Enzymes 2

Question 1

Isozyme

Answer 1

• Enzymes that catalyse the same reaction
• Multiple forms in an organism-sometimes in different tissues
• Products of different genesHow can isoenzymes be styd

Question 2

What reaction do glucokinase and hexokinase catalyse?

Answer 2

Glucose + ATP > Glucose-Phosphate + ADP

Question 3

What happens when blood glucose increases?

Answer 3

-Glucokinase activity increases but hexokinase activity does not respond as it is already working at Vmax.

Question 4

What happens when blood glucose decreases?

Answer 4

-Gluconeogenesis releases glucose from the liver, but glucokinase cannot catalyse glucose back into glucose-6-phosphate under these conditions allowing glucose to be used in the body.

Question 5

How can isoenzymes be studied?

Answer 5

Using electrophoresis

Question 6

What is creatine kinase?

Answer 6

A dimer made from 2 polypeptides B and M

Question 7

What does elevation of CK2 indicate?

Answer 7

Myocardial infarction

Question 8

How are enzymes used in diagnosis?

Answer 8

• Measure activity and compare with normal

- Separate different forms of enzymes by electrophoresis and examine pattern

Question 9

What does catalysing a reaction with 2 or more substrates usually involve?

Answer 9

Transfer of groups from one substrate to the other

Question 10

How can transferring of groups between substrates occur?

Answer 10

• Random order or Ordered with ternary complex

- No ternary complex formation

Question 11

What does lactate dehydrogenase exhibit to its catalysis of pyruvate to lactate?

Answer 11

An ordered sequential mechanism

Question 12

Describe the catalysis of pyruvate by lactate dehydrogenase.

Answer 12

-Coenzyme (NADH) binds first and the lactate is always released first

Question 13

What happens in an ordered sequential reaction mechanism?

Answer 13

The enzymes exists in a ternary complex, first with the substrates of the reaction and then (after catalysis) with the products of the reaction

Question 14

What does creatine kinase exhibit when it catalyses the formation of phosphocreatine from creatine?

Answer 14

Random sequential mechanism

Question 15

What happens in a random sequential mechanism?

Answer 15

The order of binding and release of substrate and product is random but the formation of a ternary complex still occurs first with substrates and then the products of the reaction

Question 16

What are classic examples of reactions which have no ternary complex formation??

Answer 16

Reactions where amino groups are shuttled between amino acids and ketoacids

Question 17

Where does the term ‘ping pong’ come from?

Answer 17

Substrates bounce on and off the enzyme as they are catalysed

Question 18

What do allosteric enzyme not follow?

Answer 18

M-M Kinetics

Question 19

Describe the kinetics of allosteric enzymes.

Answer 19

• One substrate binding to an enzyme subunit can cause changes in other active sites on other subunits.
• This can lead to cooperative binding of substrate molecules

Question 20

What are allosteric enzymes composed of?

Answer 20

Many subunits which contain man active sites

Question 21

What affects the activity of an enzyme?

Answer 21

• Temperature
• pH
• Inhibitors

Question 22

How does temperature affect enzyme activity?

Answer 22

Increase:

• Increases molecule collisions
• Increases energy of molecules
• Will eventually denature enzyme

Question 23

How does pH affect enzyme activity?

Answer 23

• pH changes the charge of amino acids
• If the active site amino acids charge changes the enzyme will cease to function
• Extreme pH will denature most enzymes
• pH will also affect the substances of the reaction, some of which may require H or OH groups to be involved in the reaction

Question 24

What are the 3 kinds of enzyme inhibitor?

Answer 24

• Competitive inhibitor
• Non-competitive inhibitor
• Uncompetitive inhibitor

Question 25

Competitive inhibitors

Answer 25

Bind to enzymes non-covalently and will usually resemble the substrate molecule, therefore competing with the active site

Question 26

What do competitive inhibitors lead to?

Answer 26

-Decrease in affinity between active site and substrate, so the Km of ES increases

Question 27

What can overcome competitive inhibition?

Answer 27

Increasing substrate concentration

Question 28

What is Azidothylmidine?

Answer 28

A controversial drug created in the 90s for AIDs

Question 29

How does AZT act?

Answer 29

- Acts by competitive inhibition of the reverse transcriptase enzyme - Reverse transcriptase is used by HIV to produce dsDNA molecule from its ssRNA - AZT undergoes triphosphorylation in the body and thus mimics the ordinary DNA precursor thymidine triphosphate

Question 30

What is a transition state analogue?

Answer 30

An inhibitor that mimics the transition state

Question 31

What is an example of a transition state analogue?

Answer 31

Oseltamivir (Tamiflu)

Question 32

How does oseltamivir work?

Answer 32

- In the body oseltamivir is hydrolysed in the liver to its active form which is then able to block the activity of neuraminidase enzyme. - Neuramindase normally cleaves sialic acid that allows the release of new virus particles form the cells

Question 33

What is the concept of catalytic antibodies?

Answer 33

- Antibodies can be generated against potentially any biological molecule, therefore and antibody could be made that was specific to a transition state molecule. - This would allow the production of an antibody with a structure that resembles that of the active site of the original enzymes

Question 34

What difficulty is there with catalytic antibodies?

Answer 34

Transition states are notoriously difficult to isolate as they are the intermediate step between and ES an EP

Question 35

In what disease would you find a naturally occurring catalytic antibody?

Answer 35

Lupus erythematosus | -Characterised by the autoantibodies attacking the connective tissue of the joints, skin, kidneys, heart and lungs

Question 36

What are non-competitive inhibitors?

Answer 36

Inhibitors that bind to enzymes non-covalently and will usually attach to a site other than the active site of the enzyme

Question 37

Why does the Km of the ES remain unchanged with non-competitive inhibitors?

Answer 37

The substrate is usually still able to bind to the active site

Question 38

Why does Vmax decrease with non-competitive inhibitors?

Answer 38

Increasing substrate concentration does not change the inhibition.

Question 39

What is an irreversible inhibitor?

Answer 39

An inhibitor that will usually bind to an enzyme covalently

Question 40

What is a regulatory enzyme?

Answer 40

Key enzyme in a pathway

Question 41

What are the 2 main ways regulatory enzymes modulate reactions?

Answer 41

- Allosteric enzymes | - Covalently modified enzymes

Question 42

What is feedback inhibition?

Answer 42

A build up of end product of a pathway, or key junction in a pathway can ultimately slow the entire pathway

Question 43

What can inhibit bacterial threonine dehydratase?

Answer 43

The final product of a 5 step pathway (L-isoleucine)

Question 44

How does L-isoleucine inhibit bacterial threonine dehydratase?

Answer 44

It binds to a site other than the active site which changes the conformation of the active site blocking the enzymes action

Question 45

How do allosteric enzymes work?

Answer 45

They bind non-covalently to a site other than the active site. - This changes the enzymes structure - Some effectors are activators - Binding of the effector will increase or decrease the efficiency of substrate binding and processing

Question 46

What are allosteric enzymes examples of?

Answer 46

Non-competitive inhibitors

Question 47

What does the kinetics of allosteric enzymes suggest they do?

Answer 47

Increase of [S] a low levels has less of a response than the same increase at a higher level. -This suggest that at low [S] it sensitises the enzyme

Question 48

What are the 2 models?

Answer 48

- Concerted model | - Sequential model

Question 49

Describe the concerted model.

Answer 49

- Each sub-unit can exist in 2 different conformations - One binds substrate well the other doesn't - With no substrate the enzyme flips between the 2 conformations - All sub-units must be in the same conformation

Question 50

What do allosteric enzymes do in concerted model?

Answer 50

- They will stabilise the open conformation allowing S to bind more effectively - They will stabilise the closed conformation and make it difficult for S to bind effectively

Question 51

What does the sequential model assume?

Answer 51

- No flipping between different conformation states - Sub-units exist in a conformation that can bind S, activators, inhibitors - It is the binding that causes a conformational change

Question 52

What does substrate binding in sequential model do?

Answer 52

Causes a change in one sub-unit

Question 53

How can enzymes be regulated?

Answer 53

Through reversible covalent modification

Question 54

What is a common covalent medication?

Answer 54

Phosphorylation

Question 55

Where can proteins be phosphorylated?

Answer 55

- At a single site - Multiple sites - Multiples phosphorylations at one site

Question 56

What enzymes are involved in phosphorylation?

Answer 56

- Protein kinases: add phosphate groups to proteins | - Protein phosphatases: remove phosphate groups from proteins

Question 57

What do multiple phosphorylation sites allow?

Answer 57

Very fine control of enzyme function depending on the requirement of the particular enzyme at a given time

Question 58

Other than in its active state, what other state can an enzyme exist in?

Answer 58

-Inactive precursor called a proenzyme

Question 59

How do proenzymes become active?

Answer 59

They can be cleaved by protesases

Question 60

What do digestive enzymes tend to be called?

Answer 60

Zymogens