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Enzymes & Coenzymes Flashcards

1
Q

What do enzymes do?

A

(Biological catalysts)

Speed up the rate of a reaction without altering the final equilibrium

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2
Q

What reaction does catalase catalyse? How much faster is the rate of the reaction when catalysed?

A

Hydrogen peroxide to water

10^14 times

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3
Q

What kind of molecule are enzymes?

A

Mainly proteins

Ribosomes = RNA-based

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4
Q

What does an enzyme affect which allows the reaction to occur faster?

A

(Lowers) activation energy

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5
Q

Why does the reaction reach equilibrium with an enzyme?

A

Product has (low) affinity for the enzyme

Product inhibition

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6
Q

What reactions does alcohol dehydrogenase catalyse?

A

Oxidation of primary alcohols to aldehydes

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7
Q

What is the term used to describe when an enzyme can catalyse the reaction of several similar molecules?

A

Group specificity

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8
Q

What will fit in an enzyme’s active site?

A

A substrate with the correct shape and charge

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9
Q

What is an example of a reaction of multiple enzymes acting sequentially on a substrate?

A

D-glucose to lactate/lactic acid

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10
Q

How does the IUB Commission on Enzymes classify enzymes?

A

1-6 based on type of reaction catalysed

4 number code for every individual enzyme

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11
Q

What are the main six classes of enzyme?

A
  1. Oxidoreductases
  2. Transferases
  3. Lyases
  4. Hydrolases
  5. Isomerases
  6. Ligases
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12
Q

What do enzymes of class 1 do?

A

Oxidoreductases

Catalyse redox reactions

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13
Q

Give an example of a class 1 enzyme

A

Lactate dehydrogenase

Anaerobic respiration: pyruvate + NADH + H+ –> L-lactate + NAD+

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14
Q

What do enzymes of class 2 do?

A

Transferases

Catalyse the transfer of functional groups between (different) molecules

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15
Q

Give an example of a class 2 enzyme

A

Alanine aminotransferase

Transamination: glutamate + pyruvate –> L-alanine + a-ketoglutarate

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16
Q

What do enzymes of class 3 do?

A

Lyases

Catalyse the cleavage of C-C, C-O or C-N bonds

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17
Q

Give an example of a class 3 enzyme

A

ATP-citrate lyase

ATP and CoA used in C-C cleavage citrate –> oxaloacetate + acetate

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18
Q

What do enzymes of class 4 do?

A

Hydrolases

Catalyse the cleavage of bonds by the addition of water (often ester bonds)

19
Q

Give an example of a class 4 enzyme

20
Q

What do enzymes of class 5 do?

A

Isomerases

Catalyse the transfer of functional groups within the same molecule

21
Q

Give an example of a class 5 enzyme

A

Phosphoglucose isomerase

Glucose-6-phosphate –> fructose-6-phosphate movement of C=O

22
Q

What do enzymes of class 6 do?

A

Ligases

Use ATP to catalyse the formation of new covalent bonds

23
Q

Give an example of a class 6 enzyme

A

DNA ligase

Forms phosphodiester bonds to join 2 segments of DNA

24
Q

What level of protein structure is most important in determining the enzyme’s function?

25
What is an enzyme's thermostability?
Range of temperatures within which the enzyme's structure is maintained (resists irreversible change)
26
What does the Lock and Key model describe?
Enzyme's active site and substrate's shape are always perfectly complementary to each other
27
What does the modified Lock and Key model describe?
Enzyme distorts substrate molecule as active site is not perfectly complementary (increases energy of substrate)
28
What does the Induced Fit model describe?
Both the enzyme and substrate change shape on binding to secure substrate and increase its energy (adaptive response)
29
What does chymotrypsin do?
Cleaves peptide bonds on carboxyl side of tyrosine, tryptophan, phenylalanine and large hydrophobic residues (eg. methionine)
30
What is the name given to the three amino acids in the active site involved in catalysis?
Catalytic triad
31
What amino acids are part of the catalytic triad of chymotrypsins?
Serine Histidine Aspartic acid
32
What are the two general phases of chymotrypsin catalysis?
1: serine nucleophile attacks substrate forming a covalent intermediate bonded to enzyme; first product released (PC) 2: nucleophile created from water attacks covalent intermediate breaking covalent bond to enzyme; second product released (PN)
33
Describe the process of chymotrypsin catalysis
1. Polypeptide substrate binds non-covalently with side chains of hydrophobic pocket 2. Proton transferred from Ser to His; Ser: attacks C to create a tetrahedral transition state (FIRST TRANSITION STATE) 3. Proton transferred from His to C-terminal fragment; cleavage of C-N bond and release of PC 4. Water binds to enzyme in place of PC and transfers proton to His to create OH:- 5. OH:- attacks carbon of N-terminal fragment creating another tetrahedral transition state (SECOND TRANSITION STATE) 6. Acyl bond cleaved so PN released; proton transferred back to Ser and enzyme returns to original state
34
What are metalloenzymes?
Enzymes that rely on metallic cofactors in order to work (usually transition metals)
35
What is special about flavin enzymes?
Iron-sulphur centre
36
What is special about cytochrome oxidase?
Haem group
37
What metallic cofactors does cytochrome oxidase have?
Cu2+ Fe2/3+
38
What metallic cofactor does pyruvate kinase have?
K+
39
Why does pH affect enzymes?
Affects protonation of side chains Affects weak bonds maintaining tertiary structure
40
What are isoenzymes?
Enzymes with different protein structures but catalyse the same reaction
41
Where is H4 lactate dehydrogenase found?
Heart
42
Where is M4 lactate dehydrogenase found?
Muscle
43
What is the structure of lactate dehydrogenase?
Four subunits, each of which can be heart or muscle type
44
Which form of lactate dehydrogenase has a lower Km and why?
Heart Lower Km = high affinity for lactate so reaction is faster at lower concentrations of lactate Do not want lactate to cause muscle cramping in heart

BMS02 (10 decks)

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