Session 3 ILO's - Enzymes Flashcards
Explain the effects of enzymes on chemical reactions
State the key features of enzymes
- Enzymes are catalysts that speed up the rate of reaction
- They do not alter the equilibrium of a chemical reaction (no net change in the amounts of reactants and products occurs)
- But it accelerates the rate by which the equilibrium is reached
- Enyzmes work by providing an alternative reaction pathway with a lower activation energy than the original reaction.
- They lower the activation energy by promoting formation of the transition state.
- The structure of the enzyme remains unchanged after use.
- Can be affected by pH, temperature and substrate conc ect…
Explain the role of the active site of an enzyme
- The active site allows a substrate to bind to the specific region of an enzyme & forms the enzyme-substrate complex (induced fit)
- The active site has a complementary shape to the substrate giving selectivity to this binding (highly specific)
- The active site is formed by amino acids from different parts of the primary sequence and reaction between the enzyme and the substrate is stabilised by surrounding amino acids within the active site
- The substrate is bound to the active site by multiple weak, non-covalent bonds (Van der Waals, Hydrogen bonds and ionic) and these can be dissolved to allow the substrate to unbind again and release the products
- Note - the active site occupies a small part of the enzyme (formed from a small number of amino acids)
- Active sites are clefts or crevices (in order to exclude water)
What is the transition state?
High energy intermediate
molecule that lies between
being a substrate and product
Name 3 ways that enzymes speed up a reaction
• Hydrolysis – add WATER
• Oxidation – remove H atoms
• Phosphorylation – add Pi
using ATP
Describe how reaction rates vary as a function of enzyme concentration (assuming substrate concentration stays the same)
(Describe how concentration of enzyme affect rates of reaction … I think)
Rate of an enzyme-catalysed reaction is directly proportional to the enzyme concentration i.e. if you double conc. of enzyme, the rate of reaction doubles (because the substrate is not limiting)
Describe how reaction rates vary as a function of substrate concentration (assuming enzyme concentration stays the same)
(Describe how concentration of
substrate affect rates of reaction … I think)
Rate of an enzyme-catalysed reaction increases as the substrate concentration increases, however this does plateau as the enzymes become saturated
Define Km and what are the units?
Michaelis Constant
Substrate concentration that gives half the maximal rate of reaction (Vmax). Therefore, a lower Km suggests a higher affinity of the substrate for the enzyme.
(KM IS NOT HALF OF V MAX)
- Work out Vmax
- Find half of Vmax
- Find the substrate concentration that corresponds to that value
- Measured in units of concentration (e.g. Moles, M)
Define Vmax and what are the units?
Theoretical maximum rate of reaction when all of the enzyme molecules are saturated with substrate and they are proceeding as quickly as they can
- Measured in units of rate (mol/min)
Define the 2 terms of activity & international unit of enzyme activity
Vmax and Vo(Initial Rate of reaction) are rates of activity
1 unit of activity/ 1 international unit of enzyme activity = the amount of enzyme that produces 1umol of product per min under standard conditions.
- Often expressed as a standardised rate e.g. per litre (L) of serum or per gram (g) of tissue e.g. 3ml/g of tissue
(we could also say that it’s one umol of substrate converted/min)
How do you work out the rates of an enzyme-catalysed reaction?
When the graph is product/substrate over time)
- Draw a tangent to the curve and work out the gradient of that curve and
- To work out initial ROR, you do the tangent at time = 0
Describe the effects of competitive inhibition on enzyme kinetics
- Possess a similar shape to that of the substrate molecule and compete with the substrate for the active site of the enzyme.
- Reduces proportion of E-S complexes formed
- Therefore, fewer substrate molecules can bind to the enzymes so the reaction rate is decreases the concentration of functional enzyme
Are competetive inhibitors reversible or irreversible? And explain this
• Competitive inhibitors are reversible, and their activity can be overcome by adding more substrate. The substrate can displace the inhibitor at the active site and the reaction will proceed as normal
Describe the effects of non competitive inhibition on enzyme kinetics
- In noncompetitive inhibition, the inhibitor binds at an allosteric site separate from the active site of substrate binding
- The inhibitor changes the conformation of the enzyme.
- The substrate can no longer bind, or it may be able to bind but the active site cannot catalyse the reaction, or catalyses it at a slower rate (decreases the concentration of functional enzyme)
In a chemical reaction, what do competitive inhibitors cause, in terms of Km and Vmax
- In a chemical reaction, competitive inhibitors cause: Km to increase, Vmax to stay the same
Are non-competitive inhibitors reversible or non-reversible, and explain what this means?
Non-competitive inhibitors are irreversible, and so their activity cannot be overcome by addition of the substrate
Show how an inhibitor would alter the shape of v vs [S] plot
(or Lineweaver-Burk plot)
Determine the type of inhibition from an of v vs [S] plot (or
Lineweaver-Burk plot)
What is this graph called?
Lineweaver-Burk plot
Summarise the Michaelis-Menten equation
Sketch a graph that shows the effect of substrate concentration on reaction velocities for two enzymes: enzyme 1 with a small Michaelis constant (Km) and enzyme 2 with a large Km.
Vmax = maximal velocity.
What does a competitive inhibitor substrate vs rate of reaction graph look like? Roughly describe it.
What does a non - competitive inhibitor substrate vs rate of reaction graph look like? Roughly describe it
