Enzymes Flashcards
What are enzymes?
- globular proteins with a specific tertiary structure, held together by hydrogen bonds, ionic bonds, disulphide bridges, and hydrophobic interactions
- act as biological catalysts that speed up the rate of metabolic reactions by lowering the activation energy
- can work intracellularly (in solution/membrane bound) or extracellularly (secreted)
- catalyse anabolic and catabolic reactions
What are anabolic and catabolic reactions?
Anabolic: builds up by catalysing bonds (photosynthesis, polymerisation)
Catabolic: breaks down by breaking bonds (respiration, hydrolysis)
How do enzymes work?
A small region (active site) is functional, and when it comes into contact with a substrate, an enzyme substrate complex forms
The reaction takes place and the products are released, leaving the active site free to perform another reaction
What is the Lock and Key Model for enzyme action?
- the bust rate molecule fits into the active site of the enzyme molecule like a key fitting into a lock as they are complementary shapes, forming the enzyme substrate complex
- product is then formed and is released as it no longer fits inside the activ site
- explains why enzymes are very specific
What is the Induced Fit Model for enzyme action?
- the specific substrate and the active site are not complementary shapes
- when a substrate molecule binds to the active sit, the active site changes shape to fit itself around the substrate
- this places a strain on the substrate molecule and distorts a bond, lowering the activation energy required to break the bond
- the products are formed and leave the active site which then return o its original shape
What are the properties of an enzyme?
- specific due to sequence of amino acids that make up their active site
- fast acting with a very high turnover number, they can convert many molecules of substances per unit time
- soluble, as the hydrophilic R groups are found on the outside of the molecule
What does the enzyme lysosyme do?
- kills bacteria by catalysing the hydrolysis of sugars in the peptidoglycan cel walls
- cell walls are weakened and the bacteria absorb water by osmosis, and burst
It is present in many secretions, such as tears, saliva, human milk, and is found in lysosomes
Describe the mass of product formed (y) to the time (x) graph?
- the curve is initially steep as there is a large concentration of substrate molecules, so they are more likely to successfully collide with an active site and form the product. Enzyme concentration is the limiting factor
- as the reaction proceeds, the curve gets shallower as thee is a decreasing concentration of substrate o there is less chance of successful collisions. Substrate concentration is the limiting factor
- the graph levels off as all of the substrate has been converted into product and no further product is formed
What factors can affect the rate of an enzyme controlled reaction?
- temperature
- pH
- substrate concentration
- enzyme cpmcentration
- inhibitors (competitive and non-competitive)
What happens if the tertiary 3D structure of an enzyme is changed?
- enzyme will become denatured and will not work as their active site shapeis no longer complementary to the shape of the structure
- if the change in structure is minor and reversible, the enzyme is said to have undergone inactivation
- if the change is irreversible, it can cause the enzyme to become denatured
How does temperature affect enzyme activity?
- at low temperatures the rate of reaction is low as the enzyme and substrates have a low kinetic energy so there are fewer collisions, forming less ESCs
- increasing temperatures up to the optimum increases the rate of reaction as kinetic energy increases, increasing the chance of successful collisions. More ESCs are formed
- at optimum temperature, the rate of reaction is highest as there are many successful collisions so most amount of ESC form
- above optimum temperature, the rate of reaction decreases as the increasing kinetic energy creates vibrations that break hydrogen bonds, changing the tertiary structure of the enzyme. The active site is no longer complementary to the substrate, lowering the rate of reaction as ESCs cannot form
How does pH affect enzyme activity?
Most enzymes have an optimum pH at which the rate of reaction is maximum
- only work in a narrow range of pH values
- small deviations from the optimum pH can cause reversible changes in enzyme structure - inactivation
- extreme pHs can permanently denature an enzyme
NOT ALL ENZYMES IN THE BODY HAVE THE SAME OPTIMUM PH
BUFFERS can be added to maintain a constant pH
What can a small change in pH do to an enzyme?
- may alter the electrostatic charge on the side chains of the amino acids
- if the active site has too many H+ or OH- ions, the active site and substrate may both have the same charge and the enzyme will repel the substrate
What effect does substrate concentration have on enzyme controlled reactions?
Rae of reaction will increase as substrate concentration increases
- low substrate concentration, the substrate concentration is the limiting factor, the enzyme molecules only have a few substrate molecules to collide with
- as more substrate added, there are more successful collisions and more enzyme active sites become occupied, until the rate of reaction reaches a maximum and substrate molecules are in excess. Enzyme concentration becomes the limiting factor
What effect does enzyme concentration have on enzyme controlled reactions?
Once a product leaves an active site, the enzyme molecules only can be reused
Rate of reaction will vary with changes in the enzyme concentration, as the enzyme concentration increases, there are more active sites available so the rate of reaction increases
What is enzyme inhibition?
The reduction in the rate of an enzyme-controlled reaction by anther molecule
The inhibitor combines with the enzyme and either directly or indirectly prevents it from forming an enzyme-substrate complex
What is a competitive inhibitor?
- similar in shape to a substance
- complementary to the enzyme active sites and so bind to it, blocking the active site, so less ESCs can form
- rate of reaction decreases so less product is produced
- if substrate concentration increases, the effect of the inhibitor is reduced
What is an example of a competitive inhibitor?
MALONIC ACID
- similar shape to succinic acid (reaction involved in aerobic respiration is catalysed by the enzyme succinic dehydrogenase)
- prevents a reaction in aerobic respiration
What are non competitive inhibitors?
- not a similar shape to the substrate
- binds to the enzyme at the allosteric site and changes the shape of the enzyme and therefore the shape of the active site so it is no longer complementary to the substrate
- enzyme molecule is permanently damaged, and cannot be outcompeted if substrate concentration is increased
What is an example of a non competitive inhibitor?
Cyanide
This also blocks a process in aerobic respiration/nerve transmission and is fatal
What is an immobilised enzyme?
Enzymes that are fixed, trapped, or bound on an inert matrix
- absorption onto an insoluble matrix
- covalently bonded to a solid support
- trapped within a gel
- encapsulated behind a selectively permeable membrane
Trapping an enzyme will stabilise it and prevent the shape change that would denature the active site so can be used under a wider range of conditions
What are the advantages of an immobilised enzyme?
- enzymes are easily recovered for reuse
- product is not contaminated with the enzyme
- increased stability over a wider range of pH values
- increased stability over a range of temperatures, and enzymes denature at higher temperatures, means rate of reaction can be increased by increasing the temperature
- several enzymes with differing temperature or optimum pH can be used in one process
- enzymes can be easily added or removed, giving greater control over the rate of reaction
How can immobilised enzymes be used to make lactose free milk?
- lactose free milk can be produced using immobilised enzymes trapped in an alginate bead in a column to hydrolyse the lactose to its monosaccharides glucose and galactose
What do immobilised enzymes do in biosensors?
Due to the specificity of enzymes, they rapidly detect and measure very low concentrations if a specific substrate in a complex mixture
Measurement of glucose in blood samples to help detect/manage diabetes
