Chapter 4: Enzymes Flashcards
What is the role of enzymes?
- To catalyse reactions that affect metabolism at a cellular and whole organism level
E.g. synthesising cell/organism components for growth, releasing energy through digestion - Speed up chemical reactions by lowering the activation energy of the reaction
- do not get used up
What is an example of an intracellar enzyme? What does it catalyse?
Enzymes that act within cells
E.g. Catalase:
- present inside peroxisomes (membrane-bound organelle in cytoplasm of eukaryotic cells)
- breaks down toxic hydrogen peroxide into water and oxygen
- found in the liver (high amounts of peroxisomes involved in excretion)
What is an example of an extracellular enzyme? What does it catalyse?
Enzymes that work outside of the cell that made them to make use of polymers for nutrition
E.g. Amylase:
- hydrolyses starch into maltose which is further broken down into glucose by maltase
- found in the saliva
E.g. Trypsin:
- hydrolysis of polypeptides into polypeptide fragments and then amino acids
- produced by pancreatic cells and is released into the small intestine
What is an anabolic reaction?
Build up from smaller to larger molecules
What is a catabolic reaction?
Break down from larger to smaller molecules
Mechanism of enzyme action
- Enzymes collide with substrates (at high temp. and pressure) to reduce the activation energy required for reactions to start
- active site within the tertiary structure of the enzyme has a shape that is complementary to the specific substrate molecule
- substrate binds to active site to form an enzyme-substrate complex
- substrates react and the products are formed in an enzyme-product complex
- products are released and enzymes (unchanged) are able to take part in other reactions
Lock and Key Hypothesis
Only the specific substrate will fit into the active site of a complementary enzyme
- Molecules need to collide in the right orientation so the R-groups within the active site of the enzyme are close enough to interact with the substrate (form temporary bonds)
Induced-Fit Hypothesis
Active site changes shape slightly as substrate enters
- Initial interaction between enzyme and substrate is relatively weak
- weak interactions rapidly induce change in enzyme tertiary structure that strengthens binding, strains substrate molecule
- can weaken bonds in substrate which lowers activation energy of reaction
Effect of pH
As pH increases, rate increases until optimum, then decreases until rate is O (symmetrical curve)
- Concentration of hydrogen ions alters the shape of the active site
- reduces R-group interactions in the tertiary structure
- changes ionic charges in bonds, has amphoteric properties
- if pH changes significantly enzyme cannot form enzyme-substrate complex and becomes denatured
E.g. Pepsin in gastric juice:
favours acidic conditions (low optimum pH 1-2)
Found in stomach acid (HCl)
E.g: Trypsin in pancreatic juice:
Favours slightly alkaline conditions (optimum pH 8)
Found in small intestine
Effect of temperature
As temperature increases, rate of reaction increases up to an optimum then decreases rapidly
- Increasing temperature increases the kinetic energy of the particles
- more frequent successful collisions between a substrate and enzyme, increased rate of reaction
- at high temperatures the bonds in the tertiary structure are disrupted (hydrogen bonds, ionic bonds, hydrophilic/hydrophobic interactions)
- active site changes shape and can no longer form enzyme-substrate complex, enzyme denatures
Temperature coefficient (Q10)
Measure of how much the rate of reaction increases with a 10°C rise in temperature
Q10 = rate of reaction at T + 10°C / rate of reaction at T°C
- many enzymes have a Q10 between 2-3
Effect of enzyme concentration
Rate of reaction increases as the enzyme concentration increases up to maximum rate (Vmax)
- More enzymes increases the number of available active site in an area
- enzyme-substrate complexes formed at a faster rate
- rate increases until Vmax when enzyme finishes catalysing the available substrates and the reaction stops
Effect of substrate concentration
The rate of reaction increases as the substrate concentration increases up to a maximum rate (Vmax)
- More substrate molecules increased collisions with active sites of enzymes to form enzyme-substrate complexes at a faster rate (substrate concentration is a limiting factor)
- rate of reaction increases until Vmax (point of saturation -all active sites occupied) where no more enzyme-substrate complexes can form until products are released from active sites (enzyme concentration becomes the limiting factor)
Cofactors
Inorganic non-protein component necessary for the effective functioning of an enzyme
Temporarily bind to an enzyme protein in order to activate them
- precursor enzymes are inactive and need to undergo a change in shape (of active site) to be activated
- apoenzymes (inactive) have a cofactor added to become a holoenzyme (active)
Obtained via the diet as minerals:
E.g. amylase requires a chloride ion (Cl-) to catalyse the break down of starch
Coenzymes
Organic cofactors (non-protein component necessary for the effective functioning of an enzyme) Temporarily bind to an enzyme protein in order to activate them
Obtained from vitamins in the diet:
E.g. vitamin B3 used to synthesise NAD responsible for the transfer of hydrogen between molecules during respiration / NADP in photosynthesis
E.g. vitamin B5 used to synthesise coenzyme A essential in the breakdown of fatty acids and carbohydrates in respiration
Prosthetic groups
Organic / inorganic cofactors (non-protein component necessary for the effective functioning of an enzyme)
Permanent feature of the protein structure
Obtained via the diet as minerals:
E.g. carbonic anhydrase has Zinc ions (Zn+2) for the metabolism of CO2
Inactive precursors
Enzymes are produced as inactive precursors when:
- can cause damage within cells that produce them / tissues where released
- enzyme action needs to be controlled / only activated in certain conditions
Change in tertiary structure of apoenzyme can be brought about by
- another enzyme breaks certain bonds in the molecule
- change in conditions (pH, temperature) : proenzymes
E.g. inactive pepsinogen released into stomach -> acid pH transforms to active pepsin enzyme
- protects body tissues against digestive action of pepsin
E.g. successive enzyme activations in blood clotting (coagulation cascade)
Competitive inhibition
- molecule / part of molecule with similar shape to substrate can fit into active site of enzyme
- blocks the substrate from forming an enzyme-substrate complex so enzyme cannot catalyse the reaction
- reduces number of substrate molecules binding to active sites and slows rate of reaction
- if substrate competition increases enough then Vmax can still be reached unchanged
Most competitive inhibitors bind temporarily - reversible effect
E.g. Statins used to reduce blood cholesterol concentration
E.g. Aspirin irreversibly inhibits synthesis of prostaglandins and thromboxane (chemicals produce pain + fever) at active site of COX enzymes
Non-competitive inhibitor
- inhibitor binds to allosteric site (location other than active site) which causes the active site to change shape due to the change in tertiary structure
- the active site shape is no longer complementary to the specific substrate so it is unable to form an enzyme-substrate complex
- active sites become unavailable and cannot catalyse the reaction and effect cannot be overcome by increasing enzyme/substrate concentration
Irreversible non-competitive inhibitors often very toxic:
E.g. Organophosphates (in insecticides/herbicides) inhibit enzyme acetyl cholinesterase necessary for nerve impulse transmission
- causes muscle cramps, paralysis, death
E.g. Proton Pump Inhibitors block enzyme system responsible for secreting H+ into stomach to reduce production of excess acid
- forms stomach ulcers
Product inhibition
Non competitive reversible inhibition
Product of a reaction acts as an inhibitor for enzyme that produces it
- serves as negative feedback control mechanism for reaction
metabolic pathway respiration e.g. ATP production regulation by PFK enzyme
- when ATP levels are high more binds to allosteric site on PFK
prevent addition of 2nd phosphate group to glucose
glucose cannot break down and ATP produced at slower rate
- ATP used up so less binds to PFK
enzyme is able to catalyse addition of 2nd phosphate group to glucose
glucose breaks down in respiration and more ATP is produced
