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BB10235 > Enzymes > Flashcards

Enzymes Flashcards

1
Q

What are the 4 main differences between enzymes and ordinary chemical catalysts

A
  1. Higher reaction rates- at least several orders of magnitude greater
  2. Milder reaction conditions- temp below 100 degrees, nearly neutral pH’s, atmospheric pressure. Chemical catalysis often require elevated temps and pressures and extreme pH
  3. Greater reaction specificity- rarely have side products and go to completion
  4. Capacity for regulation- activity varies in response to concentrations of substances other than their substrates- allosteric control covalent modification and variation of amount of enzyme synthesised.
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2
Q

What do catalysts do

A
  1. Speed up chemical reactions
  2. Don’t change equilibrium constants
  3. Recycled, they are not consumed in reaction
  4. Are not reagents or co-factors
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3
Q

Give an example of chemical catalysis

A
  1. Hydrogenation of ethene on a metal surface
    a) Surface chemo-adsorption of H2
    b) Surface chemo-adsorption of ethene
    c) Electrophilic addition
    d) Rearrangement
    e) Desorption of ethane product
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4
Q

Compare Kcat/Kuncat values for chemical and enzyme catalysis

A
  1. For chemical catalysis Kcat/Kuncat = 10^1 - 10^4

2. For enzyme catalysis Kcat/Kuncat = 10^4 - 10^17

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5
Q

Name 3 theories of enzyme binding

A
  1. Lock and key hypothesis
  2. Induced fit
  3. Transition state stabilisation model
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6
Q

Briefly describe the lock and key hypothesis

A
  1. Binding site is rigid and complementary to the ligand’s geometry
  2. The specificity of an enzyme for its substrate arises form their geometrically complementary shapes
  3. The shape of the active site is determined by the quaternary and tertiary structure of a protein
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7
Q

Briefly describe the induced fit model

A
  1. The ligand and binding site are flexible
  2. Binding induces a conformation change
  3. Certain enzymes catalyse similar reactions
  4. Induced fit suggests that the active site will interact with the substrate and adapt to it to make the perfect fit
  5. Hexokinase example of induced-fit
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8
Q

Generally what does a substrate binding site consist of

A
  1. An indentation or cleft on the surface of an enzyme molecule that is complementary in shape to the substrate (geometrically complementarity)
  2. Amino acid residues that form the binding site are arranged to interact specifically with the substrate in an attractive manner (electronic complementarity)
  3. Molecules which differ in shape or functional group distribution from the substrate cannot form enzyme-substrate complexes that lead to the formation of products
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9
Q

What is stereo-specificity

A
  1. Stereo-specificity is a description of the reaction path, not the selectivity for substrates or products
  2. Enzymes are highly specific both in binding chiral substrates and in catalysing their reactions
  3. This is because enzymes are chiral so form asymmetric active sites
  4. This is termed ‘enantio-selective’
  5. Enzymes are absolutely stereo-specific in the reactions they catalyse.
  6. A substrate of the wrong chirality will not fit into an enzymatic binding site
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10
Q

Give an example of stereospecificity

A
  1. E.g alcohol dehydrogenase of yeast
  2. Ethanol is pro-chiral
  3. The ADH active site determines the ethanol binding geometry
  4. ADH transfers the pro-R hydrogen of EtOH to the NAD+
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11
Q

What is geometric specificity/ Regiospecificity

A
  1. More strict requirement than stereospecificity
  2. Selective about identities of chemical groups on their substrates
  3. Enzymes vary a lot in their level of geometric specificity
  4. A few enzymes are absolutely specific for only one compound
  5. Most catalyse for reactions of a small range of related compounds e.g. YADH catalyses oxidation of primary and secondary alcohols
  6. Many enzymes particularly digestive enzymes are very permissive in their range of acceptable states
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12
Q

Define cofactor

A
  1. Small molecules required by some enzymes during catalysis
  2. Some cofactors are transiently associated with a given enzyme molecule so they function as co-substrates
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13
Q

Define coeznymes

A
  1. Organic cofactors
  2. Are chemically changed by the enzymatic reactions in which they participate in
  3. So must be returned to original state to complete catalytic cycle
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14
Q

Define prosthetic groups

A
  1. Cofactors which are permanently associated with their protein often by covalent bonds e.g heme prosthetic group of heamoglobin
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15
Q

Define holoenzyme

A
  1. A catalytically active enzyme-cofactor complex
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16
Q

Define apoenzyme

A
  1. The enzymatically inactive protein resulting from the removal of a holoenzyme’s cofactor
17
Q

Show equation linking apoenzyme and holoenzyme

A
  1. apoenzyme (inactive) + cofactor holoenzyme (active)
18
Q

Describe how vitamins are important for enzyme action

A
  1. Many organisms are unable to synthesise certain portions of essential coenzymes and therefore these substances must be present in the organism’s diet- vitamins
  2. Vitamins in the human diet which are precursors are water-soluble
  3. Lipid-soluble vitamins A and D are not components of coenzymes
19
Q

Describe how vitamin B1 (thiamine) acts as a coenzyme precursor

A
  1. Coenzyme= thiamine pyrophosphate
  2. Coenzyme function= decarboxylation reactions
  3. Human deficiency disease= Beriberi
20
Q

Describe how vitamin B2 (Riboflavin) acts as a coenzyme precursor

A
  1. Coenzyme= flavin coenzymes
  2. Coenzyme function= oxidation reduction reactions involving 2 hydrogen atoms
  3. No disease
21
Q

Describe how vitamin B3 (niacine) acts as a coenzyme precursor

A
  1. Coenzyme= NAD+
  2. Coenzyme function= oxidation-reduction reactions involving the hydride ion (H−)
  3. Human deficiency disease= pellagra
22
Q

Describe how vitamin B6 (pyridoxine) acts as a coenzyme precursor

A
  1. Coenzyme= pyridoxal phosphate
  2. Coenzyme function= variety of reactions including the transfer of amino groups
  3. Human deficiency disease= N/A
23
Q

Describe how vitamin B12 (cyanocobalamin) acts as a coenzyme precursor

A
  1. Coenzyme= Cobalamin coenzymes
  2. Coenzyme function= Intramolecular rearrangement reactions
  3. Human deficiency disease= pernicious anemia
24
Q

Describe how vitamin Biotin acts as a coenzyme precursor

A
  1. Coenzyme= biotin
  2. Coenzyme function= carboxylation reactions
  3. Human deficiency disease= N/A
25
Q

Describe how vitamin Folic acid acts as a coenzyme precursor

A
  1. Coenzyme= tetrahydrofolate
  2. Coenzyme function= carrier of one-carbon units such as the formyl group
  3. Human deficiency disease= anemia
26
Q

Describe how vitamin Pantothenic acid acts as a coenzyme precursor

A
  1. Coenzyme= coenzyme a
  2. Coenzyme function= carrier of acyl groups
  3. Human deficiency disease= N/A
27
Q

Describe how vitamin c (Ascorbic acid) acts as a coenzyme precursor

A
  1. Coenzyme= None
  2. Coenzyme function= antioxidant; formation of collagen, a protein found in tendons, ligaments, and bone
  3. Human deficiency disease= scurvy
28
Q

What are two ways that an organism can regulate catalytic activities of its component enzymes

A
  1. Control of enzyme availability- amount of a given enzyme in a cell depends on both its rate of synthesis and its rate of degradation
  2. Control of enzyme activity- Enzymes catalytic activity may be directly regulated through conformational or structural alterations
29
Q

What are the different types of regulation

A
  1. Enzyme regulation can be positive or negative- Activation or Inhibition
  2. It can involve non-covalent or covalent interactions- Covalent binding of the regulatory molecule and Non-covalent interaction of the regulatory molecule e.g. Ionic, hydrophobic, van der Waals forces
  3. It can be reversible- Non-covalent and covalent
  4. It can be irreversible- Post translational modifications, protein cleavage, irreversible regulatory molecule binding
30
Q

Describe non-covalent regulation

A
  1. E + R ER
  2. Remove R and activity returns to normal
  3. It is highly specific- So only the target enzyme is affected
  4. Two main types of non-covalent regulator- Simple inhibitors and Allosteric regulators

BB10235 (16 decks)

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