Enzymes

Question 1

Enzymes ___ reaction rate by ___ activation energy, making it easier for substrate to reach transition state. They ___ alter equilibrium constant, overall free energy change of reaction, and enthalpy change of reaction. Enzymes are sensitive to ___, ___, and ___ (in vitro).

Answer 1

• Increase, decreasing.
• DO NOT.
• Temperature, pH, salinity.

Question 2

Oxidoreductase

Answer 2

• Catalyze oxidation-reduction reactions (transfer of electrons) between biological molecules.
• Often have an electron-carrying cofactor, such as NAD+ or NADP+.
• Dehydrogenase, reductace, oxidase.

Question 3

Transferase

Answer 3

• Catalyze the movement of a functional group from one molecule to another.
• Transferase, kinase (transfers phosphate groups generally from ATP to another molecule).

Question 4

Hydrolase

Answer 4

• Catalyzes the breaking of a compound into two molecules by adding water (hydrolysis).
• Phosphatase (cleaves phosphate group from another molecule), peptidase, nuclease, lipase.
• Phosphorylase: Breaks bonds using inorganic phosphate, instead of water.

Question 5

Lyase

Answer 5

• Catalyzes the cleavage of a single molecule into two products without water (lyase) or the synthesis of small molecules into a single molecule (synthase).

Question 6

Isomerase

Answer 6

• Catalyzes the rearrangement of bonds within a molecule, such as stereoisomers and constitutional isomers.
• Mutase: Transfers functional group from one position to another on a molecule.
• Can also be classified as oxidoreductase, transferase, or lyase, depending on mechanism of enzyme.

Question 7

Ligase

Answer 7

• Catalyzes addition or synthesis reactions between large similar molecules, often with the use of ATP.
• Used for nucleic acid synthesis and repair.

Question 8

Endergonic reactions ___ energy.

Exergonic reactions ___ energy.

Answer 8

• Take in (delta G > 0).

* Give off (delta G < 0).

Question 9

___ + Cofactors/Coenzymes = ___

Answer 9

• Apoenzymes + Cofactors/Coenzymes = Holoenzymes.
• Cofactor: Inorganic molecules or metal ions; dietary minerals, such as magnesium or iron.
• Coenzymes: Small organic groups; vitamins or vitamin derivatives, such as NAD+ or coenzyme A.

Question 10

Describe this Michaelis-Menten Plot

Answer 10

• Rate of reaction of enzyme (v) depends on substrate concentration [S].
• vmax: Maximum enzymatic reaction rate at saturation. Increasing substrate concentration beyond this point will not further increase reaction rate.
• Km (Michealis Constant): Substrate concentration at 1/2 vmax (when half of the enzyme’s active sites are full); Used to measure enzyme’s affinity for substrate (low Km means high affinity).

Question 11

Catalytic Efficiency & kcat

Answer 11

• kcat: Turnover rate of substrate to product; vmax = [E]kcat.
• Catalytic Efficiency: kcat/Km; Large kcat (high turnover) or small Km (high substrate affinity) result in high catalytic efficiency.

Question 12

Give the Michealis-Menten Equation in terms of vmax.

Answer 12

Question 13

Give the Michealis-Menten Equation in terms of kcat.

Answer 13

Question 14

Describe the Lineweaver-Burk Plot.

Answer 14

• Double reciprocal graph of Michealis-Menten Plot.
• x-intercept = -1/Km.
• y-intercept = 1/vmax.

Question 15

What does a Michealis-Menten Plot of a Cooperative Enzyme look like? Describe Hill’s Coefficient.

Answer 15

• Cooperative enzymes are described by a sigmoid curve (instead of normal hyperbola) on the Michealis-Menten Plot.
• If Hill’s Coefficient > 1, positively cooperative binding is occurring (after a ligand binds, enzyme’s affinity for another ligand INCREASES).
• If Hill’s Coefficient < 1, negatively cooperative binding is occurring (after a ligand binds, enzyme’s affinity for another ligand DECREASES).
• If Hill’s Coefficient = 1, enzyme does not exhibit cooperative binding.

Question 16

Competitive Inhibition

Answer 16

• Inhibitor binds to active site, directly preventing substrate from accessing the active site.
• Can be overcome by adding more substrate.
• Adding a competitive inhibitor causes Km to increase but does NOT alter vmax. Line pivots up about y-intercept.

Question 17

Noncompetitive Inhibition

Answer 17

• Inhibitor binds to allosteric site, inducing a conformational change and thereby preventing substrate from accessing the active site.
• Can NOT be overcome by adding more substrate. Noncompetitive inhibitor binds equally well to the enzyme and the enzyme-substrate complex.
• Adding a noncompetitive inhibitor causes vmax to decrease but does NOT alter Km. Line pivots up about x-intercept.

Question 18

Uncompetitive Inhibition & Mixed Inhibition

Answer 18

• Uncompetitive Inhibition: Inhibitor binds to allosteric site of only E-S complex. Km and vmax both decrease. On L-B Plot, curves with and without inhibitor are parallel (inhibited curve is higher).
• Mixed Inhibition: Inhibitor binds to allosteric site of both enzyme and E-S complex, but with a different affinity for each. Km can increase or decrease, but vmax always decreases. On L-B Plot, curves with and without inhibitor intersect at a point that is not on either axis.