Chapter 6- Enzyme Action

Question 1

Enzymes

Answer 1

• accelerate reactions by factors of as much as a million or more
• sole purpose: facilitate the formation of the transition state (a molecular form that is no longer substrate but not yet product)
• the energy required to form the transition state from the substrate is called the activation energy (Delta G)
• nearly all enzymes are proteins

Question 2

Enzymatic Catalysis

Answer 2

• enzymes and substrates “a contact sport”
• the formation of the enzyme-substrate complex is the first step in enzymatic catalysis
• enzymes bring substrates together on a region of the enzyme called the active site
• this promotes the formation of the transition state

Question 3

Free Energy

Answer 3

• Enzymes alter the reaction rate but not the reaction equilibrium
• equilibrium is determined only by the free energy difference of reactants and products–enzymes cannot alter this difference!!!
• Free Energy (G) is a measure of energy capable of doing work: spontaneity, not rate
• it is a measure of how far a chemical reaction is from equilibrium
• you get more useful work out of a reaction if it is far from equilibrium

Question 4

Free Energy (G) relationships

Answer 4

G=0; reaction is at equilibrium (reactants and products equal each other)

G less than 0 (-); reactant concentrations are higher (Exergonic) longer arrow on top.

G greater than 0 (+); product concentrations are higher (Enderomic)

Question 5

Free Energy & Equilibrium Constant (Keq)

Answer 5

• delta G and Keq are opposite sides of the same coin!
• remember delta G and Keq is the opposite as far as negative and positive
• delta G = 0, Keq= 1 (equilibrium)

Question 6

Six major classes of Enzymes

Answer 6

• Oxidoreductase: oxidation-reduction reactions
• Transferases: move functional groups between molecules
• Hydrolyases: cleave bonds with the addition of water
• Lyases: remove atoms to form double bonds or add atoms to double bonds
• isomerases: move functional groups within a molecule
• ligases: join two molecules at the expense of ATP

Question 7

Cofactors

Answer 7

• small molecules that some enzymes require for activity
• two main classes: coenzymes (organic molecules derived from vitamins), and metals
• tightly bound coenzymes are called prosthetic groups
• an enzyme with its cofactor is a holoenzyme
• without its cofactor is an apoenzyme

Question 8

Enzyme activity

Answer 8

• temperature enhances the rate of enzyme-catalyzed reactions
• most enzymes have an optimal pH

Question 9

Kilojoule vs. kilocalorie

Answer 9

1 kJ = 0.239 kcal

Question 10

Properties of the active site

Answer 10

• 3D structure or cleft that is formed by a different amino acid sequence
• very small region of the enzyme
• unique non-polar micro environment where the active site is free from any water molecules (enhances ES formation)
• non-covalent interactions are mediated by electrostatic interactions
• allows binding of only one specific substrate

Question 11

What does an apoenzyme need to become a holoenzyme

Answer 11

A cofactor (helper molecule)

Question 12

Types of cofactors

Answer 12

Inorganic: metal ions (Zn, Ca)
Organic: flavin, heme
-prosthetic group (tightly bound to enzyme)
-coenzymes: released from the active site during the reaction (NADH, NADHP, ATP)

Question 13

What is the fundamental mechanism by which enzymes enhance the rate of chemical reactions?

Answer 13

They lower the energy of activation by stabling the transition state.

Question 14

What is meant by the term binding energy?

Answer 14

Formation of the ES complex releases free energy—this free energy is released due to the formation of the complex and is called binding energy.

Question 15

What is RT

Answer 15

R=gas constant = 8.315 J K mol

T = temperature at 25 C= 298 K