Flashcards in Enzymes Deck (32)
what are enzymes?
- biological catalysts
- globular proteins so soluble
enzymes catalyse reactions at which level?
- at cellular level e.g. respiration
- at whole organism level e.g. digestion in mammals
enzymes affect what?
- structure e.g. involved in collagen production (important protein in connective tissue of animals)
- function e.g. respiration
What is an intracellular enzyme?
enzymes that work inside cells
- H2O2 - hydrogen peroxide is a toxic by-product of several cellular reactions. can kill cells if left to build up.
- Catalase works inside cells to catalyse breakdown of H202 to harmless 02 and H20
What is an extracellular enzyme?
works outside cells
e.g. Amylase and Trypsin
- works in digestive system and is found in saliva
- secreted in the mouth by cells in salivary glands
- catalyses hydrolysis of starch into maltose (sugar) in the mouth
- works in digestive system and is produced by pancreas
- secreted in small intestine
- catalyses hydrolysis of peptide bonds (turns big polypeptides to smaller polypeptides)
What is a catalyst?
substance that speeds up chemical reactions without being used up in reaction
What is an active site?
- enzymes have active site with specific shape
- it is part of enzyme that substrate molecules bind to
- the specific shape of active site is determined by enzymes tertiary structure.
- active site is only formed by a few amino acids and is usually a cleft/depression
What must happen for an enzyme to work?
a substrate with a complementary shape to the enzyme's active site must bind to it.
if substrate shape is not complementary, reaction will not be catalysed so enzymes work with only few substrates
What is the lock and key hypothesis?
- substrate with a complementary shape to the active site binds to it forming enzyme-substrate complex.
- substrate changes into product but is not released from enzyme forming enzyme-product complex.
- products are released and the enzyme remains unchanged after the reaction.
What is the induced fit hypothesis?
- As substrate binds to active site, active site changes shape, slightly to fit substrate more closely together
- enzyme-substrate complex forms and the bond son the substrate are strained so more likely to react.
- substrate is held in the active site by temporary bonds between substrate and R-groups of amino acids.
- enzyme-product complex forms and product is released
What is activation energy?
- it is the certain amount of energy that needs to be supplied to the chemicals before the reaction will start
- often provided as heat
What do enzymes do to activation energy?
- enzymes reduce activation energy needed so reactions happen at a lower temperature than they could without enzymes.
- they speed up the rate of the reaction
How does the formation of an enzyme-substrate complex lower the activation energy?
- if 2 substrates need to be joined, attaching to an enzyme holds them close together, reducing any repulsion between the molecules so the bond more easily
- if enzyme is catalysing a breakdown reaction, fitting into active site places a strain on the bonds in the substrate. Strain means substrate molecules are more likely to break up more easily.
Which factors affect rate of reaction?
How does temperature affect rate of reaction?
- increase in temperature increases rate of reaction
- more heat - more kinetic energy - molecules move faster - higher likelihood of collisions between substrates and enzymes
What happens if temperature is too high?
- reactions stops if temperature is too high
- increase in temperature makes enzymes vibrate more - vibration breaks some of the bonds that hold the enzyme in shape if passed optimum temperature - active site changes shape so enzyme and substrate no longer complementary so don't fit together - enzyme denatured so cant function as catalyst
What is temperature coefficient (Q10)?
- Q10 value for reaction shows how much the reaction rate changes when temperature is raised by 10 degrees.
- at temps before optimum, Q10 value of 2 means rate of reaction doubles when raised by 10 degrees
- Q10 value of 3 means rate of reaction trebles.
- most enzyme controlled reactions have a Q10 value of 2.
How does pH affect rate of reaction?
- all enzymes have an optimum pH value
- most human enzymes work best at pH 7
- Pepsin works best at acidic pH 2 - useful as found in stomach
- above and below optimum pH, H+ and OH- ions found in acids and alkalis can mess up ionic bonds and hydrogen bonds which hold enzymes tertiary structure in place. This makes active site change shape so enzyme is denatured.
How does enzyme concentration affect rate of reaction?
- higher enzyme concentration increases rate of reaction
- more enzymes- greater likelihood of collisions with substrate - more likely to form enzyme-substrate complex
- if substrate amount is limited, there's a point where there is more than enough enzymes to deal with all the available substrate so adding more enzyme has no effect.
How does substrate concentration affect rate of reaction?
- more substrate, faster the reaction
- more substrate - greater likelihood of collisions with enzyme - more active sites used up
- saturation/ Vmax point
- so many substrates that all active sites are occupied so adding more substrate has no affect for a certain point
- substrate concentration decreases with time during reaction
- if no variables are changed, rate of reaction decreases over time
- initial rate of reaction is highest rate of reaction.
How to measure rate of enzyme-controlled reaction?
- measure how fast product of reaction appears
- measure how fast substrate disappears
What are cofactors?
- some enzymes work only if another non-protein substance is bound to them
- the non-protein substance is a cofactor
- some cofactors can be inorganic or ions.
- they help enzyme and substrate to bind together
- they don't directly participate in reaction so aren't used up or changed
e.g. Cl- are cofactors for amylase
What are co-enzymes?
- some cofactors are organic and are called coenzymes
- participate in reaction and are changed by it
- act as carriers by moving chemical groups between different enzymes
- continuously recycled in this process
e.g. vitamins are sources of coenzymes
- if cofactor is tightly bound to enzyme - its known as prosthetic group
e.g. Zn2+ are prosthetic group for carbonic anhydrase (enzyme in red blood cells that catalyses production of carbonic acid from H20 and CO2
- zinc ions are permanent parts of enzymes active site
What are inhibitors?
- they inhibit enzyme activity
- inhibition can be competitive or non-competitive