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Science for Medicine > Enzymes 2 > Flashcards

Enzymes 2 Flashcards

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1
Q

Isozyme

A
  • Enzymes that catalyse the same reaction
  • Multiple forms in an organism-sometimes in different tissues
  • Products of different genesHow can isoenzymes be styd
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2
Q

What reaction do glucokinase and hexokinase catalyse?

A

Glucose + ATP > Glucose-Phosphate + ADP

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3
Q

What happens when blood glucose increases?

A

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

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4
Q

What happens when blood glucose decreases?

A

-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.

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5
Q

How can isoenzymes be studied?

A

Using electrophoresis

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6
Q

What is creatine kinase?

A

A dimer made from 2 polypeptides B and M

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7
Q

What does elevation of CK2 indicate?

A

Myocardial infarction

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8
Q

How are enzymes used in diagnosis?

A
  • Measure activity and compare with normal

- Separate different forms of enzymes by electrophoresis and examine pattern

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9
Q

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

A

Transfer of groups from one substrate to the other

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10
Q

How can transferring of groups between substrates occur?

A
  • Random order or Ordered with ternary complex

- No ternary complex formation

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11
Q

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

A

An ordered sequential mechanism

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12
Q

Describe the catalysis of pyruvate by lactate dehydrogenase.

A

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

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13
Q

What happens in an ordered sequential reaction mechanism?

A

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

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14
Q

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

A

Random sequential mechanism

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15
Q

What happens in a random sequential mechanism?

A

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

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16
Q

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

A

Reactions where amino groups are shuttled between amino acids and ketoacids

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17
Q

Where does the term ‘ping pong’ come from?

A

Substrates bounce on and off the enzyme as they are catalysed

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18
Q

What do allosteric enzyme not follow?

A

M-M Kinetics

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19
Q

Describe the kinetics of allosteric enzymes.

A
  • 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
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20
Q

What are allosteric enzymes composed of?

A

Many subunits which contain man active sites

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21
Q

What affects the activity of an enzyme?

A
  • Temperature
  • pH
  • Inhibitors
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22
Q

How does temperature affect enzyme activity?

A

Increase:

  • Increases molecule collisions
  • Increases energy of molecules
  • Will eventually denature enzyme
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23
Q

How does pH affect enzyme activity?

A
  • 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
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24
Q

What are the 3 kinds of enzyme inhibitor?

A
  • Competitive inhibitor
  • Non-competitive inhibitor
  • Uncompetitive inhibitor
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25
Q

Competitive inhibitors

A

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

26
Q

What do competitive inhibitors lead to?

A

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

27
Q

What can overcome competitive inhibition?

A

Increasing substrate concentration

28
Q

What is Azidothylmidine?

A

A controversial drug created in the 90s for AIDs

29
Q

How does AZT act?

A
  • 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
30
Q

What is a transition state analogue?

A

An inhibitor that mimics the transition state

31
Q

What is an example of a transition state analogue?

A

Oseltamivir (Tamiflu)

32
Q

How does oseltamivir work?

A
  • 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
33
Q

What is the concept of catalytic antibodies?

A
  • 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
34
Q

What difficulty is there with catalytic antibodies?

A

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

35
Q

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

A

Lupus erythematosus

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

36
Q

What are non-competitive inhibitors?

A

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

37
Q

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

A

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

38
Q

Why does Vmax decrease with non-competitive inhibitors?

A

Increasing substrate concentration does not change the inhibition.

39
Q

What is an irreversible inhibitor?

A

An inhibitor that will usually bind to an enzyme covalently

40
Q

What is a regulatory enzyme?

A

Key enzyme in a pathway

41
Q

What are the 2 main ways regulatory enzymes modulate reactions?

A
  • Allosteric enzymes

- Covalently modified enzymes

42
Q

What is feedback inhibition?

A

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

43
Q

What can inhibit bacterial threonine dehydratase?

A

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

44
Q

How does L-isoleucine inhibit bacterial threonine dehydratase?

A

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

45
Q

How do allosteric enzymes work?

A

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
46
Q

What are allosteric enzymes examples of?

A

Non-competitive inhibitors

47
Q

What does the kinetics of allosteric enzymes suggest they do?

A

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

48
Q

What are the 2 models?

A
  • Concerted model

- Sequential model

49
Q

Describe the concerted model.

A
  • 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
50
Q

What do allosteric enzymes do in concerted model?

A
  • 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
51
Q

What does the sequential model assume?

A
  • 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
52
Q

What does substrate binding in sequential model do?

A

Causes a change in one sub-unit

53
Q

How can enzymes be regulated?

A

Through reversible covalent modification

54
Q

What is a common covalent medication?

A

Phosphorylation

55
Q

Where can proteins be phosphorylated?

A
  • At a single site
  • Multiple sites
  • Multiples phosphorylations at one site
56
Q

What enzymes are involved in phosphorylation?

A
  • Protein kinases: add phosphate groups to proteins

- Protein phosphatases: remove phosphate groups from proteins

57
Q

What do multiple phosphorylation sites allow?

A

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

58
Q

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

A

-Inactive precursor called a proenzyme

59
Q

How do proenzymes become active?

A

They can be cleaved by protesases

60
Q

What do digestive enzymes tend to be called?

A

Zymogens

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