L9

Question 1

the 3D structure of enzymes provides an active site for (2):

Answer 1

• binding of substrate(s)
• catalytic conversion to product (s)

Question 2

what do catalysts do?

Answer 2

speed up the attainment of reaction equilibrium - but do not affect equilibrium itself

Question 3

Properties of enzymes (substrates, stereospecific, reaction specificity)

Answer 3

substrates: highly specific reactants for enzymes
stereospecific: enzymes usually act upon only one stereoisomer of a substrate
reaction specificity: enzyme product yields are essentially 100% (no formation of wasteful by-products)

Question 4

What do enzymes do to the activation energy of a reaction?

Answer 4

Lower it

Question 5

What is ‘induced fit’?

Answer 5

the active site fits better to the substrate after bding

Question 6

The active site is most complementary in shape to (and has highest binding energy for) what?

Answer 6

the transition state = transition state stabilization (lowers energy barrier)

Question 7

What is the proximity effect?

Answer 7

reaction of substrate(s) is favoured by proximity effect (reaction on enzyme increases the effective concentration of reacting groups)

Question 8

what is the role of enzyme active site residues? (2)

Answer 8

• binding energy (non-covalent interaction)
• catalytic functional groups

Question 9

What are cofactors?

Answer 9

• Essential ions
• coenzymes (cosubstrates + prosthetic groups)

Question 10

What do kinetic experiments examine?

Answer 10

the concentration of product ( P ) formed (or substrate consumed (S)) per unit of time

Question 11

what is the rate equation

Answer 11

delta (P) / delta (t) = v (velocity or rate) = k[S] (rate constant)

Question 12

What does the Vo [S] plot usually look like

Answer 12

Linear (for chemical process) - 1st order: Vo = k[S]

Question 13

For enzyme catalyzed reactions, what does the curve look like?

Answer 13

hyperbolic - linear at low [S], independent of [S] at high [S] (reaches maximum velocity)

Question 14

What do the constants mean in the Michaelis-Menten equation? Km

Answer 14

Km ~ the enzyme-substrate dissociation constant
- measure of the affinity of the enzyme for the substrate (the lower the value of Km, the tighter the substrate binding)
- equals the concentration of substrate needed for 1/2 max velocity

Question 15

What do the constants mean in the Michaelis-Menten equation? Vmax

Answer 15

Maximum initial velocity - velocity when an enzyme is saturated with substrate (high [S])
(Vmax is proportional to enzyme concentration)

Question 16

What is Vmax equation?

Answer 16

Vmax = kcat[Et]
where Kcat is the rate constant for ES -> E + P (only process occuring since enzyme saturated)
and Et is the total enzyme concentration (i.e. no. of active sites)

Question 17

What is kcat?

Answer 17

termed the catalytic constant / turnover number
kcat = Vmax/[Et] - a measure of the number of molecules of substrate converted to product per second per active side

Question 18

What are the six classes of enzymes?

Answer 18

1. oxidoreductases (dehydrogenases)
2. transferases
3. hydrolazes
4. lyazes
5. isomerases
6. ligases (synthetases)

Question 19

What do oxidoreductases do?

Answer 19

catalyze redox rxns

Question 20

What do transferases do?

Answer 20

catalyze group transfer rxns

Question 21

What do hydrolases do?

Answer 21

catalyze hydrolysis rxs (cleave a bond (i.e. C-O, C-N), phosphoanhydride with addition of water to the products

Question 22

What do lysases do?

Answer 22

catalyze lysis: cleavage of a C-C, C-O, C-N, or other bond - leaving a double bond (distinct from hydrolase as water is not added to products)

Question 23

What do isomerases do?

Answer 23

catalyze isomerism rxns (many dif forms exist - depending on type of isomerism)
isomerism: the transfer of groups within molecules to yield different structural or geometric isomers (molecular formula doesn’t change - no loss of groups/atoms)

Question 24

What do ligases do?

Answer 24

catalyze ligation (joining) of two substrates (forming new C-C, C-O, C-N, C-S bonds) - requires chemical energy (ATP hydrolysis)