S4) Enzymes Flashcards

1
Q

What are enzymes and what do they do?

A

Enzymes are protein catalysts that mediate metabolic pathways by increasing the velocity of chemical reactions while not being consumed during the reaction

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2
Q

Identicy 5 characteristics of enzymes

A
  • Highly specific
  • Unchanged after reaction
  • Increase rate of reaction
  • Proteins
  • Does not affect reaction equilibrium
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3
Q

What is an active site?

A

An active site is the special cleft on an enzyme containing amino acid side chains that bind substrates and trigger catalysis

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4
Q

Identify 4 characteristics of active sites

A
  • Cccupies a small part of the enzyme
  • Formed by amino acids
  • Clefts/crevices
  • Bind to substrates through weak bonds
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5
Q

Identify 2 different hypotheses which govern substrates binding to an enzyme

A
  • Lock and key hypothesis – active sites have a complementary shape to the substrate
  • Induced fit hypothesis – binding of substrates can induce chages in the comformation of the active site
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6
Q

Describe the 5 steps involved in enzyme catalysis

A
  • Substrate binds to active site on enzyme
  • Conformational change occurs and ES complex forms
  • Catalysis occurs
  • ES complex is converted to EP complex
  • EP compex dissociates to enzyme and product
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7
Q

What is an allosteric site?

A

An allosteric site is a cleft on an enzyme where an activating/inhibiting molecule may bind, changing the shape of the enzyme and influencing its ability to be active

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8
Q

Outline the specificity of enzymes

A
  • Enzymes are highly specific, interacting with 1/few substrates and catalysing only one type of chemical reaction
  • The set of enzymes made in a cell determines which metabolic pathways occur in that cell
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9
Q

Can enzymes be regulated?

Why is this needed?

A

Enzyme activity can be regulated (increased or decreased) so that the rate of product formation responds to cellular need

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10
Q

Why are many enzymes localised in specific organelles within the cell?

A

Compartmentalisation serves to:

  • Isolate the reaction substrate/product from other competing reactions
  • Provides a favourable environment for the reaction
  • Organises thousands of cell enzymes into purposeful pathways
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11
Q

What is activation energy?

A

Activation energy is the minimum energy required by a chemical system to start a chemical reaction

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12
Q

How do enzymes impact activation energy in a chemical reaction?

A
  • Activation energy is usually very high, leading to slow rates in uncatalyzed chemical reactions
  • Enzymes work to lower activation energy, so more particles are able to overcome the transition state and react
  • Rate of reaction increases
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13
Q

What is the rate of reaction?

A

The rate of a reaction is the number of substrate molecules converted to product per unit time ( [product]/time )

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14
Q

How does the rate of reaction vary in the presence of an enzyme?

A

The rate of an enzyme-catalysed reaction increases with substrate concentration until a maximal velocity (Vmax) is reached

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15
Q

Why does the rate of reaction level off in enzyme-catalysed reactions?

A

The levelling off of the reaction rate at high substrate concentrations reflects the saturation of all available enzyme active sites with substrate

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16
Q

Identify and describe 2 factors which increases the rate of reaction

A
  • Temperature: increases no. of molecules with activation energy
  • Concentration: increases chance of molecular collisions
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17
Q

In Michaelis-Menten kinetics, what type of curve is shown when measuring the rate of reaction for an enzyme-catalysed reaction?

A

Rate of reaction is hyperbolic

18
Q

For allosteric enzymes, what type of curve is shown when measuring the rate of reaction for an enzyme-catalysed reaction?

A

Rate of reaction is sigmoidal e.g. oxygen dissociation curve of haemoglobin

19
Q

Discuss the effect of temperature on reaction velocity

A
  • Initially, an reaction velocity increases with temperature due more molecules surpassing the energy barrier of the reaction
  • Reaction velocity then decreases due to the temperature-induced denaturation of the enzyme
20
Q

What is the optimum temperature for most human enzymes?

A

Between 35 and 40°C

21
Q

Discuss the effect of pH on enzyme function

A

Extremes of pH lead to enzyme denaturation due to a conformational change the structure of enzyme (active site)

22
Q

What is the optimum pH for human enzymes?

A

The optimum pH varies for different enzymes and often reflects the [H+] at which the enzyme functions in the body

23
Q

State the Michaelis-Menten equation

A
  • V = initial reaction velocity
  • Vmax = maximal velocity
  • Km = Michaelis constant
  • [S] = substrate concentration
24
Q

What is Km in Michaelis-Menten kinetics?

A
  • Km is numerically equal to the substrate concentration at 1⁄2 Vmax
  • Km does not vary with the concentration of enzyme
25
Q

What is the significance of Km?

A
  • Km is characteristic of an enzyme and its particular substrate
  • It reflects the affinity of the enzyme for that substrate
26
Q

Compare and contrast the enzyme affinity in terms of a small Km and large Km

A
  • Small Km: high affinity of the enzyme for a substrate as a low [substrate] is needed to half-saturate the enzyme
  • Large Km: low affinity of enzyme for substrate as a high [substrate] is needed to half-saturate the enzyme
27
Q

Describe the relationship of velocity to enzyme concentration in Michaelis-Menten kinetics

A

The rate of the reaction is directly proportional to the enzyme concentration at all substrate concentrations

E.g. if the [enzyme] is halved, the rate of reaction is halved

28
Q

What is a Lineweaver-Burk plot?

A

A Lineweaver-Burk plot plots 1/V0 versus 1/[S] to obtain a straight line graph for Michaelis-Menten kinetics

  • It is used to calculate Km and Vmax, as well as to determine the mechanism of action of enzyme inhibitors
29
Q

What are the values of the x and y intercepts in a Lineweaver Burk plot?

A
  • X-intercept is equal to −1/Km

- Y-intercept is equal to 1/Vmax

30
Q

What is an inhibitor?

A

An inhibitor is any substance that can diminish the velocity of an enzyme-catalysed reaction

31
Q

Distinguish between the bonding of irreversible and reversible inhibitors

A
  • Irreversible inhibitors bind to enzymes through covalent bonds
  • Reversible inhibitors bind to enzymes through non-covalent bonds
32
Q

Identify 2 forms of reversible inhibition

A
  • Competitive inhibition
  • Non-competitive inhibition
33
Q

Outline competitive inhibition

A
  • Inhibitor binds reversibly to the same site the substrate would occupy
  • Inhibitor competes with the substrate for the active site
  • Increasing [substrate] reduces the effect of inhibitor
34
Q

Describe the effect of competitive inhibition on the Vmax and Km

A
  • Effect on Vmax: effect is reversed by increasing [S] & rate of reaction reaches Vmax
  • Effect on Km: increases the Km for a given substrate as more substrate is needed to achieve 1⁄2 Vmax
35
Q

Describe the Lineweaver-Burk plot for competitive inhibition

A
  • Inhibited and uninhibited reactions intersect on the y-axis at 1/Vmax (Vmax is unchanged)
  • The inhibited and uninhibited reactions show different x-axis intercepts (Km is increased)
36
Q

Outline non-competitive inhibition

A
  • Inhibitor binds reversibly to allosteric site on the enzyme
  • Inhibitor does not compete with the substrate for the active site
  • Increasing [substrate] has no effect on inhibition as active site has changed shape
37
Q

Describe the effect of competitive inhibition on the Vmax and Km

A
  • Effect on Vmax: decrease the Vmax of the reaction as inhibition is not overcome with increasing [substrate]
  • Effect on Km: same value of Km as inhibitors do not interfere with the binding of substrate to enzyme
38
Q

Describe the Lineweaver-Burk plot for non-competitive inhibition

A
  • Inhibited and uninhibited reactions intersect on the x-axis at 1/Km (Km is unchanged)
  • The inhibited and uninhibited reactions show different y-axis intercepts (Vmax is decreased)
39
Q

What are amyloid fibres?

A

An amyloid fibre is the misfolded, insoluble form of a normally soluble protein

40
Q

Describe the structure of amyloid fibres

A
  • Highly ordered with a high degree of ß-sheet
  • Core ß-sheet forms before the rest of the protein
  • Stabilised by hydrophobic interactions between aromatic amino acids