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Molecular Cell Biology FoM > Enzymes > Flashcards

Enzymes Flashcards

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1
Q

What are enzymes for?

A
  • Most reactions in biological systems don’t occur at observable rates unless enzymes are present
  • Vital for cellular and body functions
  • Most enzymes are proteins except for ribozymes (RNA)
  • 25% of all genes code for enzymes
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2
Q

Describe enzyme nomenclature

A
  • Suffix -ases to root name of substrate it acts upon
  • Some enzymes discovered early also have common or tribial names
  • Latest system is international enzyme commission (EC) system based on type of reaction catalysed
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3
Q

How do enzymes work?

A
  • Increase reaction rates by factors of at least a million
  • They do this by lowering the activation energy for a reaction
  • Bring substrates/domains in favourable orientations into an enzyme substrate complex
  • Anabolic: building of molecules
  • Catabolic: breaking down substrate
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4
Q

What is the active site?

A
  • Determines substrate binding- residues that participate in making/breaking bonds
  • Substrates are bound by multiple weak attractions
  • Specificity depends on precise arrangement of atoms in the site- non-polar residues
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5
Q

Describe the lock-and-key model of enzyme action

A
  • Substrate fits enzyme
  • Forms ES complex
  • Highly specific
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6
Q

Describe the induced fit model of enzyme action

A
  • Enzyme folds around substrate
  • Breaks it down
  • Enzyme returns to original shape
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7
Q

What evidence is there for the active site?

A
  • At a constant conc. of enzyme, reaction rate increases with increasing substrate conc. until maximal velocity reached
  • Occurs due to all catalytic sites being filled (Leanor Michaelis, 1913)- Vmax
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8
Q

What is the Vmax?

A
  • The maximum velocity of enzyme reaction

- When the substrate concentration increases, there are no enzymes left

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9
Q

Describe the Michaelis-Menten plot

A
  • Sub conc (x) against Reaction rate (y)
  • Curve that is steep at first, then plateaus
    V = Vmax[S]/ (Km+[S])
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10
Q

What is Km?

A
  • A constant that is equal to [s] at half the Vmax

- Represents the most efficient substrate concentration at a specific enzyme concentration

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11
Q

What is the Lineweaver-Burke plot?

A
  • Reciprocal of the Michaelis-Menten plot

- Kinetic

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12
Q

What types of catalysis are there?

A
  • Metal ion catalysis
  • Electrostatic catalysis
  • Covalent catalysis
  • Acid-base catalysis
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13
Q

Describe metal ion catalysis

A

Metal ion serves as an electrophilic catalyst (copper, zinc, iron)

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14
Q

Describe electrostatic catalysis

A

Active site residues or cofactor form ionic bonds with intermediate

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15
Q

Describe covalent catalysis

A

Active site residues or cofactor from transient covalent bond with the substrate to stabilise the intermediate

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16
Q

Describe acid-base catalysis

A

Active site contains chemicals which proton donors or acceptors

17
Q

Describe mechanisms of enzyme regulation

A
  • Feedback inhibition
  • Gene expression controlling enzyme protein synthesis
  • Proteolytic regulation through zymogen activation
  • Cofactor binding
  • Post-translational modification
  • Substrate availability
  • Control protein binding
18
Q

Describe feedback regulation

A
  • Downstream reaction products inhibit initial enzymic reaction
  • Essential process or regulatory enzymes at the beginning of reaction series, TCA cycle enzymes such as pyruvate dehydrogenase are inhibited by NADH
  • Enzyme pathway leads to final product that eventually inhibits original enzyme
19
Q

Describe allosteric regulation

A
  • ‘Other site’- i.e. binding at remote site affects normal substrate binding
  • Saturation curve does not follow Michaelis-Menten characteristics, e.g. sigmoidal
  • E.g. feedback regulation
  • ATP allosteric inhibitor of pyruvate kinase during glycolysis
  • Can be activating- allows substrate to bind
  • Or inhibiting- prevents substrate binding
20
Q

Describe enzyme regulation through synthesis

A
  • Transcription/translation, e.g. methlyation
  • Changes in total amount of enzyme
  • Post-translational modifications
    • Glycosylation, hydroxylation, sulphation
21
Q

Describe proteolytic regulation

A
  • Examples include pepsinogen conversion to pepsin in acidified gastric juice
  • Protein cleavage
  • Positive feedback
22
Q

What are co-factors?

A

Many enzymes require co-factors ‘helper molecules’- non protein chemical compounds/metal ions

23
Q

What are co-enzymes?

A

Soluble and may diffuse between different enzymes, i.e. ATP, NADH

24
Q

What metal ions?

A

Often bound by dipole interactions with histidine and other amino acids with lone pairs, e.g. iron 2 in catalase

25
Q

What are prosthetic groups?

A

Strongly bound to enzyme, either covalently or by other means, i.e. haem

26
Q

Describe pharmacological interventions

A
  • Many diseases result from enzyme mutations
  • Leading to loss of enzyme activity- causing potential harmful effects
  • Many drugs that alter enzyme activity improve health conditions
27
Q

Give examples of diseases caused by enzyme mutations

A
  • Niemann-Pick disease- sphingomyelinase deficiency- accumulation of unmetabolised sphingomyelin and cholesterol- lipid build up in organs- increased CVD risk
  • Krabbe disease- galactosylceramidase deficiency- affect myelin- degeneration of motor skills
28
Q

Describe non-specific inhibitors of enzyme activity

A
  • Denaturing
    • Extreme pH
    • Heavy metals
    • Heat
    • Reducing agents
  • Tightly bound to enzyme can be artificial/poison/suicide
29
Q

Describe specific inhibitors of enzyme activity

A
  • Competitive
  • Reversible
  • Uncompetitive
  • Non-competitive
30
Q

Describe irreversible inhibitors

A
  • Often tightly bound to enzyme, either covalently or non-covalently, e.g. Thiol group inhibitors like iodoacetamide
  • Often artificial
  • Often posions
  • Include ‘suicide’ inhibitors
31
Q

Describe reversible inhibitors

A
  • Rapid dissociation of enzyme/inhibitor complex
  • Usually specific
  • 3 major categories
  • Comp, uncomp., non-comp.
32
Q

What is the difference between non-competitive and uncompetitive?

A
  • Non-comp binds allosterically to the enzyme equally well whether substrate is bound or not
  • Uncomp binds allosterically to the ES complex and not the free enzyme
33
Q

Give examples of inhibitor use

A
  • Alcohol poisoning

- NSAIDs

34
Q

Describe alcohol metabolism

A
  • Alcohol dehydrogenase converts ethanol to acetaldehyde
  • Aldehyde dehydrogenase converts acetaldehyde to acetic acid
  • Both of these enzymes are oxidoreductases

Molecular Cell Biology FoM (12 decks)

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