:
- respiration
- DNA replication
- protein production
Other enzymes are released from cells and work elsewhere.
Digestive enzymes are good examples.
They are produced by cells but released into parts of the digestive system where they act on food.
Enzymes Are Reusable
Consider the simplified reaction:
E + S → ES → E + P
where:
E = enzyme
S = substrate
ES = enzyme-substrate complex
P = product
Notice that the enzyme appears both:
before
and:
after
the reaction.
It is not permanently consumed.
This allows one enzyme molecule to catalyze many substrate conversions.
Does an Enzyme Change the Final Products?
An enzyme changes the rate of a reaction.
It does not normally change which products are favored by the underlying reaction or the overall energy difference between reactants and products.
Instead, it provides a lower-activation-energy pathway to the same reaction outcome.
This distinction is important:
Enzyme → changes reaction rate
not:
Enzyme → supplies the reaction's energy
Why Enzyme Shape Matters
An enzyme's function depends strongly on its three-dimensional structure.
The active site must have appropriate:
- shape
- charge
- chemical properties
for the substrate and reaction.
If the enzyme's structure changes significantly, its ability to bind substrates and catalyze reactions may decrease.
This becomes especially important when studying:
- temperature
- pH
- denaturation
Enzymes and Temperature
Increasing temperature generally increases molecular motion.
Up to an enzyme's suitable operating range, this can increase the frequency of productive interactions between enzymes and substrates.
However, sufficiently high temperatures can disrupt the enzyme's structure.
The active site may change shape.
The enzyme may become:
denatured
We can therefore expect enzyme activity to depend strongly on temperature.
Enzymes and pH
Enzymes also have pH conditions under which they function effectively.
Different enzymes can have different optimum pH ranges.
For example:
Pepsin
functions effectively in the acidic environment of the stomach.
Other enzymes function better near neutral or alkaline conditions.
Large changes in pH can alter interactions within an enzyme and affect the active site's structure and chemistry.
Enzyme Concentration
If more enzyme molecules are available, more active sites may be available for substrate molecules.
Under suitable conditions:
more enzyme → potentially faster reaction
However, this depends on whether enough substrate is available.
If substrate becomes limiting, adding additional enzyme may have less effect.
Substrate Concentration
Increasing substrate concentration can increase reaction rate because enzyme molecules encounter substrate more frequently.
Eventually, however, most available active sites may be occupied much of the time.
The reaction approaches a maximum rate for that enzyme concentration.
This is called enzyme saturation.
Enzymes in Everyday Life
Enzymes are not restricted to laboratories or textbooks.
They are used in:
- food production
- cheese making
- baking
- brewing
- biological detergents
- fruit juice production
- medicine
- biotechnology
- diagnostic tests
Their ability to catalyze specific reactions makes them extremely useful.
Enzymes in Biological Detergents
Some laundry detergents contain enzymes.
Examples include:
Proteases
help break down protein-containing stains.
Lipases
help break down fatty stains.
Amylases
help break down starch-containing stains.
This is an example of applying biological chemistry to an everyday problem.
Enzymes in Food Production
Enzymes are widely used in food processing.
Examples include:
- lactase for producing lactose-reduced products
- enzymes used during cheese production
- amylases used in baking and brewing
- pectinases used in fruit juice processing
Enzyme specificity allows particular chemical changes to be targeted.
Comparing Enzymes
| Enzyme | Substrate or Target | Example Role |
|---|---|---|
| Amylase | Starch | Carbohydrate digestion |
| Protease | Proteins | Protein digestion |
| Lipase | Lipids | Fat digestion |
| Lactase | Lactose | Lactose digestion |
| Catalase | Hydrogen peroxide | Cellular protection |
| DNA polymerase | DNA building blocks | DNA replication |
| Lysozyme | Bacterial cell-wall components | Defense |
| RuBisCO | Carbon dioxide-related substrate in carbon fixation | Photosynthesis |
These examples demonstrate the enormous variety of reactions controlled by enzymes.
Example: Digestion of Starch
Consider eating a piece of bread.
Bread contains starch.
In the mouth:
salivary glands release amylase
Amylase begins breaking down starch into smaller carbohydrates.
The enzyme allows this chemical digestion to occur rapidly under normal body conditions.
Example: Protecting a Cell
Hydrogen peroxide forms during some cellular processes.
High concentrations can be harmful.
Catalase converts hydrogen peroxide into:
water + oxygen
Because catalase works rapidly, cells can remove hydrogen peroxide before excessive accumulation causes damage.
This shows that enzymes can be important for maintaining stable internal conditions.
Example: Copying DNA
Before a cell divides, its DNA must be copied.
DNA polymerase helps assemble a new DNA strand using an existing strand as a template.
Without enzyme-controlled DNA replication, cells would not be able to pass genetic information accurately to new cells.
Enzymes are therefore directly connected to:
- growth
- repair
- reproduction
- inheritance
Why Can't Cells Simply Use Higher Temperatures?
Higher temperatures can make many chemical reactions occur faster.
However, cells cannot simply become extremely hot because high temperatures can damage:
- proteins
- membranes
- DNA
- cellular structures
Enzymes solve this problem by allowing reactions to occur rapidly at biologically suitable temperatures.
Enzymes and Homeostasis
Homeostasis is the maintenance of relatively stable internal conditions.
Enzymes contribute to homeostasis by controlling reactions involved in:
- energy release
- waste removal
- blood chemistry
- nutrient processing
- cellular repair
- signaling
Because enzymes regulate metabolic reactions, they are fundamental to maintaining stable cellular conditions.
A Simple Enzyme Investigation
One common enzyme experiment uses:
catalase + hydrogen peroxide
Catalase can be obtained from biological tissues such as potato or liver.
When hydrogen peroxide is added:
oxygen gas is produced
Possible measurements include:
- foam height
- oxygen volume
- reaction time
- gas production rate
The experiment can be modified to investigate the effects of:
- temperature
- pH
- enzyme concentration
- substrate concentration
Another Enzyme Investigation: Amylase
Amylase activity can be investigated using starch and iodine.
Iodine can be used to test for starch.
A typical investigation examines how quickly starch disappears when amylase is present.
Variables might include:
- temperature
- pH
- amylase concentration
This provides a practical way to measure enzyme activity.
Common Misconception: Enzymes Are Living
Enzymes are produced by living organisms, but individual enzyme molecules are not living organisms.
They are biological molecules.
They do not:
- reproduce
- grow
- contain cells
- carry out all life processes independently
They catalyze chemical reactions.
Common Misconception: Enzymes Are Used Up
Enzymes are not consumed as reactants in the reactions they catalyze.
After products are released, the enzyme can usually interact with another substrate molecule.
However, enzymes can eventually become damaged, degraded, or denatured.
Common Misconception: One Enzyme Can Catalyze Every Reaction
Enzymes are generally highly specific.
Amylase does not perform the same function as:
- lipase
- catalase
- DNA polymerase
Different chemical reactions require different enzymes.
Common Misconception: Enzymes Make Impossible Reactions Happen
Enzymes do not make any imaginable chemical reaction possible.
They accelerate reactions that are chemically possible by lowering the activation-energy barrier.
They do not change the fundamental thermodynamic outcome of a reaction.
Common Misconception: All Enzymes Work Best at 37°C
Many human enzymes function effectively around human body temperature, but enzymes occur in organisms living in many different environments.
Some organisms live in:
- cold oceans
- hot springs
- acidic environments
- alkaline environments
Their enzymes can be adapted to different conditions.
There is no single optimum temperature that applies to every enzyme.
Enzyme Names
Many enzyme names end in:
-ase
Examples:
- amylase
- lactase
- lipase
- catalase
- DNA polymerase
Often the name provides information about the enzyme's substrate or function.
For example:
lactase → acts on lactose
lipase → acts on lipids
However, some enzymes, such as pepsin and trypsin, have traditional names that do not end in -ase.
Did You Know?
Enzymes can be remarkably efficient.
Some enzyme molecules can catalyze enormous numbers of reactions in a short period of time.
This allows cells to coordinate vast networks of chemical reactions while maintaining temperatures and conditions compatible with life.
Your body therefore depends on enzymes every moment — whether you are:
- digesting food
- moving muscles
- repairing cells
- copying DNA
- producing energy
- removing harmful substances
Life is fundamentally dependent on enzyme-controlled chemistry.
Key Terms
- Enzyme: Biological catalyst that increases the rate of a chemical reaction.
- Catalyst: Substance that speeds up a reaction without being permanently consumed.
- Substrate: Molecule upon which an enzyme acts.
- Product: Substance produced by a chemical reaction.
- Active site: Region of an enzyme where substrate binding and catalysis occur.
- Enzyme-substrate complex: Temporary combination formed when a substrate binds to an enzyme.
- Activation energy: Minimum energy barrier that must be overcome for a reaction to proceed.
- Specificity: Tendency of an enzyme to catalyze particular reactions involving particular substrates.
- Metabolism: All chemical reactions occurring within an organism.
- Catabolism: Metabolic reactions that break larger molecules into smaller ones.
- Anabolism: Metabolic reactions that build larger molecules from smaller ones.
- Metabolic pathway: Series of connected chemical reactions in a cell.
- Denaturation: Loss of normal protein structure that can reduce or destroy enzyme function.
- Optimum: Conditions under which an enzyme shows particularly high activity.
- Saturation: Condition in which increasing substrate concentration has little additional effect because enzyme active sites are highly occupied.
- Homeostasis: Maintenance of relatively stable internal conditions.
- Hydrolysis: Reaction involving water that can be used to split larger molecules.
- Ribozyme: RNA molecule capable of catalyzing a chemical reaction.
Key Relationships
The general enzyme reaction can be represented as:
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
Enzymes:
lower activation energy → increase reaction rate
Most enzymes:
specific active site → specific substrate interactions → specific reaction
Metabolism includes:
Catabolism + Anabolism
Enzyme activity can be influenced by:
temperature + pH + substrate concentration + enzyme concentration
Key Takeaways
- Enzymes are biological catalysts.
- Most enzymes are proteins, although some RNA molecules can also act as catalysts.
- Enzymes increase the rate of chemical reactions without being permanently consumed.
- Enzymes work by lowering the activation energy required for reactions.
- Enzymes do not simply supply energy to reactions.
- Substrates are the molecules upon which enzymes act.
- Substrates interact with enzymes at active sites.
- Enzyme specificity depends on the structure and chemistry of the active site.
- The lock-and-key model provides a simple explanation of enzyme specificity.
- The induced-fit model recognizes that enzymes can change shape slightly during substrate binding.
- An enzyme and substrate temporarily form an enzyme-substrate complex.
- Products are released after the reaction, leaving the enzyme available for reuse.
- Enzymes can break molecules down or help build larger molecules.
- Metabolism consists of all chemical reactions occurring in an organism.
- Catabolic pathways break molecules down.
- Anabolic pathways build molecules.
- Metabolic pathways often involve several enzymes working in sequence.
- Enzymes allow metabolic reactions to occur rapidly under conditions compatible with life.
- Digestive enzymes include amylases, proteases, and lipases.
- Lactase breaks down lactose.
- Catalase breaks down hydrogen peroxide.
- DNA polymerase is essential for DNA replication.
- Enzymes are involved in cellular respiration, photosynthesis, defense, blood clotting, and many other biological processes.
- Enzyme activity can be affected by temperature and pH.
- Excessive temperature or unsuitable conditions can alter enzyme structure and reduce activity.
- Increasing substrate concentration can increase reaction rate until enzyme activity approaches saturation.
- Enzymes have important applications in food production, detergents, medicine, and biotechnology.
- Enzymes themselves are not living organisms.
- Different enzymes catalyze different reactions.
- Not all enzymes have the same optimum conditions.
- Enzymes are essential because life depends on thousands of controlled chemical reactions occurring rapidly and efficiently.