2.1.4 Enzymes Flashcards
What is an enzyme?
- A globular protein with complex tertiary structure (some have a quaternary structure)
- Biological catalyst
- Speeds up a reaction without being used up
How do enzymes affect the activation energy?
They lower it
What happens when an enzyme is catalysing a breakdown?
- Fitting into the active site puts a strain on the bonds of the substrate
- They can be broken more easily
How are two substrate molecules joined?
- Enzyme holds them together
- They can bond more easily
What is intracellular enzyme action?
When enzymes are made and retained inside the cell
e.g. catalase, polymerase, lysosomal enzymes, ATPase, ATP synthase
What is catalase?
- An intracellular enzyme
- Breaks down hydrogen peroxide into water and oxygen
- 2H2O2 –> O2 + 2H2O
- Found in peroxisomes (vesicles in liver cells)
What is extracellular enzyme action?
- Secreted by cells and work outside of the cells
E.g. amylase
What is amylase?
- An extracellular enzyme
- Works outside of cells in saliva to catalyse the hydrolysis of starch into maltose
What is trypsin?
- An extracellular enzyme
- Hydrolyses peptide bonds of large polypeptides into smaller ones
- Smaller ones are eventually broken down to amino acids by other enzymes
What enzymes are in fungi?
- Extracellular
- Cells in fungal hyphae synthesise enzymes that are secreted outside the cell to digest organic matter
- Absorbed and used for growth
What is the structure of an enzyme?
Globular protein:
- 3D shape
- Almost spherical
- Soluble in water
Active site:
- Specific shape
- Where substrate binds to
What is the lock and key model?
- The old model
- Enzymes will only catalyse certain reactions because the active site is a specific shape to fit the substrate
- There is no change as active site is exactly complimentary
What is the induced fit model?
- New model
- Enzyme isn’t complimentary to the substrate
- Enzyme changes shape slightly as the substrate binds
How does temperature affect enzyme activity?
- At low temperature, enzyme is deactivated (temporary)
- As temperature increases so does kinetic energy
- The number of successful collisions with the active site increases the rate of reaction
- Optimum temperature is where the enzyme activity is at its highest as there will be the maximum amount of enzyme-substrate complexes
- When temperature is too high, vibrations break bonds that hold enzyme together so enzyme denatures and activity decreases
What is denaturation?
- Irreversible change to the tertiary structure of the active site
- No longer complementary to substrate
- No E-S complezes
How are enzymes in thermophiles adapted?
- Thermophiles live in very hot environments
- Enzymes have more hydrogen bonds and disulfide bridges in the enzyme protein molecules
- More resistant to temperature rises
- Cyanobacteria live between 40 and 85 degrees
- Spirochetes live between 40 and 73 degrees
What is Q10?
A factor by which the rate of enzyme activity increases when the temperature is raised by 10 degrees
What is the formula for Q10?
Q10 = rate at higher temperature/rate at lower temperature
How does pH affect enzyme activity?
- As you move away from the optimum temperature, enzymes start to denature and activity decreases
- H+ and OH- ions break the ionic and hydrogen bonds that hold the enzymes tertiary structure in place
- No longer able to form E-S complexes
- Either side of the optimum is permanent denaturation
- Optimum pH is the pH the enzyme works best at
What are the optimum pHs of enzymes?
- Pepsin is in the stomach with HCl so optimum is low (1-2)
- Amylase is in the mouth so needs to be neutral (6-7)
How do you control the pH in an investigation?
Buffer solutions
How does enzyme concentration affect enzyme activity?
- As enzyme concentration increases, rate of reaction increases as there are more active sites
- Substrate concentration is a limiting factor so eventually increasing enzyme concentration doesn’t increase ROR
How does substrate concentration affect enzyme activity?
- As substrate concentration increases, the rate of reaction increases
- More active sites are engaged
- More successful collisions with active sites
- Point of saturation (Vmax) is when all the active sites are filled
- Past this, enzymes concentration is a limiting factor so ROR doesn’t increase (graph plateaus)
What is a cofactor?
A non-protein which binds to enzymes to make them work
What is an inorganic cofactor?
- An inorganic ion or molecule
- Helps the enzyme and substrate to bind together but doesn’t directly participate in the reaction
- Not used up or changed
- Not inhibitory and enables the reaction
E.g. Chloride ions (Cl-) are cofactors for amylase
What are organic cofactors/coenzymes?
- Participate in the reaction and are changed
- They often act as carriers, moving chemical groups between enzymes
- Continually recycled during the process
E.g. NAD
What is a prosthetic group?
- A cofactor that is tightly bound to an enzyme
E.g. Zinc ions (Zn2+) are a prosthetic group for carbonic anhydrase so are a permanent part of the enzyme’s active site
What are competitive inhibitors?
- Similar shape to active site and substrates
- Bind to the active site but no reaction takes place
- This blocks the active site and leads to a slower reaction
E.g. Malonate is a competitive inhibitor for succinate dehydrogenase
What are non-competitive inhibitors?
- These bind to the allosteric site
- Causes a change in shape of the active site
- The substrate molecule can no longer fit so enzyme-substrate complexes are no longer able to be formed
E.g. Copper sulfate is a non-competitive inhibitor for catalase
How do the graphs of inhibitors compare?
- A competitive inhibitor will have a slower rate of reaction but still reach the same end point
- A non-competitive inhibitor has a slower rate of reaction and lower end point as the enzyme’s active site is permanently damaged
- All 3 lines start from the same point
What is a reversible inhibition?
- Inhibitor can be removed
- Due to weaker hydrogen bonds or weak ionic bonds
What is a non-reversible inhibition?
- Inhibitor cannot be removed easily
- Due to strong covalent bonds
What do metabolic poisons do?
They interfere with metabolic reactions in cells
What is cyanide?
- Non-competitive inhibitor of cytochrome C oxidase
- Involved with respiration
What is arsenic?
- Non-competitive inhibitor of pyruvate dehydrogenase
- Involved with glycolysis
What is malonate?
- Competitive inhibitor of succinate dehydrogenase
- Involved with the kreb cycle
