Lecture 12 - Enzymes II - Catalysis

Question 1

What is catalysis?

L12 S1

Answer 1

An increase in rate of a chemical recreation induced by a substance that is not chemically changed by the reaction

Question 2

What does a catalyst do with regards to the energy of a reaction? What does it no do?

L12 S2

Answer 2

It lowers the amount of energy needed for a reaction to proceed (activation energy) by stabilizing the transition state.

It does not change the ΔG or difference between energy of product and reactant.

Question 3

Differentiate between Q and K in the variations of the equation of ΔG.
What relationship is desired between Q and K in order to drive a reaction forward?

L12 S4

Answer 3

Q is the concentration of products over reactants in general while K is only while at equilibrium.

Q should be less than K for a reaction to proceed.

Question 4

For a chain of related reactions, their ΔG values can be summed.

Answer 4

Understand this concept.

Question 5

What is the induced fit model?

L12 S11

Answer 5

When a substrate binds an enzyme, the enzyme changes shape to force the substrate into transition state.

Question 6

What are the mechanisms of catalysis?

L12 S12

Answer 6

• proper substrate orientation
• straining of substrate bonds
• favorable microenvironment
• covalent and/or noncovalent interactions between enzyme and substrate

Question 7

What are the covalent/noncovalent mechanisms by which a catalyst interacts with its substrate?

L12 S13-16

Answer 7

• covalent catalysis (electron transfer)
• acid-base catalysis (proton transfer)
• electrostatic catalysis (stabilization of unfavorable charges

Question 8

What reaction does chemotrypsin affect and what changes does it have on its catalyzed reaction?

L12 S18

Answer 8

Cleaves peptide bond associated with serine residues.

Reduces reaction time from years to milliseconds.

Question 9

What mechanisms does chymotrypsin use to catalyze it’s reaction?

L12 S23

Answer 9

Catalytic triad:

• serine (S195) is a nucleophile
• histidine (H57) is a base (proton acceptor)
• aspartic acid (D102) is an acid (proton donor)

Oxyanion hole:

• stabilizes transition state
• serine (S195) and glycine (G193)

Specificity pocket:

• selective for certain R group characteristics
• large nonpolar pocket which selects for large, nonpolar R groups

Question 10

What reaction does carbonic anhydrase affect and what changes does it have on its catalyzed reaction?

L12 S18

Answer 10

Converts CO2 into HCO3- and vice-versa.

Reduces reaction time from seconds to microseconds.

Question 11

What mechanism does carbonic anhydrase use to catalyze it’s reaction?

L12 S27

Answer 11

Active site:
contains a Zn ion that binds 3 histidine residues and a water molecule that is deprotonated and catalyzes reaction
-lowers the pKa of water

Entry channel:
-small and weakly polar which limits substances that can enter (such as CO2)