Core Concepts: Enzymes Flashcards
What is an ESC?
Enzyme-substrate complex
- intermediate structure formed where enzyme and substrate temporarily bind
What is the effect of pH on the rate of enzyme action?
Small changes to the pH (around the optimum)
- causes small reversible changes to enzyme structure and reduced its activity
Extreme changes to pH
- Charges on amino acid side-chains of active site affected by H+ and OH- ions
- at a very LOW pH excess H+ ions are attracted to negative charges on active site
- at a very HIGH pH excess OH- ions neutralise positive charges
- both disrupt ionic and hydrogen bonds holding the tertiary shape, changing the enzyme shape, no ESC, denatures enzyme and enzyme activity is lost
Describe the effect of a low substrate concentration?
- fewer substrate molecules
- fewer enzyme-substrate complexes being formed
- results in fewer products being produced
- the time taken for the reaction is longer
- substrate concentration is the limiting factor
Describe the effect of high substrate concentration?
- a point is reached where all the enzymes are saturated (all active sites are full)
- rate of reaction is at a maximum so a further increase in substrate concentration will not change the rate of reaction, stays constant
- enzyme concentration is the limiting factor
Describe the effect of low enzyme concentrations?
- fewer enzyme molecules, fewer active sites
- fewer ESCs formed
- fewer products are produced
- time taken for the reaction is longer
- the enzyme concentration is the limiting factor
Describe the effect of very high enzyme concentration?
- a point will reach when there are more active sites available than substrate molecules
- therefore increasing the enzyme concentration will decrease time taken, remains constant
- substrate concentration is the limiting factor
What is an inhibitor?
A molecule or ion that binds to an enzyme and reduces the rate of the reaction it catalyses
What is a competitive inhibitor?
- has a complementary shape to the active site, similar to the substrate
- binds to the active site and forms an enzyme-inhibitor complex blocking the active site from the substrate
- this causes less ESCs to form and the rate of the reaction is lowered
- however increasing the concentration of substrate, reduces the effects of the inhibitor as there is a greater chance of the substrate binding instead of the inhibitor
What is an example of a competitive inhibitor?
- ethanol is a inhibitor/antidote for ethylene glycol poisoning (antifreeze)
- ethylene glycol metabolises into oxalic acid if ingested, crystallises major organs (e.g. brain) which can be fatal
What is a non-competitive inhibitor?
- binds to an allosteric site on the enzyme (doesn’t compete with the substrate)
- this binding distorts the tertiary structure, which changes the shape of the active site
- enzyme is no longer complementary to its specific substrate, no ESC
- rate of reaction is decreased
- increase in substrate concentration has no effect
What is an example of a non-competitive inhibitor?
Cyanide-respiratory inhibitor
- inhibits the enzyme cytochrome C oxidase which releases ATP in respiration (fatal)
How are enzymes made?
- Culturing microbe in fermentation vessels as microbes produce enzymes as their normal metabolic activity
- microbes are the killed and enzymes are extracted and purified for use
Uses of immobilised enzymes: Biosensors
- Detects particular molecules (even at very low temperatures) turning a chemical signal into an electrical signal to accurately detect, identify and measure a substance
- immobilised enzymes have the substrate bind to them (the particular substance) which produces the chemical signal
- used for medical or environmental monitoring
- also used on test strips (e.g. detecting glucose in urine)
What is the effect of temperature on the rate of enzyme action?
- Increasing the temperature higher than the optimum increases the KE of molecules, molecules vibrate more
- This weakens and breaks H-bonds in enzymes changing the tertiary structure
- This alters the shape of the active site, substrate will never fit and rate of reaction drops as no ESCs form
- Enzyme is therefore denatured, permanent change in structure (irreversible)
- At low temperatures the enzyme is inactivated as enzymes have little kinetic energy (shape is unchanged and is reversible)
Explain the shape of the graph (mass of product v time - steep gradient which then plateaus)
- When the enzyme and substrate are first mixed together there are many substrate molecules so more active sites are filled by many successful collisions
- If conditions are optimal and there’s excess substrate, the enzyme concentration is the LF as there aren’t enough free active sites
- As the reaction continues there is less substrate (enzyme concentration is constant) substrate concentration is the limiting factor
- Eventually the line plateaus as all the substrate is used up
What are the 3 ways enzyme reactions can be measured?
- concentration of products produced (moldm^3)
- time taken for reaction to occur
- rate of reaction
How do enzymes change the activation energy?
- Enzymes lower the activation energy as the molecule shape changes when it enters the active site (less energy needed to break down the substrate)
- Reactions can occur at lower temperatures (heating would speed up the reaction but cause enzymes to be denatured)
What is an example enzyme for induced fit theory?
Lysozyme
- an anti-bacterial enzyme in human saliva, mucus and tears
Describe the ‘Induced Fit’ Model?
- substrate collides with a non-complementary active site
- active site fits more closely around substrate molecule, held in position by oppositely charged groups
- an enzyme-substrate complex is formed
- the change in enzyme shape places a strain on substrate which weakens bonds, breaks easily (lowers activation energy)
- products don’t fit into the active site anymore so products are released
Describe the ‘Lock and Key’ model?
Enzymes have a uniquely shaped active site which is complementary to the shape of one substrate type
- bind together to form an ESC and the the products leave the active site
Where are the 3 sites where enzymes act?
- Extracellular - outside the cell (e.g. digestive enzymes like amylase which is secreted from the salivary glands to mouth)
- Intracellular in solution - inside cells in a solution
- Intracellular membrane bound - attached to membranes
Uses of immobilised enzymes: High-fructose corn syrup (HFCS) manufacture
- multi-step process from starch
- many several immobilised enzymes used which requires different physical conditions
What is meant by ‘metabolism’?
All the reactions in the body, in a sequence called metabolic pathways, including anabolic and catabolic reactions which are catalysed by enzymes
- Anabolic = larger molecules synthesised
- Catabolic = larger molecules broken down into smaller molecules
What is a metabolic pathway?
A sequence of enzyme-controlled reactions in which a product of one reaction is a reactant of the next
What are the general properties of enzymes?
- globular proteins that are specific to one substrate and are biological catalysts
- R-groups determine bonds amino acids make with each other
- speed up reactions
- not used up or changed in the reaction
- high turn-over number
- require specific conditions
- lower activation energies
Where does a substrate bind on the enzyme?
The active site -> a pocket or cleft area which is complementary to a specific substrate
Why are enzymes used in large scale industrial production?
- speed up rate of reaction
- lower activation energies (lowers temperature and increases efficiency)
- less waste products from fewer side reactions
What are immobilised enzymes?
Enzyme molecules bound to an inert material, over which substrate molecules move
What are the advantages of immobilised enzymes?
- polymer matrix creates a microenvironment for enzymes, increases stability and function over wider ranges of temperature and pH
- products are not contaminated
- enzymes are easily recovered for reuse
- a sequence of columns can be used for continuous process of enzymes with different optimum conditions
- enzymes can be easily or removed, greater control of reaction
What are the disadvantages of using immobilised enzymes?
- if enzymes are held inside beads it would take time for the substrate to diffuse into it
- in absorption the enzyme may become detached
- alginate gel may alter the active site, reducing activity
- chemically bonding the enzymes is a complex and expensive process
- any contamination is costly because the whole system has to be shut down and the vessel re-sterilised
Uses of immobilised enzymes: lactose free milk
- this is an important industrial use of immobilised enzymes
- milk is passed down a column containing immobilised lactase
- lactose is hydrolysed into glucose and galactose
