Biochemistry Lecture 11 - Enzyme Regulation

Question 1

What does high ALT (alanine transaminase) levels in the blood indicate?

Answer 1

Liver damage

Question 2

What does decreased alpha-1 antitrypsin levels in the blood indicate?

Answer 2

Liver damage - a1 antitrypsin is made in the liver

Question 3

How is a zymogen activated?

Answer 3

Proteolytic cleavage at a specific site that either unblocks the active site or induces a conformational change to confer its catalytic ability.

Question 4

What are the steps in the final common pathway of the blood clotting cascade?

Answer 4

Tissue damage –> activated enzyme on a membrane surface –> Prothrombin (II) –> Thrombin (IIa) –> Fibrinogen (I) –> Fibrin (Ia) –> cross-linked fibrin clot

Question 5

What type of enzyme is thrombin?

Answer 5

A serine protease.

Question 6

How many times is prothrombin cleaved to generate active thrombin? What do the cleavages entail?

Answer 6

Twice. The first cleavage gets rid of a shitload of N-terminal AA residues, and the second one allows for proper active site formation when residue 16 H-bonds with residue 194 to form the catalytic triad.

Question 7

Is prothrombin cleavage reversible?

Answer 7

Nope.

Question 8

Where does prothrombin cleavage occur?

Answer 8

On a membrane surface.

Question 9

What allows for prothrombin to bind to the membrane surface?

Answer 9

N-terminal glutamate residues are modified by gamma-carboxylation, which allows ions such as Ca 2+ to bind, allowing membrane anchoring.

Question 10

What is an important cofactor for gamma-carboxylation of the N-terminal glutamate residues of prothrombin? What is a commonly prescribed analogue for this cofactor?

Answer 10

Vitamin K. Warfarin is a Vitamin K analogue.

Question 11

How does Warfarin work?

Answer 11

Warfarin competes with Vitamin K as a cofactor for gamma-carboxylation of glutamate residues on prothrombin. Without the proper cofactor, the glutamates are not carboxylated and cannot bind Ca2+, and prothrombin cannot dock on the membrane for cleavage to form mature thrombin.

Question 12

Is the action of protein enzyme inhibitors reversible?

Answer 12

Nope.

Question 13

What is the protein enzyme inhibitor for thrombin? What drug is used to alter the affinity of the inhibitor-enzyme? How is the inhibitor-enzyme complex degraded?

Answer 13

Antithrombin. Heparin promotes antithrombin-thrombin binding. The complex is degraded as a whole unit.

Question 14

What physiological enzyme has the potential to degrade lung tissue? What kind of enzyme is it?

Answer 14

Elastase can destroy elastin in the lungs. It is a serine protease.

Question 15

What inhibitor regulates elastase activity? Where is this inhibitor synthesized?

Answer 15

Alpha-1 antitrypsin inhibits elastase, its synthesized in the liver.

Question 16

How does smoking affect alpha-1 antitrypsin activity?

Answer 16

The enzyme is oxidized, rendering it unable to effectively inhibit elastase, leading to lung degradation.

Question 17

Which three amino acids can be modified by phosphorylation?

Answer 17

Serine, threonine, and tryosine.

Question 18

What is the class of enzymes called that catalyze the addition of phosphate groups? What is the class of enzymes called that catalyze the removal of phosphate groups?

Answer 18

Kinases add phosphates, phosphatases remove them.

Question 19

How does phosphorylation of enzymes alter activity?

Answer 19

Through conformational changes.

Question 20

Do competitive inhibitors change Vmax of an enzyme? Do they change Km?

Answer 20

Competitive inhibitors do NOT change Vmax. They INCREASE Km (effectively lower binding affinity).

Question 21

Do non-competitive inhibitors change Vmax of an enzyme? Do they change Km?

Answer 21

Non-competitive inhibitors LOWER Vmax, but do NOT change Km.

Question 22

What does DIFP do?

Answer 22

It is a component of nerve gas. It covalently binds to acetylcholine esterase, which degrades Ach in the synaptic cleft. Without it, impaired nerve function results.

Question 23

At what substrate concentration do cells like to regulate enzyme activity? Why?

Answer 23

Near Km the Velocity vs. [S] curve is steep so slight changes in [S] result in large changes in enzyme activity.

Question 24

What is product inhibition? Name an example.

Answer 24

When the immediate product of a catalyzed reaction inhibits the same enzyme that catalyzed the reaction. Example: hexokinase (glycolysis) is inhibited by its product (glucose-6-phosphate competes with ATP).

Question 25

Do allosteric enzymes follow M-M kinetics?

Answer 25

Nope.

Question 26

What is special about allosteric enzymes?

Answer 26

They are multimeric and exhibit cooperativity with Taut and Relaxed conformations. Their V vs. [S] graphs are sigmoidal, and at Km the slope is very steep.

Question 27

Why are allosteric enzymes often used to regulate the first step of reaction pathways? What is this called?

Answer 27

Because the V vs [S] curve is so steep at Km, small changes in substrate binding cause dramatic changes in enzyme activity. The ultimate compound of a reaction pathway often will regulate the allosteric enzyme involved in the first step of the pathway so no excess energy is wasted in producing the intermediate compounds. It is called feedback inhibition.

Question 28

What type of regulation is Phosphofructokinase 1 (PFK1) involved in? What else is involved?

Answer 28

It is involved in a feedback inhibition loop in the committed step of glycolysis. ATP inhibits PFK1 because high ATP levels indicate that the cell has enough energy.