Enzyme Regulation

Question 1

Reversible reactions are controlled by changes in ______.

Answer 1

Substrate concentration

Question 2

Irreversible reactions are controlled by

Answer 2

Allosteric Enzymes
Feedback Inhibition
Feed Forward Regulation
Enzyme Levels

Question 3

Flux is controlled by _______.

Flux through a pathway is never _______ than the ________. Thus it is the _______.

Answer 3

Enzymes in the pathway
Supply of Starting Material
Rate of Product Utilization

faster, slowest step, rate-limiting step

For example, a series of reaction take Glucose to G-6-P. G-6-P then uses several enzymes to either become ATP or glycogen.

Question 4

Enzyme regulated steps are usually _____.

Answer 4

Irreversible

Question 5

Feedback Regulation

Answer 5

The product of a pathway can control its synthesis

Question 6

Effect of substrate concentration on enzyme reaction rate.

When [S] > Km ______
When [S] < Km

For most enzymes [S] ___ Km

Answer 6

1. Vmax does not change
2. Velocity changes in proportion to [S]
3. For many enzymes [S] is near Km.

Question 7

v0 =

Answer 7

(Vmax)[S]/ Km

Question 8

According to _______ a ___ change in substrate concentration is necessary to access the full dynamic range of enzyme initial velocity.

Answer 8

Michealis-Menten, 100-fold

0.1Vmax - 0.9Vmax

Question 9

What is the difference between product inhibition vs feedback inhibition?

Answer 9

Product inhibition is when the product of an enzyme inhibits the enzyme function.

Feedback inhibition is when a product during the metabolic pathway; not the final product; influences its production

Question 10

What is allostery?

Answer 10

Biological macromolecules transmit the effect of binding at one site to another from a site that is NOT the active site.

There are 2 requirements.
1. Binding
2. Shape Change

Question 10

Describe the regulation of Hexonkinase and Glucokinase

Answer 10

Hexokinase cataylzes the first step in glycolysis by adding a phosphate from ATP into glucose to for G6P.

Hexokinase has a low Km and is inhibited by its product G6P.

Glucokinase is directly regulated by glucose concentration. It has a high Km.

Question 11

What is cooperative allostery?

Answer 11

Allostery where there are two or more binding sites for allosteric enzymes.

Question 12

What is positive cooperativity?

Answer 12

First ligand bound has lower affinity for active site and increases affinity for subsequent ligands.

First ligand has larger Kd and hill slope is more than one

Question 13

What is negative cooperativity?

Answer 13

Substrate binding at one active site decreases affinity for subsequent ligands.

Question 14

What are homotropic allosteric modifiers?

Answer 14

Substrates for active site

Question 15

What are heterotropic Allosteric Modifiers

Answer 15

Do NOT bind to active site, they are not substrates

Question 16

Heterotropic inhibitors stabilize the _______ while activators stablize the ____.

Answer 16

T-conformation, R-Conformation

Question 17

PFK-1 cooperates with _____ F6P ________. Explain this concept.

FGP is a ________ of PKF-1
ATP is a _________ of PFK-1

*PFK is a key enzyme is glycolysis

Answer 17

F6P, positively

PFK-1 binds a phosphate from ATP to F6P to form F1,6,bisphosphate. The curve is sigmoidal indicating + cooperativity.

This is because when F6P is absent, PFK-1 is locked in the T-state. Binding of F6P at one active site switches all other sites to an active R-state. This means F6P is a homotropic activator of PFK-1.

ATP is a substrate and heterotropic inhibitor of PFK-1.
PFK-1 has two different ATP binding sites on each subunit. One ATP site is at the active site, the other is not.The one that is NOT stabalizes the T-conformation which is why ATP is a HETEROTROPIC inhibitor.

Question 18

ATP binds at one active site and one non-active site.

F6P binds at an active site

F26BP is a _______ that binds at a ______.

Answer 18

heterotropic activator, non-active

F26BP stabilizes the R-state of PFK-1 thus increasing its activity. Km decreases.

Question 19

PFK-1 Cooperativity

F6P
ATP
AMP
F26BP
Citrate

Answer 19

1. Substrate, homotropic activator
2. Heterotropic Inhibitor
   , signals no more ATP is needed
3. Heterotropic Activator; Signal ATP is low
4. Heterotropic Activator, Signal that cells needs to use glucose for energy
5. Heterotropic Inhibitor, Citric acid cycle is replenished

Question 20

What are the two forms of post-translational modification of enzymes?

Answer 20

1. Phosphorylation/Dephosphorylation of serine, thereonine or tyrosine residues by kinases.
2. Proteolysis that activate a zymogen or proenzyme by proteases

Question 21

Protein kinases transfer a ____ from ______ to ____ of _____. Protein _________ remove the ______ from the enzyme

Answer 21

phosphate, ATP/NTP, hydroxyl groups of Serine,Threonine, Tyrosine

Phosphatases

Question 22

Describe Glycogen Phosphorylase

Answer 22

Glycogen Phosphorylase is activated heterotropically by AMP. Phosphorylation of glycogen phosphorylase stabilizes its active form.

Question 23

Describe Zymogens. What is a synonym?

Answer 23

Zymogens are digestive enzymes. They are translated inactive and are activated by proteolysis

Proteases

Question 24

Describe the role of zymogens in the blood clotting cascade.

Answer 24

Prothrombin is an inactive protease or zymogen. It is converted to thrombin via a protease called Factor Xa which is activated by another protease.