Enzymes Flashcards
Enzymes are “biological catalysts.” Define this term.
- Biological = they function in living systems
- Catalyst = they speed up the rate of chemical reaction without being used up or chemically changed - they provide an alternative pathway for the reaction that they catalyse with a lower activation energy.
What are enzymes?
- Globular proteins - always have at least tertiary structure (some even have quaternary eg trypsin)
- Control metabolic pathways
- All metabolic reactions within any living organism is catalysed by an enzyme (therefore they are essential to life)
Talk about the active site?
- Specific substrates bind to it
- Forming an enzyme-substrate complex
- Complementary to only one specific substrate
- Shape is determined by the tertiary structure of the protein
Give the 3 types of Enzymes, define them and give an example of each
1) Intracellular - produced and function inside the cell (eg DNA Polymerase, ATP hydrolase)
2) Extracellular - secreted by cells and catalyse reactions outside of cells (eg lipase, pepsin)
3) Membrane-bound - in the phospholipid bilayer (eg dipeptidases, maltase)
- Refer to SME example on amylase and catalase
Difference between catabolic and anabolic enzymes and give an example
Catabolic = catalyse breakdown of larger, complex molecules into smaller, simpler ones eg amylase
Anabolic = catalyse building up of larger, complex molecules from smaller, simpler ones eg DNA Polymerase or glycogen synthase
Define denaturation
- When extreme temperature or pH change the shape of the active site
- prevents substrate binding
How does an enzyme lower the activation energy?
- They put a strain on the bonds in the reactant(s)
- Making the bonds less stable and therefore more reactive
- Therefore increasing the rate of reaction
Conditions needed for an enzyme-controlled reaction to occur?
- Enzyme and substrate must be in solution
- Substrate must collide with the enzyme’s active site at the correct orientation and speed
Describe the Emil Fischer Lock and Key Hypothesis (1890)
- Substrate fits precisely into the enzyme’s active site
- Enzyme-substrate complex is formed
- Reaction takes place and enzyme-product complex is formed
- Products are released
This was modified to the induced-fit hypothesis, which is very similar. Describe this.
- The enzyme’s active site does not fit perfectly into the active site
- But when substrate is near/entering, the active site changes shape slightly (conformational change) allowing substrate to bind
- Enzyme-substrate complex is formed
- Reaction takes place and enzyme-product complex is formed
- Products are released
Why are enzyme-controlled reactions slow at low temperatures?
- Molecules move slowly therefore don’t have much kinetic energy
- Lower frequency of successful collisions between substrate molecules and active sites of enzymes
- Less frequent enzyme-substrate complex formation
Why do higher temperatures speed up the rate of enzyme-controlled reactions?
- Molecules move faster and so kinetic energy increases
- Higher frequency of successful collisions between substrate molecules and active sites of enzymes
- More frequent enzyme-substrate complex formation
Why does the rate drop sharply past the optimum temperature?
- Kinetic energy increases to the point that molecules vibrate so much that the hydrogen bonds break
- Therefore enzymes denature
- The tertiary structure of the protein changes
- Permanently damaging the active site
- Prevents substrate from binding
Optimum temperature of most enzymes in the human body
37-40°C
Optimum pH meaning
- The pH at which the enzyme operates best
- Specific to each enzyme
What happens either side of the optimum pH?
- Enzyme denatures (but more gradually than past optimum temperature)
- Hydrogen and ionic bonds in the tertiary structure break due to an increase in H+ or OH- ions
- Alters the shape of the active site
- Enzyme substrate complexes form less easily (or not at all if pH difference is extreme)
Equation for pH
pH = -log₁₀ [H⁺]
Describe the relationship between enzyme concentration and rate of reaction
- At first, as the enzyme concentration increases, the RoR increases LINEARLY
- This is because there are more active sites available, increasing the likelihood of enzyme-substrate complexes forming
- However, if the substrate concentration is limited, and the enzyme concentration continues to increase, the RoR stays the same as substrate concentration becomes the limiting factor
Describe the relationship between substrate concentration and rate of reaction
- At first, as the enzyme concentration increases, the RoR increases LINEARLY
- This is because more active sites are filled, increasing the likelihood of enzyme-substrate complexes forming
- However, if the enzyme concentration is limited and the substrate concentration is increased past a certain point, the RoR stays the same
- This is because eventually all active sites become saturated
- So substrates have no site to bind to
Define competitive inhibitor
- A molecule with a similar shape to the substrate
- That reversibly/irreversibly binds to the active site of the enzyme
- Therefore competing with the substrate for the active site
- Can be countered by increasing the concentration of substrate
Define non-competitive inhibitor
- A molecule which binds to an allosteric site of the enzyme
- Which alters the shape of the active site
- And prevents the substrate from binding to it
- Cannot be overcome by increasing substrate concentration
Are reversible inhibitors ever useful? (Obviously irreversible ones aren’t)
- Yes
- Can act as regulators in metabolic pathways
- So that the reaction is tightly controlled and balanced and no enzyme “runs wild”
- Refer to the negative feedback loop on SME
