Enzymes Flashcards
What is a catalyst?
- A substance that speeds up the rate of a reaction by lowering the activation energy without itself undergoing a permanent change
- Since enzymes are not permanently altered by the reactions they catalyse they can be used again many time over
What is an enzyme?
- Enzymes are biological catalysts
- They are globular proteins that interact with substrate molecules causing them to react at much faster
rates without the need for harsh environmental conditions
What are the properties of enzymes?
(1)
- Enzymes are globular proteins
- Their function is dependent on their exact structure
- If the structure is changed (or denatured), the activity is reduced/lost
(2)
- The enzyme catalyses a change in a substrate to convert it into a product
- Substrate = chemical to be altered in reaction
- Product = altered chemical/outcome of reaction
- Only a small part of the enzyme (= the active site) is in contact with the substrate
(3)
- The shape of the acrtive site is complementary to the shape of the substrate
- The substrate fits fits ‘exactly’ into the active site
- Denaturation of the enzyme protein loses the shape of the active site which results in lowering the rate of reaction
(4)
- Enzymes are extremely specific
- Only one type of substrate molecule may be altered by each enzyme e.g. the substrate that fits into the active site
(5)
- Enzymes have a very high turnover number (number of substrate moelcules turned into products per second)
- A small amount of enzyme can affect large amount of substrate
(6)
- Enzymes may be intracellular (within cells) or extra cellular (e.g. digestive enzymes)
What is an active site?
- A 3D crevice in the enzyme structure (usually on its surface) formed by the folding of the protein chain
- It is the region of the enzyme where the substrate molecules binds and undergoes a chemical reaction to form a product
What is catalase?
- An intracellular enzyme
- It breaks down hydrogen peroxide into water and oxygen
- It is found animal and plant tissue
What are three examples of extracellular enzymes that work in the digestive system?
Amylase, maltase and trypsin
What enzymes are involved in breaking down starch into glucose?
- Amylase breaks starch down into maltose
- Maltase breaks maltose down into glucose
Which type of enzyme (intracellular or extracellular) can work at a greater range of temperatures and pH? And why?
Extracellular enzymes as the conditions are not as tightly controlled outside cells
What is the mechanism of enzyme action? And draw a labelled diagram.
Answer on revision card
- Substrate binds to to the enzyme at the active site to make a temporary and short-lived enzyme-substrate complex (ES complex)
- Substrate(s) is changed to product(s) in the active site
- An enzyme product complex is formed
- Product(s) are released by the enzyme => enzyme can function again i.e. allow another substrate to bind to the active site
What is metabolism? And what are its two broad categories?
- Metabolism is all the chemical reactions occuring in a living organism
The two broad categories include:
Anabolism
=> synthetic reactions which turn monomers to polymers
=> condensation reactions
=> building up/synthesis
=> requires energy (endothermic/endergonic)
Catabolism
=> breakdown reactions which turn polymers to monomers
=> hydrolysis reactions e.g. in the digestive system
=> releases energy (exothermic/exergonic)
What are the two different models for binding of substrate to active site?
- Lock and key hypothesis
- Induced-fit hypothesis
What is the lock and key hypothesis?
- The shape of the active site is exactly complementary to the shape of the substrate
- When the substrate is bound to the active site and ES complex is formed
- The substrate(s) then react and the product(s) are formed in an EP complex
- The product(s) are released leaving the enzyme unchanged
- The substrate is held in a way by the enzyme that the right atom-groups are close enough to react
- The R-groups within the active site of the enzyme will also interact with the substrate, forming temporary bonds
- These put strain on the bonds within the substrate, which also helps the reaction along
What is the induced-fit hypothesis?
- Induced-fit indicates a change in the shape of the active site in response to substrate binding
- This intial reaction between the enzyme and substrate is relatively weak, but these weak interactions rapidly induce changes in the enzyme’s teritary structure that strengthen binding, putting strain on the substrate molecule
- This can weaken a particular bond or bonds in the substrate, therefore lowering the activation energy for the reaction
What are the factors that can affect enzyme activity?
- Temperature
- Substrate
- Enzyme concentration
- pH
How does substrate concentration affect enzyme activity? And draw the graph.
Answer on revision card
- Rate of reaction increases as substrate concentration increases i.e. the substrate is rate-limiting:
=>As substrate concentration increases
=> Rate of successful collisions increases
=> Rate of formation of ES complexes increases
=> Rate of reaction increases - This occurs until all the active sites are occupied/saturated, so subsequent substrate molecules must wait for an available active site
- Substrate is no longer rate-limiting so any further increase in its concentration has no effect on the rate of reaction. This is known as Vmax (‘V’ meaning velocity)
How does enzyme concentration affect enzyme activity? And draw the graph.
Answer on revision card
- Rate of reaction increases as enzyme concentration increases i.e. the enzyme concentration is rate-limiting
** Under physiological conditions the concentration of enzyme is much lower than substrate therefore the graph does not level off and the enzyme remains rate-limiting. Under experimental conditions however, enough enzyme could be added for it to be in excess
How does pH affect enzyme activity? And draw the graph.
- A slight change in pH disruptis ionic and hydrogen bonds involved in the formation of the tertiary structure of the protein enzyme
- So the enzyme is denatured (structure of the active site is lost/no longer complementary to substrate), so the substrate no longer fits
- This causes the rate of reaction to decrease
- Extracellular enzymes have a broader range of pH than intracellular enzymes (buffers are very important in living systems)
How does temperature affect enzyme activity? And draw the graph.
- As temperature increases so does rate of reaction:
=> as temperature increases
=> kinetic energy increases
=> rate of successful collisions increases
=> number of ES complex formed increases
=> rate of reaction increases - Until optimum temperature is reached
- As temperature increases past the optimum temperature, the increased vibrational energy (kinetic energy of the bonds within the enzyme) causes hydrogen bonds involved in the formation of the 3D structure break i.e. the enzyme is denatured
- The structure of the active site is lost and is no longer complementary to the substrate
- The substrate no longer fits so the ES complex cannot form causing the rate of reaction to decrease
What is the temperature coefficient for an enzyme catalysed reaction?
The ratio between the rate of that process at two different temperatures which are 10°C apart
How do you calculate the temperature coefficient (Q10)?
Temperature coefficient (Q10) = Rate of reaction at t°C/Rate of reaction at (t-10)°C
What is an inhibitor?
Molecules that prevent enzymes frim carrying out their normal function of catalysis, reducing the rate of reaction
What are the two types of inhibitors?
Competitive and non-competitive inhibitors
How does a competitive inhibitor work?
- The competitve inhibitor has a very similar structure to the substrate so can fit into the active site
- However no reaction occurs as enzymes are specific to their substrates
- Active site becomes occupied (inhibitor blocks substrate from entering the active site) so substrate cannot enter
- ES complex cannot be formed, preventing the enzyme from catalysing the reaction, so rate of reaction is reduce
- Substrate and inhibitor compete for the active site
- The degree of inhibition will depend on the relative concentrations of substrate, inhibitor and enzyme
Why is the effect of competitve inhibitors reversible?
Most competitive inhibitors only bind temporarilty to the active site
How can the effect of competitive inhibitors be reversed?
The effect of competitive inhibotrs can be reversed by increasing the concentration of the substrate
Draw a graph to show the effect of a competitive inhibitor on enzyme activity vs substrate concentration.
Answer on revision card
How does a non-competitive inhibitor work?
- The inhibitor binds tightly to the enzyme at a location other than the active site. This alternative site is called the allosteric site
- The binding of the inhibitor causes the tertiary structure of the enzyme to change, meaning the active site changes shape. This is called the allosteric effect
- So the active site is no longer complementary to the substrate
- Substrate cannot enter the active site so no ES complexes are formed
- Rate of reaction is reduced
- The effects of non-competitive inhibitors is non-reversibly: it cannot be reversed by increasing the substrate concentration
What are some examples of a competitive inhibitor?
- Statins are competitive inhibitors of an enzyme used in the synthesis of cholesterol
- Aspirin irreversibly inhibits the active site of COX enzymes, preventing the synthesis of prostaglandins and thromboxane (chemicals responsible for producing pain and fever)
What are some examples of irreversible non-competitve inhibitors?
- Organophosphates used as insecticides and herbicides irreversibly inhibit the enzyme acetyl cholinesterase
- Proton pump inhibitors (PPIs) are used to treat long-term indigestion. They irreversibly block an enzyme system responsible for secreting hydrogen ions into the stomach
Draw a graph to show the effect of a non-competitive inhibitor on enzyme activity vs substrate concentration.
Answer on revision card
What is end-product inhibition?
It is the term used for enzyme inhibition that occurs when the product of a reaction acts as an inhibitor to the enzyme that produces it
How does a metabolic pathway work in end-product inhibition?
- If the pathway continues uregulated the end-product will be surplus to the cell’s requirements (and energy wasted in its making)
- In most pathways, the end-product is a reversible non-competitve inhibitor of the first enztme of the pathway
- The whole pathway stops and intermediates and energy are not wasted unnecessarily
If pathways stop:
- The concentration of the end product decreases
- Enzyme inhibition is reduced
- Metabolic pathway starts again
What is a cofactor?
A non protein compound required for the enzyme’s activity to occur
What does an enzyme cofactor do?
They may transfer atoms or groups from one reaction to another in a multi-step pathway or they may actually form part of the active site of an enzyme
What are the 3 catergories of enzyme cofactors? And what are they?
Prosthetic groups:
- These are components such as inorganic ions, permanently attached to the proteins (e.g. haem of haemoglobin) which are required to allow the enzyme to function
Inorganic Cofactors:
- In organic ions, not permanently attached to the protein, which are required to allow the enzyme to function (e.g. chloride ion is the cofactor for amylase)
Coenzymes:
- Organic molecules which can be removed from the enzyme
- They enter the active site along with the substrate to be used in its conversion to the product
Which enzyme breaks down hydrogen peroxide into oxygen and water?
Catalase
What is the true rate of reaction?
The initial rate of reaction
What is the precursor for a coenzyme?
Vitamin B series
