Regulation of enzyme activity

Question 0

What is an isoenzyme?

Answer 0

-Same enzyme with different kinetic properties, produced by alternate splicing of the same gene which preoduces a different a’a sequence

Question 1

How does substrate/product concentration control enzyme activity?

Answer 1

-[Substrate] affects the rate of activity of the enzyme

Question 2

Give an example of isoenzymes and why these are clinically important

Answer 2

• Hexokinase and Glucokinase
• Glucokinase has high Km; Hexokinase has low Km
• Clinically important because glucokinase predominantly found in liver and only reaches effective enzyme saturation at high concentrations of glucose, i.e. after a meal

Question 3

What type of enzymes does allosteric regulation occur with?

Answer 3

-Multicomplex enzymes which have more than one active site

Question 4

Why do enzymes which are allosterically regulated have sigmoidal rate curves?

Answer 4

-Positive cooperativity -> binding one active site increases the affinity of the other active sites

Question 5

What are the two conformations of allosterically controlled enzymes?

Answer 5

• R and T forms
• Allosteric inhibitors stabilise the low affinity T form
• Allosteric activators stabilise the high affinity R form

Question 6

How are some enzymes controlled by covalent modification?

Answer 6

• Covalent attachment of various groups to the enzyme which results in activation or inhibtion of the enzyme
• Eg phosphoryl, adenyl, acetly etc

Question 7

Which is the most important form of covalent modification?

Answer 7

-Phosphorylation which usually activates an enzyme and is catalysed by a kinase enzyme

Question 8

What is proteolytic activation of enzymes?

Answer 8

-Proteolytic cleavage of part of the polypeptide

Question 9

What is the most frequent function of proteolytic activation?

Answer 9

-Activate zymogens

Question 10

What are zymogens?

Answer 10

-Inactive precursor molecules eg pepsinogen -> pepsin, Insulin precursors, blood clotting factors, proteolytic enzymes

Question 11

Why are some proteins synthesised in an inactive form?

Answer 11

• Ensures safe transport to site of action

- Only want certain enzymes to be active under certain conditions, eg insulin and high blood glucose

Question 12

What is meant by a proteolytic cascade?

Answer 12

-proteolytic activation of one enzyme will lead to proteolytic activation of a series of other enzymes

Question 13

Give an example of a proteolytic cascade in the pancreas

Answer 13

-Master regulator Enteropepsidase proteolytically cleaves trypsin which in turn proteolytically activates a series of other enzymes, including itself

Question 14

How is chymotrypsinogen activated?

Answer 14

• Proteolytic cleavage by trypsin leaves a partially active chymotrypsin and pi-chymotrypsin
• pi-chymotrypsin activates itself
• leads to cleavage of the partially active chymotrypsin
• the three segments are bonded together producing fully active chemotrypsin

Question 15

How is proteolytic activation regulated?

Answer 15

• Regulated by endogenous inhibitors binding to protease stoppong cleavage
• One inhibitor can inhibit a range of proteases

Question 16

Give an example of a clinical condition caused by a deficiency in protease inhibitors

Answer 16

-emphysema -> elastase not regulated causing destruction of alveoli

Question 17

Name the long-term regulation of enzyme activity

Answer 17

• change in transcription/translation
• Change in the rate of protein degredation -> upregulation of ubiquitin proteosome pathway -> unbiquitin regulates protein breakdown by tagging proteins to be destroyed.

Question 18

What activates the intrinsic pathway of the clotting cascade?

Answer 18

-Damage to endothelial lining promotes binding of factor XI

Question 19

What is the blood clotting cascade a proteolytic activation cascade?

Answer 19

-Proteolytic activation of one factor ptoteolytically activates many others

Question 20

What causes the extrinsic pathway of the clotting cascade to begin?

Answer 20

-Membrane damage exposes the extracellular domain of factor III which causes autocatalytic activation of factor VII

Question 21

At what point do the two cascade converge?

Answer 21

-On factor X

Question 22

What are the important cofactors?

Answer 22

-V and VIII

Question 23

How are blood clotting factors normally present in the blood and why?

Answer 23

-In very low concentrations and in zymogen form to prevent accidental activation

Question 24

What do kringle domains do?

Answer 24

-Help keep the factor in its zymogen form

Question 25

How are all the factors in the blood clotting cascade activated?

Answer 25

Proteolytic cleavage

Question 26

What do Gla domains do?

Answer 26

-They are highly negative and allow binding of the factors to the damaged epithelium as they are attracted to the positive Ca2+ which is released from the damaged vessel wall. This attracts the factors to the site of damage

Question 27

Describe the intrinsic pathway

Answer 27

• Factor XI->Xia
• Factor IX-> IXa
• Factor X->Xa
• Prothrombin->Thrombin
• Fibrinogen->Fibrin

Question 28

Describe the extrinsic pathway

Answer 28

• Factor III causes autocatalytic activation of factor VII
• Factor VII->VIIa
• Factor X->Xa
• Prothrombin->Thrombin
• Fibrinogen->Fibrin

Question 29

What is the main regulatory clotting factor?

Answer 29

-Thrombin

Question 30

What does thrombin regulate in the clotting cascade?

Answer 30

• Enhances conversion of factors V, VIII and XI to active form
• Feedsback to ensure cascade is continual
• Regulates digestion of clotting factors

Question 31

How does a clot actually form, what role does thrombin play?

Answer 31

• Under normal conditions, fibrinogen has highly negative domains on the N-Terminals of its a and b chains which cause fibrin to repel one another
• Thrombin cleaves the negative fibrinopeptides
• Fibrin molecules no longer repel each other
• C-terminal a’a residues interact with the newly exposed N-terminal residues forming a clot
• The clot becomes stabilised via the formation of amide bonds between side chains

Question 32

How is the fibrin clot/clotting factors broken down?

Answer 32

• Proteolytic digestion -> protein C digests cofactors under thrombin regulation
• Plasmin causes fibrinolysis
• Dilution of factors via blood flow
• Removal by liver
• Binding of inhibitors such as heparin -> heparin under the control of antithrombin, under the control of thrombin